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Federal Ministry
of Labour and Social Affairs
Forschungs
penavi
Sozialforschung
400-E
Electromagnetic
fields at workplaces
Final Report
ISSN 0174-4992
Impressum:
Herausgeber: Bundesministerium furArbeit und Soziales
Referat Information. Publikation, Redaktion
53107 Bonn
Stand: NOvember 2011
Artikel-Nr.: FB 400-E
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Bericht der EMF-Arbeitsgruppe des
Bundesministeriums für Arbeit und Soziales
Elektromagnetische Felder am Arbeitsplatz
Ein neuer wissenschaftlicher Ansatz für die Sicherheit
und den Gesundheitsschutz der Beschäftigten
Electromagnetic fields at workplaces
A new scientific approach to occupational health and safety
F. Börner
H. Brüggemeyer
S. Eggert
M. Fischer
H. Heinrich
K. Hentschel
H. Neuschulz
Stand: November 2011
Executive summary
This report provides an in-depth analysis of the physical and physiological background for an
effective protection of the health and safety of workers with respect to occupational exposure
to electric, magnetic and electromagnetic fields (EMF), based on current scientific knowledge.
Answers are given to the concerns being raised by stakeholders and to shortcomings within Directive
2004/40/EC. Therefore, information provided in this report, especially the figures and tables in
section 4.1 and 4.2, can serve as a sound base for a review of the risk-related provisions of Directive
2004/40/EC.
A revised concept of exposure limit values for the low frequency electric and magnetic fields is
based on the physiologically relevant parameter of the peak electric field strength in the tissue and
represents common scientific understanding. Based on this concept a set of exposure limit values
has been laid down guaranteeing the health and safety of workers without the need for unnecessary
and costly measures or unduly impacting the use of certain technologies or industrial processes. For
an easy and also cost-effective assessment of the risks due to the exposure to low frequency electric
and magnetic fields and in order to avoid unnecessary complex and time-consuming calculations
currently necessary to show the compliance of an exposure situation with the exposure limit values,
two sets of easier-to-implement action levels are given. These action levels can be compared directly
with measurable electric field strengths or magnetic flux densities.
Because all EMF-related biological effects in the low frequency range are linked to peak values
of the internal electric field strength in the tissue, all exposure limit values and lower and upper
action levels are given as peak values and not as rms-values as in Directive 2004/40/EC.
The report also addresses the risks of workers with respect to the movement and the projectile risk
in static magnetic fields. For the low frequency range it provides a sound solution on how to deal
with pulsed electric and magnetic fields, multi-frequency electric and magnetic fields and contact
currents. Contact currents are now classified as exposure limit values because of the biological
relevance.
For both the static and the low frequency range, effects of localized exposure and time or spatial
averaging are considered in the report. So far, no changes have been proposed for frequencies
higher than 100 kHz.
Preface
Due to the ongoing technological development and scientific research regarding occupational exposure to electric, magnetic and electromagnetic fields, this report presents the current knowledge
and understanding of open questions and concerns on a solid and well established scientific foundation. This report provides an in-depth analysis and the most up-to-date information available for
the ongoing discussion concerning occupational health and safety with regard to workers exposure
to static and low frequency electric and magnetic fields. If necessary, it will be updated when new
technologies emerge, new studies and results become available or new questions and concerns are
being raised.
Apart from the considerations in this document, additional guidance and information may be
necessary to assist the employer in risk assessment, thus saving time and money while guaranteeing
the safety and health of workers at the same time.
Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2
Physiological effects of EMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2.1
Direct effects of electric, magnetic and electromagnetic fields . . . . . . . .
2
2.1.1
2
2.1.2
Static electric fields . . . . . . . . . . . . . . . . . . . . .
2
2.1.1.2
Low-frequency electric fields . . . . . . . . . . . . . . . .
3
Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
Static magnetic fields . . . . . . . . . . . . . . . . . . . .
4
2.1.2.2
Low-frequency magnetic fields . . . . . . . . . . . . . . .
5
High-frequency electromagnetic fields . . . . . . . . . . . . . . . .
6
Indirect effects of electric and magnetic fields . . . . . . . . . . . . . . . . .
6
2.2.1
Electric fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.2.2
Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
Body models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Neurophysiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3.1
Mechanisms and facts for the creation of action potentials . . . . . . . . . .
8
3.2
Electrical stimulation of excitable tissues
2.2
2.3
4
2.1.1.1
2.1.2.1
2.1.3
3
Electric fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
12
3.2.1
Basic facts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
3.2.2
Long stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
3.2.3
Short stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
3.2.4
CNS tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
3.2.5
Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
3.2.6
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
Limiting occupational exposure to static and low frequency electric and magnetic
fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.1
Exposure limit values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.1.1
Static electric fields . . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.1.2
Static magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.1.3
Low frequency electric and magnetic fields . . . . . . . . . . . . .
20
4.1.4
Contact currents . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
Upper and lower action levels . . . . . . . . . . . . . . . . . . . . . . . . . .
23
4.2.1
25
4.2
Upper action level . . . . . . . . . . . . . . . . . . . . . . . . . . .
i
4.2.2
4.2.1.1
Electric fields . . . . . . . . . . . . . . . . . . . . . . . . .
25
4.2.1.2
Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . .
26
Lower action level . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
4.2.2.1
Electric fields . . . . . . . . . . . . . . . . . . . . . . . . .
28
4.2.2.2
Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . .
29
Special exposure situations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
5.1
Simultaneous exposure to electric and magnetic fields . . . . . . . . . . . .
31
5.2
Simultaneous exposure to multiple field sources operating with the same
frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
Simultaneous exposure to multiple frequency fields . . . . . . . . . . . . . .
31
5.3.1
Summation formulae . . . . . . . . . . . . . . . . . . . . . . . . . .
31
5.3.2
Assessment of fields with arbitrary temporal behaviour . . . . . .
32
5.3.3
Harmonic content . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
5.4
Localized exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
5.5
Movement in static magnetic fields . . . . . . . . . . . . . . . . . . . . . . .
34
5.6
Interference with active implanted medical devices (AIMD) . . . . . . . . .
34
5.7
Projectile risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
Annex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
A
Quantities, variables, abbreviations and SI-units . . . . . . . . . . . . . . . . . . .
44
B
Tissue data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
5
5.3
ii
1
Introduction
On 29 April 2004, the European Parliament and the Council adopted Directive 2004/40/EC on the
minimum health and safety requirements regarding the exposure of workers to the risks arising from
physical agents (electromagnetic fields). This directive is commonly referred to as the EMF Directive. It establishes minimum health and safety requirements for the protection of workers against
the risks arising from exposure to static and time-varying electric, magnetic and electromagnetic
fields (EMF). The frequency range extends from 0 Hz to 300 GHz.
Directive 2004/40/EC obliges the employers to assess the risks arising from electric, magnetic and
electromagnetic fields at the workplace and to take adequate measures to eliminate or to minimize
such risks where necessary. The Directive refers to a set of exposure limit values (ELV) listed in
table 1 in its Annex. The exposure to electromagnetic fields cannot be measured directly because
the physiologically relevant physical quantities, e.g. current density and specific absorption rate,
only exist inside the human body. In order to facilitate the application of the directive a set of
so called action values (AV) was given to simplify the determination of the level of exposure at a
workplace. If these action values are not exceeded, an inherent compliance with the exposure limit
values is guaranteed.
However, the exceedance of the action values does not automatically lead to an exceedance of the
exposure limit values. Where action values are exceeded, employers can make further efforts to
assess and, if necessary, prove that the exposure is still below the exposure limit values.
Since the adoption of Directive 2004/40/EC scientific knowledge with regard to
• the concept of limit values,
• risks related to the movement in a static magnetic field,
• the projectile risk,
• risks related to pulsed electric and magnetic fields,
• risks related to multi-frequency electric and magnetic fields,
• risks related to contact currents,
• risks related to implanted medical devices
has significantly improved.
This report will address these aspects on a solid and well established scientific and technological
basis.
1
2
Physiological effects of EMF
The physiological effects of electrical, magnetic and electromagnetic fields on the human body are
dependent on the frequency.
The effects of static electric fields are limited to the surface of the human body and can cause
motion of body hair and corona discharges.
Static magnetic fields exert forces on ferro- and dia-magnetic materials as well as charged moving
particles. This may lead to acceleration, torque effects and the induction of electric fields in the
tissue.
In the low-frequency range up to some 100 kHz the main physiological effect is the electrical
stimulation of excitable body tissues like muscles, nerves and sensory organs.
In the frequency range between several 100 kHz and some MHz electrical stimulation and tissue
heating occurs. The higher the frequency, the more the tissue heating effects increase and the
stimulation effects decrease. Tissue heating effects are dominant for frequencies above several
MHz.
A further distinction is made with regard to the interaction with the human body. If there is a
direct interaction between EMF and the human body, e.g. stimulation of muscles, nerves or sensory
organs or tissue heating, this type of interaction is called an direct effect.
If there is an interaction between EMF and objects outside the human body, e.g. contact currents,
projectile risk or the interference with implanted medical devices, this type of interaction is called
a indirect effect.
There are no confirmed long-term health effects related to the exposure to EMF.
2.1
Direct effects of electric, magnetic and electromagnetic fields
2.1.1
Electric fields
The relationship between the external undisturbed electric field strength E0 and the electric field
strength hereby induced in the body tissue Ei is established through the condition that the normal
component of the displacement current must remain steady at the surface boundary of the human
body [30, 41].
For a simple homogeneous ellipsoid model of the body it is expressed by:
E0 · k · ε0 · 2π · f = κ · Ei
with
k
ε0
f
κ
(2.1)
field distortion factor; for human beings k ≈ 13 . . . 18
A2 ·s4
permittivity of free space (vacuum); ε0 = µ01·c2 ≈ 8, 854 · 10−12 kg·m
3
0
frequency of the field
(mean) conductivity of the body tissue(s)
In principal eqn. 2.1 remains valid even for more natural and anatomically correct body models.
However, the variables k and κ become parametric functions.
2.1.1.1
Static electric fields
As an immediate result of eqn. 2.1 for static fields (f = 0) it follows that the electric field strength
inside the tissue Ei is (nearly) zero, regardless of the electric field strength of the external static
electric field. The external static electric field breaks down completely at the surface of the human body, the inner body is totally shielded from any effect of the external static electric field.
Therefore, no direct physiological effect can occur inside the human body.
External static electric fields, exceeding E0 ≈ 30 kV/m, can cause corona discharges at the surface
of the human body, e.g. fingers, nose, ears, hairs [41, 63]. Those corona discharges depend on the
2
external field strength, posture, the size and form of the body and climatic factors, e.g. relative
humidity. Such discharges can be annoying, startling or even painful.
Significant external static electric fields can only occur where high DC-voltage is used (DCpowerlines, including switchyards and inverter stations) or can be produced by triboelectricity,
e.g. plastics production and other industrial processes where highly insulating solids or liquids are
handled and charge separation can occur.
2.1.1.2
Low-frequency electric fields
External low frequency (LF) electric fields can generate internal electric fields in the tissue. Consulting eqn. 2.1 the relationship between external and internal field strength can be rewritten
as:
Ei =
k · ε0 · 2π · f
· E0
κ
(2.2)
As (k ·ε0 ·2π ·f ) is very small compared to κ in the low frequency range, there still exists a shielding
effect from the outside to the inside of the human body. However, it is not a complete shielding
like in static electric fields.
Therefore there is the potential for adverse effects inside the human body. However, external
electric fields used by technological processes or near electric powerlines are generally not strong
enough to cause adverse health effects.
Especially for power frequency fields with an external electric field strength of some (kV/m) the
electric field strength in the body tissue is in the range of some (mV/m). With very high electric
field strengths currently in use by technological processes (mean value ≈ 180 kV/m for experimental
high-voltage transmission lines with voltages >1500 kV [46]) and limited by the breakdown field
strength of air (≈ 3000 kV/m for homogeneous fields [41, 46]), it is not possible to generate
electrical fields in the tissue of the human body that can trigger any adverse physiological effects
like electrophosphenes (E0 >200 kV/m at 50 Hz) or peripheral nerve stimulation (E0 >4000 kV/m
at 50 Hz).
Very strong electric fields higher than 30 kV/m can also cause corona discharges on the surface of
the human body.
However, these electrical field strengths in the tissue can interfere with the proper operation of
active medical implants, e.g. pacemakers or cardioverter defibrillators, see section 5.6.
2.1.2
Magnetic fields
Magnetic fields exert physical forces on electric charges, but only when such charges are in motion.
There are three physical effects of magnetic fields on biological tissues:
• Electrodynamic forces and magnetic induction
• Magneto-mechanical effects
• Electron spin interaction
The main effect of a magnetic field is the Lorentz force F~ on a point charge q moving with velocity
~v as described by:
~
F~ = q ~v × B
(2.3)
Due to these forces charge transfers develop in the biological tissue. They generate differences in
the electrical potential and thus an electrical field strengths in the tissue of the human body. The
3
connections between electrical field strength and the magnetic flux density describes Faraday’s law
of induction [30].
Z
I
~ · dA
~
~ · d~l = − d
B
(2.4)
E
dt
The left-hand side of eqn. 2.4 is a line integral over a closed loop and the right hand side is the
time derivative of a surface integral of the normal component of the magnetic flux. The equation
only calculates the average electric field over the loop, but it is often the only measure available
when the actual local field in a complex system can only be estimated with numerical methods
that require very detailed knowledge about the fields on the system boundary and its material
properties. If we assume that the bulk conductivity of the material is relatively homogeneous, we
can also infer the average induced current by Ohm’s law.
The equation will register an average electric field when the integral changes with time. If we
consider the loop to be of fixed dimensions, this can happen in several different ways:
• the magnetic field itself varies with time. This is the typical situation for many field studies
in which a spatially homogenous field is modulated, e.g. with a sine wave;
• by motion in a field that has spatial variation. This is, for instance, relevant when transporting a person into or out of a magnetic resonance imaging (MRI) machine that has very
strong spatial gradients at the opening to the bore;
and
• the relative orientation between the loop and the field vector is changed. This happens when
we rotate the loop in a static field.
The field gradients are decisive and can enhance the induction effect.
It makes no difference whether a person is stationary in a field changing over time or whether a
person moves in a constant magnetic field. In both cases the effect is the same: Induction of an
electrical field in the body tissue.
2.1.2.1
Static magnetic fields
Current data suggests possible pathological effects of static magnetic fields, e.g. induced blood flow
potentials around the heart which might interfere with the autonomous heart action, increases
in blood flow resistance due to magneto-hydrodynamic effects, can occur only in magnetic flux
densities exceeding 10 T [54, 55, 56, 71, 99, 112, 107].
However, it must he noted, that the actual data base is very small for flux densities exceeding 8T.
Available studies for flux densities exceeding 8 T are often not replicated.
Detailed information of direct effects of static magnetic fields, especially for MRI and magnetic
resonance spectroscopy (MRS) applications can be found in [54, 55, 56, 99, 112]. However, all
conclusions must be carefully examined to determine if they are really due to the direct physiological
effects of static magnetic fields, because they are sometimes mixed up with effects from time-varying
magnetic fields or movements in static magnetic fields.
Changes in a static magnetic field, e.g. time-variation or movement, induces electric fields in body
tissues. The induced field may interact with the human body by several mechanisms. The main
mechanisms are sensory or nerve stimulation – see section 3. The occurrence of these effects is
dependent on the temporal gradient of the field or the spatial gradient of the field and the movement
speed of the subject.
Movement in a static magnetic field – see section 5.5 – causes a low frequency internal electric field
in the tissue.
4
The strongest static magnetic fields are currently used in MRI with flux densities up to 14 T and
MRS with magnetic flux densities up to 25 T.
Other sources of strong static magnetic fields are thermonuclear reactors, magnetohydrodynamic
systems, particle accelerators, and superconducting generators. Industries where strong magnetic
field exposure can occur are those involving electrolytic processes such as chlorine or aluminum
production and in the manufacture of permanent magnets and magnetic materials. The typical
exposures in these industries are a few mT of the working day with peak exposures up to several
tens of mT.
2.1.2.2
Low-frequency magnetic fields
The main physiological effect of low-frequency (LF) magnetic fields is the induction of electrical
fields in the human body and the stimulation of excitable body tissues, like sensory organs, nerves
and muscles.
Because different excitable body tissues have different maximum sensitivities with respect to the
frequency, the major points of interaction change with the frequency. Table 2.1 shows for some
physiological effects their major point of interaction and their frequency range of maximum sensitivity.
Maximum sensitivity
Physiological effect
Point of interaction
Metallic taste
Various receptors in the tongue
(shift in ion gradients)
Vertigo, nausea
Blood flow induced electric
fields in tissue
Inner ear (vestibular system)
Nerve, muscle excitation
(interference with heart action)
≈ 20 Hz
Magnetophosphenes
Retina
≈ 50 Hz
Tactile and pain sensations
Loss of muscle control
Interference with
autonomous heart action
Peripheral nerves
Peripheral nerves, muscles
1 Hz
< 0.1 . . . 2 Hz
Table 2.1:
Heart
Frequency range of maximum sensitivity and major point of interaction for
some physiological effects
Extremely low frequency sensory effects caused by movements in a strong static magnetic field are
experienced with flux densities above 2 - 3 T [26, 36, 56]. The maximum sensitivity is expected at
frequencies around 0.1 Hz. Sometimes these effects last longer than the actual field exposure and
can be detrimental to work performance and quality.
Pathological effects of blood flow induced electric fields in the tissue causing nerve and muscle excitation in the immediate vicinity of these blood vessels or that may interfere with the autonomous
heart action are expected for flux densities exceeding 8 - 10 T [99].
However, exposure during typical industrial processes, including e.g. electrolysis, electroplating or
welding is well far below these threshold values.
Magnetophosphenes give the magnetically evoked appearance of light spots in vision. They have a
very sharp response peak (maximum sensitivity) at ≈ 20 Hz. For lower frequencies the sensitivity
decreases approximately proportional with f , for higher frequencies the decrease in sensitivity is
proportional to nearly f 3 .
At the frequency 50 Hz there is the maximum sensitivity for nerve and muscle stimulation. However, the response curve is very flat in the frequency range from 10 Hz to some hundreds of Hz.
5
Only for frequencies higher than 3 - 5 kHz the sensitivity decreases approximately proportional
with f .
In general, the thresholds for the direct stimulation of muscles are much higher than for nervous
structures. However, the exposure at most industrial workplaces is far below both threshold values.
All these physiological effects have a clearly defined threshold. Any stimuli below the threshold
value will not cause an adverse effect, even when applied for a long time [50] – see also section 3.
A comprehensive compilation of direct physiological effects of LF magnetic fields can be found in
[51, 54, 55, 57, 63, 64, 68, 70, 82, 89, 90, 91, 108].
Low frequency magnetic fields are ubiquitous at workplaces where electric energy is used. Magnetic fields will be produced e.g. by transmission lines, underground cables, distribution lines,
transformers, electric railway systems, household appliances, resistance and induction heating systems, hand-held electric tools and arc, spot and resistance welding equipment.
Exposure to low frequency magnetic fields at workplaces in terms of magnetic flux density ranges
from some nano- or microtesla, e.g. in office buildings, up to several tens or hundreds of millitesla,
e.g. at industrial workplaces.
The frequency range covered reaches from fractions of 1 Hz, e.g. movement in static magnetic
fields, up to some tens or hundreds of kHz, e.g. induction heating.
2.1.3
High-frequency electromagnetic fields
The direct effect of high frequency (HF) electromagnetic fields is the penetration of HFelectromagnetic fields in the body and the absorption of energy in tissues. The energy absorption
causes an increase of temperature in the tissue which could lead to an increase in body temperature.
To prevent adverse health effects the increase in tissue and body temperature must be limited. A
commonly used value is to limit the temperature increase in the tissue caused by an electromagnetic
field to a maximum value of 1 ℃ [51, 106].
The penetration depth into the biological tissue depends on the frequency of the electromagnetic
field and the electric properties of the body tissue. The higher the frequency of the electromagnetic
field and the electrical permittivity of the tissue, the shorter the penetration depth.
For continuous-wave exposures with frequencies exceeding 10 GHz the penetration depth is very
short and the total energy is absorbed in the top layers of the skin.
2.2
2.2.1
Indirect effects of electric and magnetic fields
Electric fields
Static electric fields can accelerate dust particles towards the worker and therefore enhance the
dust deposition on the worker. This can lead to allergic and inflammatory reactions in sensitive
personnel.
Movement or vibration of body hair can also occur in static and time-varying electric fields, creating
a possible annoyance. However, the perception threshold of hair vibration shows a wide individual
variation [10, 108].
Contact currents occur, if a worker touches a charged object or touches a grounded object while
being charged himself, due to exposure to a electric field or due to triboelectricity. The resultant
physiological effect is largely dependent on the size of the contact area, e.g. touch or grasp contact,
and on the amount of discharge energy and transferred charge, as well as the amplitude and
frequency of the continuously flowing contact current. These effects can be annoying, painful or
can have life threatening consequences [18, 58, 59, 60].
In general, two different phases of a contact current event can be distinguished:
6
• a spark discharge, i.e. an initial discharge current impulse
• a continuous contact current
Depending on the specific exposure scenario only one or both phases of the contact current event
might be present. Usually, the initial discharge current with a duration in the sub-millisecond
range is only present for exposure situations involving either a static or time-varying electric field.
In general, a continuous contact current is linked to time-varying electric or magnetic fields, but
can also occur in conjunction with ongoing triboelectric processes. The frequency of the continuous
contact current depends on the frequency of the causal time-varying electric field, but can also be
a DC current in case of triboelectric processes.
Therefore it is necessary to limit both phases of the contact current event.
The thresholds for perception and pain are lower for touch contact when compared to grasping
contact. For a frequency of 50 Hz the perception thresholds for such touch and grasp currents are
in the range of 1 . . . 3.5 mA (rms). For frequencies in the 100 kHz and MHz range, the thresholds
are up to 40 . . . 50 mA (rms) [3, 10, 13, 21, 22, 39, 58, 59, 60, 106, 108].
If in a certain workplace environment, e.g. high-voltage switchyards, spark discharges or contact
currents cannot be avoided by technical measures, workers should be trained to always make grasp
contact or instructed to use special work techniques, e.g. equalization of potentials, or work gear,
e.g. insulating or conductive gloves.
2.2.2
Magnetic fields
Indirect effects of static and time-varying magnetic fields are translational and rotational forces
on ferromagnetic and conductive objects, interference with AIMDs and the heating of conductive
objects.
A quantitative solution for the translational and rotational forces on a ferromagnetic object being
placed in a static magnetic field can be found in chapter 5.7.
For the magnetic field characteristic (spatial magnetic gradient) of an unshielded magnet a minimum magnetic flux density of Bz ≈ 60 mT is needed to overcome the initial frictional force, which
in turn makes it possible that a sphere is accelerated in the magnetic field and a so-called projectile
risk can occur. This result is in good agreement with the value given in [21].
In general, shielded superconducting magnets have higher spatial gradients at their openings to
the bore. This leads to a lower minimum magnetic flux density which could constitute a so-called
projectile risk. Current data for shielded systems indicates a minimum magnetic flux density in
the central axis of a superconducting cylindrical magnet in the range from 30 . . . 40 mT necessary
for a projectile risk to occur.
For non-spherical objects not only a translational force can exist, but a torque as well. Needleshaped rotational ellipsoids try to turn their long axis parallel to the direction of the field. The
magnitude of the torque is proportional to the square of the static magnetic flux density Bz2 , so
the maximum torque is to be expected in the center of the magnet and can be higher than the
maximum translational force. Personnel working in areas with high static magnetic fields, e.g.
MRI, MRS, electrolysis, electroplating, particle accelerators, superconducting generators, should
be informed that these torques can occur and trained to avoid any interference with the proper
handling of tools and material.
Interference mechanisms for static and time-varying magnetic fields with AIMDs are covered in
section 5.6.
High time-varying magnetic fields can also heat up conductive objects, e.g. passive medical implants
and tools. A detailed risk assessments needs to be carried out for workers with passive medical
implants who are exposed to high time-varying magnetic fields or who need to handle conductive
objects in these fields.
7
2.3
Body models
Directly measurable external quantities, e.g. electric field strength, magnetic flux density or contact current, are linked to the exposure-limiting body internal quantities, e.g. peak electric field
strength in the tissue, by using analytical and numerical body models with different resolution and
complexity. All calculations throughout this report are done using simple ellipsoid models [63, 75]
– mainly used for validation purposes –, detailed anatomical models based on the Visible Human
data set [81] and on CAD models of the Virtual Family [23] with voxel sizes in the range from 1 to
5 mm3 . For calculations inside of the eye and the inner ear custom made high resolution models
with spatial resolutions of up to 0.1 mm3 were used.
3
3.1
Neurophysiology
Mechanisms and facts for the creation of action potentials
The main physiological effect of electrical fields in the body tissue created by low-frequency electric
or magnetic fields is electrical stimulation of excitable body tissues, like sensory organs, nerves and
muscles.
It is therefore of utmost importance to understand the underlying neurophysiological processes
which lead to the generation of action potentials, their thresholds, time behavior and other important parameters, in order to limit the exposure to low frequency electric and magnetic fields, thus
protecting the health and safety of workers while being exposed to these physical agents.
Figure 3.1:
Schematic structure of a typical CNS or motor neuron
(In part from [111])
Fig. 3.1 shows the simplified structure of a typical neuron. Basic components of a neuron are one or
more dendrites, a single soma with the cell nucleus, a single axon and one or more axon terminals.
The information is passed in form of an electrical signal, i.e. the action potential, between the
dendritic inputs and the axon terminal outputs. Coupling to other neuronal structures usually
happens in the form of neurotransmitters, i.e. chemical agents, which are released at the axon
terminals and picked up by receptor sites on the postsynaptic dendritic spines.
The axon hillock is the anatomical part of a neuron that connects the cell body, i.e. the soma,
to the axon. It is described as the location where the summation of inhibitory and excitatory
postsynaptic potentials from numerous synaptic inputs on the dendrites or cell body occurs. The
axon hillock also has a high concentration of voltage-gated ion channels, which are also common
on the surface of the soma and at the nodes of Ranvier, but not on the dendritic spines. Most of
the length of the axon is insulated by a myelin sheath, i.e. Schwann cells in the peripheral nervous
8
system and oligodendrocytes in the central nervous system, which wrap themselves around the
axonal segment forming a thick fatty layer that prevents ions from entering or leaving the axon.
The internodal distance d between two nodes of Ranvier lies in the range of 0.2 . . . 2 mm and is
linked to the fiber diameter D by the empirical equation:
d ≈ 100 · D
(3.1)
The length of the uninsulated gap G at a node of Ranvier usually is only a few micrometers
wide (G ≈ 1 . . . 2 µm) [100]. This myelin insulation increases both the energy efficiency of the
propagation process since the ionic currents are confined to the nodes of Ranvier – see fig. 3.1 –
and the conduction velocity of an action potential va through so-called saltatory conduction – see
table 3.2.
Table 3.1 gives some rough estimates for the electric properties of the cell membrane of a nerve
fiber at a nodal gap and the cell membrane plus the Myelin sheath between two nodes of Ranvier
[84].
Specific leakage resistance
[kΩ · cm2 ]
Specific capacitance
[µF/cm2 ]
1
100
1
0.01
Cell membrane
Myelin sheath
Table 3.1:
Electrical properties of cell membrane and Myelin sheath
A classification of peripheral nerve fibers according to Erlanger and Gasser [29] together with some
basic fiber properties is given in table 3.2. The autonomic, motor and sensory nervous system use
different kinds of peripheral nerve fibers.
Fiber class
Diameter D
[µm]
Conduction velocity va
[m/s]
Myelin sheath
Aα
Aβ
Aγ
Aδ
10 - 20
7 - 15
4-8
3-5
60 - 120
40 - 90
15 - 30
5 - 25
very thick
thick
normal
thin
B
1-3
3 - 15
partial
C
0.3 - 1
0.5 - 2
none
Table 3.2:
Classification and properties of peripheral nerve fibers
Class B and C fibers are found in the autonomic nervous system.
Class C fibers can also be found in the sensory nervous system innervating nociceptors for slow
pain and warmth receptors. Class Aδ fibers are associated with touch and pressure receptors (free
nerve endings) as well as thermoreceptors for cold and nociceptors for slow pain. Aα and Aβ fibers
of the sensory nervous system are the primary and secondary connections of proprioreceptors, e.g.
muscle spindles, with the CNS. Aβ fibers also innervate all cutaneous mechanoreceptors.
The lower motor neurons of the motor nervous system consist of Aα and Aγ fibers which innervate
the extrafusal and intrafusal muscle fibers, respectively.
The distribution of peripheral nerve fibers in the human body comprises fiber diameters in the
range from 0.3 . . . 17 µm with relative maxima in the fiber number at diameters of 0.6 µm for
unmyelinated fibers and 2.3 µm, 3.8 µm, 6.3 µm, 8.6 µm and 12.8 µm for myelinated fibers
9
[15, 80, 90, 94]. The distribution of myelinated fiber diameters in the central nervous system has
significantly different numbers. In the human pyramidal tract more than 89 % of nerve fibers are
in the diameter range from 1 . . . 4 µm, approximately 9 % in the diameter range from 5 . . . 10 µm
and less than 2% in the diameter range from 11 . . . 20 µm [74, 90].
Another important component of the neuron is its cell membrane.
Figure 3.2:
Schematic structure of a cell membrane
(Adapted from [110])
Fig. 3.2 shows the schematic structure of a cell membrane and its basic components. A key
component is the phospholipid bi-layer which prevents molecules and ions from leaving or entering
the cell through uncontrolled diffusion. Channel proteins form controlled gateways for substances
entering or leaving the cell. For neurons two ionic pathways through the membrane are of special
interest:
• Active ion pumps create and maintain an ionic concentration gradient between the inside of
the neuron, i.e. the cytoplasm, and the outside of the neuron, i.e. the extracellular fluid
• Voltage-gated ion channels use this concentration difference to selectively transport ions along
their concentration gradients
Directly linked to these ionic concentration differences between the cytoplasm and the extracellular
fluid, i.e. the inside (index ’i’) and the outside (index ’e’) of the neuron, is the existence of a potential
difference UM = Φi − Φe or an electric field EM across the cell membrane. Any transport of ions –
and therefore charges – across the membrane by pumps or channels changes the difference of the
electric potentials and the electric field across the membrane.
The concentration of potassium (K+ ) ions inside the neuron is approximately 20-fold larger than
the outside concentration, whereas the concentration of sodium (Na+ ) ions on the outside is roughly
9-fold larger than on the inside of the neuron. Similarly, ionic gradients across the cell membrane
of a neuron also exist for calcium (Ca++ ), chloride (Cl− ) and magnesium (Mg++ ) [48].
The equilibrium membrane potential – the resting potential Ur – at which the net flow of all ions
across the membrane is zero can be calculated with the Goldman equation [38] and leads to a
typical electrical potential difference of Ur ≈ −70 . . . 80 mV across the membrane. Membrane
potentials are always measured relative to the exterior of the cell. This membrane potential in
turn leads to a strong directional electrical field EM across the membrane.
Fig. 3.3 shows the various phases of an idealized action potential passing a single point on the
cell membrane of an axon. As soon as a stimulus increases the transmembrane potential to more
positive values both the voltage-gated sodium and potassium channels begin to open, leading
to an increase of both the inward sodium ionic current, causing further depolarization, and the
10
Figure 3.3:
Phases of an idealized action potential
(In part from [109])
outward potassium ionic current, responsible for repolarization/hyperpolarization. If the change
in membrane potential is only small and does not exceed the threshold, the higher potassium
ionic current is counterbalancing the lower sodium ionic current, thereby returning the electrical
potential across the membrane to its resting value. These so-called failed initiations of a action
potential describe one part of the fundamental “all-or-none” principle which is a key element to
the behavior of excitable structures. In other words, action potentials either occur fully or do not
occur at all. That means that larger stimuli do not create higher action potentials than smaller
stimuli. Instead, the frequency of the action potentials is used to encode the intensity of a stimulus.
However, if the change in membrane potential is large enough to exceed a typical threshold level
of about 15 . . . 25 mV above the resting voltage, a positive feedback from the already open sodium
channels opens even more sodium channels and in rapid succession leads to a runaway condition,
where the electrical potential difference across the membrane nearly reaches the levels of the sodium
equilibrium potential UNa ≈ +55 mV. Because some of the slower acting potassium channels are
also open at this point in time, the peak membrane potential is lower than the sodium equilibrium
potential UNa and reaches typical values of approximately +40 mV. This rising phase of the action
potential has a time duration of typical 1 ms.
The positive feedback of the rising phase finally slows, comes to a stop and is finally transformed
into a negative feedback by a special behavior of the sodium channels. Every sodium channel has
a built-in shut-off feature which automatically closes an open channel after a certain amount of
time. The probability for a sodium channel to stay open decreases with higher potentials across
the membrane. This inactivation of the sodium channels occurs much slower than the transition
from a closed to open state and takes some additional time for being reset to a normal closed state
of the channel. The inactivation of the sodium channels lowers the membrane’s permeability to
sodium, thus driving the membrane potential back down. At the same time, the slower acting
potassium channels, which lack an automatic inactivation feature, become fully open causing the
membrane potential to drop quickly, thus repolarizing the membrane and creating the falling phase
of the action potential.
Because the potassium channels act much more slowly than the sodium channels, it takes some
time to close them again, resulting in a hyperpolarization of the cell membrane (undershoot). Only
when the membrane’s permeability to potassium returns to its usual value, the potential across
the membrane assumes the resting value again.
A previous action potential leaves many sodium and potassium channels in a refractory state,
11
in which they are unable to open again, regardless of any stimulus being present. This absolute
refractory period, where no action potential can be created, is maintained until the membrane
potential reaches sufficient negative values or even is hyperpolarized for a certain length of time.
In the relative refractory period enough ion channels have recovered that a new action potential can
be created, however requiring a stimulus, i.e. an initial depolarization of the cell membrane, much
larger than usual. These refractory periods guarantee that the action potential usually travels
only in one direction along the axon, but also limits the maximum frequency of generating action
potentials.
For mammalian nerve fibers the absolute refractory period is in the range of 0.4 . . . 1 ms for class A
fibers and ≈ 2 ms for class C fibers, whereas the relative refractory period is in the range of several
milliseconds. Under lab conditions the maximum repetition rate for action potentials created by
externally applied electric stimuli is ≈ 2000 per second. However, the maximum repetition rate for
action potentials in the human body is typically in the range of 10 . . . 100 per second and rarely
exceeds a value of 500 action potentials per second [16, 90].
The ions exchanged during an action potential make only a negligible change to the total internal
and external ionic concentrations. Even with blocked sodium-potassium-pumps a typical axon
can generate up to hundreds of thousands of action potentials before a degeneration in amplitude
occurs.
Because of the thermal motion it is not possible to predict whether a certain channel will be open
or closed at any given time. However, the laws of probability allow to make certain predictions of
the average behavior of a channel. Typically, a large number (≈ 102 ) of channels contribute to the
generation of an action potential.
Sodium channel
Potassium channel
Faster than potassium channel
(up to a factor of ten)
Time constant: ≈ 10 µs (range: 5 . . . 200 µs)
Slower than sodium channel
(probability of being open
increases with depolarization)
Automatic inactivation
(slow recovery, ≈ 10 ms at -70 mV)
No automatic inactivation
3 distinct states:
open, closed, inactivated
2 distinct states:
open, closed
9 internal states (1 open / 8 closed)
when not inactivated
16 internal states (1 open / 15 closed)
Table 3.3:
Fact sheet for sodium and potassium ionic channels
Some important data for sodium and potassium channels is summarized in table 3.3 and can also
be found in [44, 47, 84].
As already shown in table 3.2 and discussed in the previous paragraphs, myelinated class A fibers
have higher conduction rates, shorter action potential durations, shorter refractory periods and
lower electrical stimulation thresholds when compared to unmyelinated class C fibers [94].
3.2
3.2.1
Electrical stimulation of excitable tissues
Basic facts
Because of the lower electrical stimulation thresholds of myelinated class A fibers, due to the longer
internodal distance d, these fibers are an excellent choice for studying their behavior with regard
to setting safety limits.
12
A quantitative solution for the generation and propagation of action potentials as well as a description of the underlying ionic mechanisms in an unmyelinated nerve fiber, e.g. squid giant axon,
was first given by Hodgkin and Huxley [49]. Frankenhaeuser and Huxley reformulated the classical
Hodgkin-Huxley equations, in terms of electrodiffusion theory, and computed action potentials
specifically for saltatory conduction in myelinated axons [31].
The whole mathematical framework is well beyond the scope of this report but some key equations
will be presented, which give a very detailed insight into the whole process of electrical stimulation
of excitable tissue and the generation of action potentials in nerve fibers. Additional background
information can be found in the literature [24, 31, 47, 49, 84, 90, 94, 103, 104].
For a first approach, an individual nerve fiber of infinite length, the center of which is oriented
along the spatial z-axis lying in an unbounded extracellular medium (conductivity κe ) is selected.
For subthreshold conditions where the excursion of the transmembrane voltage uM = UM − Ur
from its resting value are small, the electric properties of the membrane are those of a passive
admittance described as a parallel RC network with constant R and C values. Assuming steady
state conditions, i.e. ∂uM /∂t = 0, the relationship between the membrane potential uM and the
potential of the external stimulus ϕe normalized to their respective baselines is expressed by the
differential equation:
uM
∂ϕe
∂ 2 uM
− 2 =− 2
(3.2)
∂x2
λ
∂z
p
with λ = rM /ri , where rM is the membrane resistance per unit length and ri the resistance of
the intracellular medium per unit length.
With the electric field being the negative spatial derivative of the corresponding function for the
electric potential, eqn. 3.2 can be rewritten as
uM
∂Ez
∂ 2 uM
− 2 =
2
∂x
λ
∂z
(3.3)
The term ∂Ez /∂z on the right hand side of eqn. 3.3 is often called the activation or forcing function
in the differential equation.
Eqn. 3.3 describes some important facts for changes in the membrane potential (hyperpolarization,
depolarization) and in the second case the successful initiation of an action potential:
• A gradient along the fiber axis in the electric field of the external stimulus must exist.
This finding is proven by experimental results, that the electric stimulation of excitable
tissue is facilitated, if the electric field of the stimulus is parallel to elongated cells or fibers.
A perpendicular field orientation is rather inefficient and requires a much higher stimulus in
order to be successful [9, 65, 79, 84, 86, 87, 88, 90].
• The spatial field gradient does not necessarily have to originate from the external stimulus but
can also be created by boundary conditions, e.g. beginning, termination, bends or branches,
changes in diameter of the fiber, adjacent tissues and structures with different electrical
properties.
This fact is especially of interest when studying complex excitable tissue structures, e.g.
brain, retina.
• Peak depolarization or hyperpolarization are expected at locations where ∂Ez /∂z attains its
maximum value.
• The overall reaction of the cell membrane depends on the entire course of the function
E(t, z, . . . ) not just the location or amplitude of its initial or peak values.
It is therefore expected that different forms of stimuli, e.g. rectangular, trapezoid, triangular,
sinusoidal, exponential, mono- or biphasic, even those having the same amplitude, will have
different effects on the overall behavior of the nerve cell membrane.
13
The following sections apply these basic findings to the mechanism of electrical stimulation of
peripheral nerves (PNS) and central nervous system of the head (CNS).
A review of the current literature reveals that these factors are often not well controlled and are
not sufficiently documented. Especially for experimental data it is very difficult to find relevant
parameters in the published documents.
3.2.2
Long stimuli
However, a careful review of the literature and additional numerical simulations at membrane level,
including parameter variation studies, reveals a threshold for the electric field strength in the tissue
for the onset of peripheral nerve stimulation (PNS) in the range of 6 . . . 7 V/m for stimulation
pulses longer than 1 . . . 2 ms [14, 17, 25, 42, 66, 83, 90, 101]. This value is quite conservative because
many experiments and calculations use point sources for the stimulation current or voltage, which
can cause a high spatial field gradient in the tissue, especially for small distances between the field
source and the axon under investigation. Because these high spatial field gradients in the tissue
are difficult to obtain by using external electric or magnetic fields for stimulation, even higher
threshold levels for peripheral nerve stimulation are to be expected in those cases.
Lapicque’s law [72, 73], also known as the modified Weiss equation [105], gives the fundamental
relationship between the stimulation strength – historically given as a rectangular stimulation
current Is – and the duration of the stimulus T with respect to physiological parameters like the
rheobase – also historically given as a current IR – and an empirical time constant τe which is linked
to the membrane time constant τM = RM · CM (approximately in the range of 1 ms), defined by
membrane resistance RM and membrane capacity CM , and the spatial distribution of the stimulus
current or the spatial gradient of the electrical field strength in the tissue:
Is =
Figure 3.4:
IR
1 − e−T /τe
(3.4)
Graphical representation of Lapicque’s law given by eqn. 3.4
As shown in fig. 3.4 and according to eqn. 3.4 Is , the minimum stimulation strength (or current)
with duration T , is required to reach the stimulation threshold. For long stimuli (T → ∞) the
value for Is is identical to the rheobase value IR , which defines the stimulation threshold. The time
T = τc where the minimum stimulation strength required is twice the rheobase value Is = 2 · IR
was named chronaxie by Lapicque. It must be noted that the rheobase value is dependent on
physiological parameters and individual exposure conditions.
Three fundamental statements can be derived from eqn. 3.4:
1. Stimuli must exceed a threshold, i.e. a minimum stimulation current or minimum electric
field strength in the tissue, in order to create an action potential
14
2. Stimuli below the threshold, i.e. the rheobase value, cannot create an action potential even
if they are of very long duration
3. Stimuli with shorter durations must be of higher intensity in order to be effective, i.e. create
an action potential
Some documents [3, 51, 57] present a U-shaped stimulation threshold or exposure limit value curve,
which allows for higher values of the current density or electric field strength in the tissue due to
accommodation of the nerve fiber for frequencies below 10 Hz. However, this is not endorsed by
Lapicque’s law or eqn. 3.4 and is based on a misinterpretation of physiological data as explained
below.
If a stimulus is constant at a sub-threshold value or increases only slowly with time, e.g. sinusoidal
waveform at a low frequency starting at a zero amplitude value, the sodium channels can open
gradually which leads to a small rise in membrane voltage and also to an increase in stimulation
threshold. This creates a chase condition between the stimulus and the stimulation threshold
which can only be overcome with a higher amplitude of the stimulus or a faster rate of change.
The behavior of a nerve to adapt to a constant or slowly varying stimulus is called accommodation.
However, this behavior is only present if there is a slowly changing stimulus, e.g. sinusoidal or
triangular waveforms beginning at a zero value. It is not encountered with long rectangular,
exponential or even trapezoid waveforms with steep rising and falling slopes. It is also absent if
the sinusoidal waveform starts at its peak value. Therefore, the usage of those higher values must
be restricted to certain waveforms and should not be given as a general option without stating the
limitations.
3.2.3
Short stimuli
When it comes to short stimulus durations (T → 0), the stimulus charge or the integral of electrical
field strength in the tissue ET over the stimulus duration T becomes the new threshold value:
ET · T ≥ cs for rectangular stimuli, where cs is a constant threshold value. It must also be noted
that the threshold value cs is nearly invariable to dET /dt and therefore does not largely depend on
the form of the stimulus, i.e. rectangular, trapezoid, triangular, sinusoidal and exponential stimuli
nearly give the same results.
From analytical calculations and numerical parameter variation studies and for stimulus durations
of less than 10 µs (T ≤ 10µs) a value cs > 2 · 10−3 Vs/m can be obtained, which translates to an
electric field strength in the tissue in excess of 200 V/m for a 10 µs stimulus.
3.2.4
CNS tissue
Experimental data in the literature gives lower threshold rheobase values when it comes to the
stimulation of CNS tissue of the head, e.g. electro- and magnetophosphenes [6, 76, 77, 90, 91, 95]. It
must be noted that this data are not highly reliable because of incomplete dosimetric documentation
and often gives only average values for current densities or electric field strength in the tissue or
does not take spatial gradients of the electric field in the tissue into account.
However, from smaller fiber diameters and shorter fiber lengths higher threshold values would have
been expected. As already pointed out in section 3.2.1 and shown in eqn. 3.3, high spatial field
gradients resulting from boundary conditions, neighboring tissue structures with different electrical
properties and a possible influence from highly specialized receptors, e.g. the rods of the retina of
the eye and their neural interface, can make up for a seemingly lower total stimulation threshold
value. As a preliminary result and a rough estimate, simulations indicate a total factor in the order
of 20 . . . 40 when comparing the threshold levels in the frequency range of maximum sensitivity –
see table 2.1 – with those for peripheral nerve stimulation.
15
Applying this factor to the threshold value for the electric field strength in the tissue for the onset
of peripheral nerve stimulation for long stimuli – see section 3.2.2 – in the range of 6 . . . 7 V/m
gives a threshold value for CNS tissue in the range of 0.15 . . . 0.35 V/m for the same type of stimuli.
Similar results are expected when it comes to vertigo and nausea, but high-resolution numerical
models, needed to link any exposure to extremely low frequency magnetic fields to those effects,
are sparse or not existent.
Other CNS tissue, e.g. spinal cord, can be disregarded in this context, because due to the electric
properties of the surrounding tissues an ’electric shielding effect’ occurs which results in generally
higher threshold values for electrical stimulation [12, 43].
3.2.5
Uncertainties
A reduction factor fr =
√
10 is introduced in order to address uncertainties
• in modeling, e.g. body models [23, 81]
• physiological data, e.g. tissue data [33, 34, 35]
• due to individual health status and possible pathological conditions
3.2.6
Summary
Summarizing the findings of this section on neurophysiology, mechanisms and electrical stimulation,
some important facts for threshold-level stimuli have to be noted:
• The relevant physiological parameter to describe the electrical stimulation of excitable body
tissues, like sensory organs, nerves and muscles is the peak electric field strength in the tissue
together with its spatial and temporal derivatives.
• The location where an electric field ET in the tissue, caused by an external (low frequency)
electric or magnetic field, depolarizes or hyperpolarizes the cell membrane of an axon is
dependent on its gradients in space and time. This important fact means that exposures to
different sources of electric and magnetic fields and to different frequencies in general have
different points of interaction with the cell membrane and are therefore independent of each
other. In other words, there is hardly any additivity of the different spectral components
under practical exposure conditions.
• In the case of stimuli with repetition frequencies of less than 300 . . . 800 Hz, every peak value
of the electric field strength in the tissue can create an instantaneous action potential. For
these stimuli, a threshold for the electric field strength in the tissue for the onset of peripheral
nerve stimulation in the range of 6 . . . 7 V/m applies. Due to high spatial field gradients
resulting from boundary conditions and a possible influence from highly specialized receptors,
the thresholds for CNS tissue stimulation appear to be lower than those for peripheral nerve
stimulation by a factor in the order of 20 . . . 40.
• For stimuli with repetition frequencies exceeding several kHz many stimuli, e.g. periods of
a sinusoidal waveform, are necessary in order to create an action potential. This behavior
is caused by a slow drift in the membrane potential due to subsequent stimulation and is
attributed to the different time behavior of the sodium and potassium channels, leading to
a so-called delayed action potential. Published data [44, 90] shows that for stimuli with a
repetition frequency of 5 kHz a delayed action potential is evoked after 5 . . . 10 stimuli or
periods, whereas for a repetition frequency of 50 kHz approximately 50 . . . 100 stimuli or
periods are necessary to create a delayed action potential.
16
• The probability of creating an action potential is very small for frequencies exceeding
≈100 kHz and requires a high electric field strength in the tissue (>200 V/m). Especially
for continuous-wave signals these field strengths in the tissue can lead to significant tissue
heating effects, which must be controlled.
• The generation of action potentials is instantaneous or nearly instantaneous. However, with
time frames of less than 1 . . . 2 ms, no time averaging can be justified. This also means that
root-mean-square (RMS) values, which by definition are an average, are a poor metric and
should be avoided. The usage of peak values for measurement and calculation purposes is
highly recommended. However, for single-frequency, continuous-wave sinusoidal waveforms,
√
the peak values can be derived from RMS values by multiplication with a factor of 2.
• The geometric dimensions of the main areas of field interaction and the neurological structures
involved in the generation of an action potential are very small and therefore do not allow
for any spatial averaging. However, from a practical point of view, e.g. for measuring and
calculation purposes, some spatial averaging is inevitable, but has to be controlled carefully.
A detailed analysis of this issue has to take into account several parameters, e.g. location (in
the tissue or outside the body) and source (dimension, distance), and is beyond the scope of
this document.
All these important facts need to be taken into account when limiting the exposure to low frequency
electric and magnetic fields for the protection of the health and safety of workers.
17
4
Limiting occupational exposure to static and low frequency electric and magnetic fields
The major goal of Directive 2004/40/EC is the protection of the health and safety of workers. This
means, that any physiological effects caused by an exposure to EMF must be limited in such a
way, that they do not pose a potential threat to the health and safety of workers.
Any
• interference with autonomous heart action
• loss of muscle control
• significant pain
• severe form of vertigo and nausea
• whole-body heat stress and excessive localized tissue heating
qualifies as a potential threat to the health and safety of workers and the risk of such an occurrence
should therefore be controlled.
Other effects, like phosphenes, may or may not pose a potential safety threat, depending on the
working environment and the duty of the worker. The same is true for effects like metallic taste
and minor tactile sensations at threshold level.
The proposed exposure limit values and action values presented in the next subsections of this
document are based on this valuation.
4.1
Exposure limit values
As described in section 3 the relevant metric to quantify physiological effects based on electrical
stimulation of excitable body tissue is the electric field strength in the tissue together with its
spatial and temporal derivatives.
Because these effects are threshold-based, the peak value of the electric field strength in the tissue
is the relevant parameter which needs to be limited. If this peak field strength in the tissue remains
below the identified stimulation threshold at all times, no stimulation will occur [42, 43, 44, 63,
89, 90].
4.1.1
Static electric fields
The exposure limit values for static electric fields are indicated in table 4.1.
As stated in section 2.1.1.1, the external static electric field cannot penetrate the body surface.
Therefore the exposure limit value is solely based on indirect effects of the static electric field and
is indicated as direct measurable external field quantity.
4.1.2
Static magnetic fields
The exposure limit values for static magnetic fields are indicated in table 4.2.
Sections 2.1.2.1, 2.2.2 and 5.7 give the rationale for setting these exposure limit values stated as
directly measurable external field quantities.
It has to be noted that these exposure limit values only apply if the worker is stationary with
respect to the static magnetic fields. For all time-varying exposures, including movements in static
magnetic fields, the exposure limit values as indicated in section 4.1.3 do also apply. Additional
information can be found in section 5.
18
External electric field strength
[kV/m]
(a,b)
30
Note:
(a)
Value refers to the spatial maximum
(b)
If there is a risk that the worker touches any grounded or ungrounded object,
additional restrictions due to contact currents – see section 4.1.4 – may apply
Table 4.1:
Exposure limit value for static electric fields
Maximum magnetic flux density
Exposure of head and trunk (a,b,c,d) Exposure of limbs
[T]
[T]
2
Note:
Table 4.2:
(a,b,c,e)
8
(a)
Value refers to the spatial maximum
(b)
Personnel with active medical implants, e.g. pacemakers, cardioverter
defibrillators, should not be exposed to static magnetic fields with flux
densities higher than 0.5 mT at the location of the implant. For additional
information see section 5.6
(c)
Magnetic flux densities in excess of 30 mT are allowed if any projectile risk
or any risk from translational or rotational forces on metallic objects or
implants can be excluded
(d)
For controlled environments where access is limited to specially instructed
and trained workers, where special work practices and measures are in force
and where a detailed risk analysis shows that any risks to the health and
safety of the workers or any negative impact on their duties or the safety
of others with regard to vertigo, nausea and phosphenes can be excluded,
magnetic flux densities up to 8 T are allowed
(e)
For controlled environments magnetic flux densities in excess of 8 T are acceptable for a limbs only exposure
Exposure limit values for static magnetic fields
19
4.1.3
Low frequency electric and magnetic fields
100
Peak electric field strength in the tissue [V/m]
Exposure of the trunk / Controlled environment
Whole body exposure / Exposure of the head
10
1
0.1
0.01
<0.01
Figure 4.1:
0.1
1
10
100
Frequency [Hz]
1k
10 k
100 k
Exposure limit values for time varying, low frequency electric and magnetic
fields given as peak electric field strength in the tissue
The exposure limit values are indicated as peak electric field strength in the tissue and are based
on the results presented in section 3 and the valuations in section 4 of this document.
As outlined in section 3.2.5 uncertainties in modeling, physiological data and due to individual
health√status and possible pathological conditions are addressed by applying a reduction factor
fr = 10 to the values derived in sections 3.2.2, 3.2.3 and 3.2.4. The resulting exposure limit
values are given in fig. 4.1 and table 4.3.
Applying the reduction factor fr to the threshold for peripheral nerve stimulation of 6 . . . 7 V/m
– see section 3.2.2 – gives 2 V/m as the exposure limit value in the frequency range up to 3 kHz.
For short stimuli, i.e. frequencies exceeding 100 kHz – see section 3.2.3 –, with a threshold value
of at least 200 V/m, the application of the reduction factor fr yields for an exposure limit value
of ≈ 67 V/m for a frequency of 100 kHz.
According to section 3.2.4 the thresholds for the stimulation of CNS tissue, e.g. magnetophosphenes,
vertigo, nausea, appear to be a factor of 20 . . . 40 lower than those for peripheral nerve stimulation
due to boundary and other special conditions. Dividing the exposure limit value for peripheral
nerve stimulation and long stimuli of 2 V/m by 40 gives the exposure limit value for CNS tissue
of 0.05 V/m. According to section 2.1.2.2 and tab. 2.1 magnetophosphenes have a very sharp
maximum sensitivity peak at ≈ 20 Hz, which decreases rapidly for higher frequencies. Therefore
the corner frequency of 25 Hz for the exposure limit value curve for exposure of the head or whole
body exposure together with a frequency proportional behavior is chosen quite conservatively.
In order to keep the exposure assessment as simple as possible, only two intermediate data points
are chosen in order to describe the frequency behavior of the exposure limit values in the frequency
range up to 100 kHz. However, this leads to larger reduction factors, especially for frequencies in
the range between approximately 100 Hz and several kHz.
Fig. 4.1 – lower (green) curve – gives the exposure limit values for whole body exposures to time
varying, low frequency electric and magnetic fields in the frequency range up to 100 kHz as peak
electric field strength in the tissue. These exposure limit values address all direct adverse effects
20
Peak electric field strength in the tissue
Frequency range
f / Hz
Whole body exposure
or exposure of the head (a,b,d)
[V/m]
Exposure of the trunk /
Controlled environment (a,b,c,d)
[V/m]
0 < f ≤ 25
25 < f ≤ 1000
1000 < f ≤ 3000
3000 < f ≤ 100 · 103
0.05
f /500
2
f /1500
2
2
2
f /1500
Note:
Table 4.3:
(a)
Value refers to the spatial maximum
(b)
Value given is the peak permissible electric field strength in the tissue and
must not be exceeded. Uncertainties linked to measurement or calculation
procedures must be subtracted
(c)
For controlled environments where access is limited to specially instructed
and trained workers, where special work practices and measures are in force
and where a detailed risk analysis shows that any risks to the health and
safety of the workers or any negative impact on their duties or the safety of
others with regard to vertigo, nausea and phosphenes can be excluded, this
value also applies to whole body exposures
(d)
Peak electric field strength in the tissue exceeding 49 V/m must also
be checked for compliance with the exposure limit values for whole- and
partial-body SAR in order to prevent inadmissible tissue heating
Exposure limit values for time varying, low frequency electric and magnetic
fields given as peak electric field strength in the tissue
based on electrical stimulation of body tissues and do also apply to partial exposures of the head,
which is the main area of interaction for effects like vertigo, nausea and phosphenes.
For partial body exposures of the trunk of the human body and for controlled environments,
where access is limited to specially instructed and trained workers, where special work practices
and measures are in force and where a detailed risk assessment shows that any risks to the health
and safety of the workers or any negative impact on their duties or the safety of others, with regard
to vertigo, nausea and phosphenes are controlled, the use of the exposure limit values shown in fig.
4.1 – upper (red) curve – could be allowed. However, due to the possibility that annoying indirect
effects, e.g. movement or vibration of body hair, sparc discharges and contact currents, might occur
more frequently at these exposure levels, their use should be time-restricted to fractions of a whole
work shift.
Both the exposure limit values for time varying, low frequency electric and magnetic fields for whole
body exposures and for partial body exposures of the head, applicable in general, and the exposure
limit values for partial body exposures of the trunk and for special controlled work environments
are summarized in table 4.3.
4.1.4
Contact currents
If a worker touches a charged object or touches a grounded object while being charged himself due
to exposure to a electric field or due to triboelectricity, a contact current will flow. The same can
happen, if the worker closes an induction loop when touching a conductive object in a time-varying
magnetic field.
The physiological effect is largely dependent on the size of the contact area, e.g. touch or grasp
contact, and on the amount of discharge energy and transferred charge, as well as the amplitude
and frequency of the continuously flowing contact current. These effects can be annoying, painful
or can have life threatening consequences [18, 58, 59, 60, 106].
21
In general, two different phases of a contact current event can be distinguished:
• an initial discharge current impulse, e.g. spark discharge
• a continuous contact current
Depending on the specific exposure scenario, only one or both phases of the contact current event
might be present. Usually, the initial discharge current is only present for exposure situations
involving either a static or a time-varying electric field. In general, a continuous contact current is
linked to time-varying electric or magnetic fields, but can also occur in conjunction with ongoing
triboelectric processes.
The initial discharge current usually is a very fast event, present only in the sub-millisecond range.
According to the results from section 3.2.3, these effects are best described and limited by the
integral of electrical field strength in the tissue over the duration of the initial discharge or the
transferred charge. If the voltage difference between the object and the worker is known, the
discharge energy can also be used.
The frequency of the continuous contact current depends on the frequency of the causal timevarying electric or magnetic field, but can also be a DC current in case of triboelectric processes.
Again, the limiting value is the electric field strength in the tissue at the contact site – see section
3.2 –, is directly related to the contact current for touch and grasp contact. Presenting the limit
in form of a contact current is preferred, because this quantity is directly measurable.
Maximum discharge energy
[mJ]
(a)
Maximum transferred charge
[µC]
350
Note:
Table 4.4:
(a)
50
Continuous contact current, if any, needs to be limited according to the values
given in table 4.5
Exposure limit values for the initial discharge pulse of a contact current
Peak contact current
Frequency range
f / Hz
0
3000
45 · 103
Note:
Table 4.5:
(a)
≤f ≤
≤f ≤
<f ≤
Grasp contact
[mA]
3000
45000
100 · 103
5
f /600
75
(b)
(a)
Touch contact
[mA]
1
f /3000
15
(a)
Initial discharge impulse, if any, needs to be limited according to the values
given in table 4.4
(b)
In order to avoid shocks and burns, contact currents exceeding touch current
limits are permitted only, if the workers are properly trained to always make
grasp contact or instructed to use special work techniques or work gear
Exposure limit values for continuous touch and grasp contact currents
Therefore it is necessary to provide limits for both phases of the contact current event. These
are given in table 4.4 for the initial discharge current impulse and in table 4.5 and fig. 4.2 for the
continuous contact current.
Higher values for grasp contact currents can be allowed, because both the peak electric field strength
in the tissue and its spatial gradient are lower due to the larger contact area. However, these values
should only be used if the workers are properly instructed and trained.
22
100
Peak contact current [mA]
Grasp contact
Touch contact
10
1
<0.01
Figure 4.2:
0.1
1
10
100
Frequency [Hz]
1k
10 k
100 k
Exposure limit values for touch and grasp contact currents
If in a certain workplace environment, e.g. high-voltage switchyards, spark discharges or contact
currents cannot be avoided by technical measures, workers should be trained to always make grasp
contact or instructed to use special work techniques, e.g. equalization of potentials, or work gear,
e.g. insulating or conductive gloves.
4.2
Upper and lower action levels
The metrics, e.g. basic restrictions and exposure limit values that best describe the onset of adverse
physiological reactions [3, 57], are mainly quantities that only exist in the biological tissue, e.g.
peak electric field strength in the tissue for stimulation effects and specific energy absorption rate
(SAR) for tissue heating, and are therefore not directly measurable.
EMFs are the only physical agent where this problem exists and therefore require a special solution.
For the assessment of possible health effects of electromagnetic fields a differentiation must be made
between basic restrictions (connected with exposure limit values) and reference levels (connected
with action levels) [3, 57].
According to [57] basic restrictions are defined as ’mandatory limitations on the quantities that
closely match all known biophysical interaction mechanisms with tissue that may lead to adverse
health effects’. [3] calls these values exposure limit values and states: ’Compliance with these limits
will ensure that workers exposed to electromagnetic fields are protected against all known adverse
health effects’. Because these exposure limit values mainly represent physical parameters that
exist only inside the human body, thus making them unavailable for direct measurements, a set of
reference levels [57] or action values [3] is derived from these basic restrictions or exposure limit
values, which is given as directly measurable field quantities.
Reference levels are defined as ’the . . . peak electric and magnetic fields and contact currents to
which a person may be exposed without an adverse effect and with acceptable safety factors. The
reference levels for electric and magnetic field exposure . . . may be exceeded if it can be demonstrated
that the basic restrictions are not exceeded. Thus, it is a practical or ’surrogate’ parameter that
may be used for determining compliance with the basic restrictions’ [57]. Directive 2004/40/EC
23
Unacceptable
health and
safety
related risks
Exposure Limit Value
Compliance check with exposure
limit values required
Upper Action Level
Safety measures
required
Safety related risks
Level, duration and type of exposure
calls these values action values. Compliance with the action values also guarantees compliance
with the exposure limit values.
Lower Action Level
No further action/measures required
Figure 4.3:
Schematic relationship between exposure limit value and the upper and
lower action levels with respect to the level, duration and type of exposure
Fig. 4.3 shows an extension to this concept, by introducing an upper action level and a lower
action level. Compared to the existing situation, this concept will allow a higher flexibility and
the reduction of unnecessary costs for employers for determining workers’ exposure to EMF.
Compliance with the lower action level ensures that all direct and indirect effects of EMFs, including phosphenes, which may represent a potential threat to the health and safety of workers –
see section 4 – are safely avoided. At the same time minor indirect effects at threshold level, e.g.
touch currents, are also eliminated as far as possible.
Because this lower action level will not be exceeded for approximately 90 % of all workplaces,
there is also no need for additional measures, thus reducing costs while guaranteeing the health
and safety of workers at the same time. This is a very important fact for employers, especially
with regard to small and medium-sized enterprises (SMEs).
However, health risks associated with the interference of EMFs with the proper function of active
implanted medical devices (AIMD), e.g. pacemakers, must always be considered, even if there is
compliance with the lower action values.
The upper action level is installed to simplify the determination of compliance with the exposure
limit value. At an exposure level connected with the upper action level, mildly annoying field
24
effects, e.g. phosphenes, vertigo and contact currents, are possible but adverse health effects are
excluded. The workers have to be informed on how to avoid or reduce these effects by using proper
working techniques and tools. However, measures have to be installed in order to avoid potential
threats from some indirect effects, e.g. projectile risk.
If the upper action level is exceeded, health and safety related risks can no longer be excluded. In
those cases it is mandatory to check whether or not the exposure limit values are exceeded and to
take corrective action, if necessary, in order to prevent exposures that might exceed the exposure
limit values.
This is in accordance with the current concept of Directive 2004/40/EC and means: If the exposure
is below the exposure limit value, workers will be protected against the established adverse health
effects of EMFs on the human body.
4.2.1
Upper action level
The upper action level is given in order to make the exposure assessment simpler and cheaper.
This level is derived by converting the body-internal exposure limit values into directly measurable
external field quantities, e.g. external electric and magnetic field strength and magnetic flux density, assuming worst case exposure conditions. Therefore compliance with the upper action level
guarantees that the exposure limit values are not exceeded.
The mandatory action to check whether or not the exposure limit values are exceeded – and to
take corrective action, if necessary – is coupled with the instance that the upper action level is
exceeded.
4.2.1.1
Electric fields
Peak external electric field strength [V/m]
Whole body
10 k
1k
100
<0.01
Figure 4.4:
0.1
1
10
100
Frequency [Hz]
1k
10 k
100 k
Upper action level for occupational exposures of the whole body to external
static and time-varying electric fields
Fig. 4.4 and table 4.6 show the upper action level for the external electric field strength. All values
refer to the spatial maximum and are given as the peak external electric field strength.
25
Frequency range
f / Hz
0
300
3000
Note:
Table 4.6:
≤f ≤
<f ≤
<f ≤
Peak external electric field strength
Whole body exposure
[V/m]
300
3000
100 · 103
(a,b,c)
30000
9 · 106 /f
3000
(a)
Value refers to the spatial maximum
(b)
The peak values given in this table can be exceeded if compliance with the
exposure limit values given in tables 4.1 or 4.3 is shown. However, a peak
electric field strength of 30000 V/m should never be exceeded because of the
risk of severe indirect effects
(c)
If there is a risk that the worker touches any grounded or ungrounded object,
additional restrictions due to contact currents – see section 4.1.4 – may apply
Upper action level for occupational exposures of the whole body to external
static and time-varying electric fields
It has to be noted that the electric field strength in the tissue given in fig. 4.1 and table 4.3 does
not impose a practical limit on the peak external electric field strength due to the shielding effect
of the body – see section 2.1.1.2 –, but does provide information on the frequency dependency.
The upper action level is defined by the exposure limit values for static electric fields and extrapolated values from the high frequency range.
Because no different values for partial body exposures can be allowed, the upper action level given
for whole body exposures to external electric fields do also apply to partial body exposures.
The peak values indicated in fig. 4.4 and table 4.6 can be exceeded if compliance with the exposure
limit values listed in tables 4.1 or 4.3 is shown.
Additional restrictions due to contact currents – see section 4.1.4 – may apply, if the worker can
touch any grounded or ungrounded object.
4.2.1.2
Magnetic fields
Fig. 4.5 and table 4.7 show the upper action level for the static and time-varying magnetic fields.
All values refer to the spatial maximum and are given as the peak magnetic flux density.
This upper action level for magnetic fields is derived from the exposure limit values for static
magnetic fields given in table 4.2 and the exposure limit values for the electric field strength in the
tissue given in fig. 4.1 and table 4.3 assuming worst-case exposure conditions.
The upper action level given in fig. 4.5 and table 4.7 can be exceeded if compliance with the
exposure limit values given in tables 4.2 or 4.3 is shown.
For controlled work environments where any negative impact on the worker’s duties or the safety of
others with regard to vertigo, nausea and magneto-phosphenes can be excluded, higher exposures
to magnetic fields in a certain frequency range can be justified. These values also apply to trunk
only exposures and are shown in fig. 4.5 as the curve marked ’Trunk only / Controlled environment’.
Exposures of the whole body or the head to static or time-varying magnetic fields are limited by
the upper action level shown in fig. 4.5 as the curve marked ’Whole body / Head’.
Magnetic flux densities in excess of 30 mT are only allowed, if any projectile risk or any risk from
translational or rotational forces on metallic objects or implants can be excluded.
Workers with active implanted medical devices, e.g. pacemakers or cardioverter defibrillators,
should not be exposed to static magnetic fields with flux densities in excess of 0.5 mT at the
location of the implant. Additional information is given in section 5.6.
26
Trunk only/Controlled environment
Whole body/Head
Peak magnetic flux density [T]
1
100 m
10 m
1m
100 µ
<0.01
Figure 4.5:
0.1
1
10
100
Frequency [Hz]
1k
10 k
Upper action level for occupational exposures to static and time-varying
magnetic fields of the whole body, head or trunk and for controlled environments
Peak magnetic flux density
Frequency range
f / Hz
0
0.024
0.96
25
1000
3000
Note:
Table 4.7:
100 k
≤f
≤f
≤f
<f
<f
<f
≤
≤
≤
≤
≤
≤
0.024
0.96
25
1000
3000
100 · 103
(a,b,c,d,e)
Whole body exposure
or exposure of the head
[T]
Exposure of the trunk /
Controlled environment
[T]
2
48 · 10−3 /f
48 · 10−3 /f
1.92 · 10−3
1.92/f
0.64 · 10−3
2
2
1.92/f
1.92/f
1.92/f
0.64 · 10−3
(a)
Value refers to the spatial maximum
(b)
The peak values given in this table can be exceeded if compliance with the
exposure limit values given in tables 4.2 or 4.3 is shown
(c)
Magnetic flux densities in excess of 30 mT are allowed, if any projectile risk
or any risk from translational or rotational forces on metallic objects or
implants can be excluded
(d)
Workers with active implanted medical devices, e.g. pacemakers or cardioverter defibrillators, should not be exposed to static magnetic fields with
flux densities in excess of 0.5 mT at the location of the implant – see also
section 5.6
(e)
The values for the magnetic field strength H can be calculated from the
values of the magnetic flux density B by using the formula H = B/µ0 with
µ0 = 4π · 10−7 T·m
A
Upper action level for occupational exposures to static and time-varying
magnetic fields of the whole body, head or trunk and for controlled environments
27
4.2.2
Lower action level
If the lower action level is not exceeded, no further actions or measures are required. Exceptions
to this rule are safety measures for workers with AIMD – see section 5.6.
Compliance with the lower action level excludes the occurrence of any adverse direct or indirect
effects – apart from disturbance of AIMD – and does not require a detailed exposure assessment for
related workplaces, thus avoiding unnecessary actions and measures and therefore reducing costs.
4.2.2.1
Electric fields
Peak external electric field strength [V/m]
Whole body
10 k
1k
100
<0.01
Figure 4.6:
0.1
1
0
600
3000
Table 4.8:
1k
10 k
100 k
Lower action level for occupational exposures of the whole body to external
static and time-varying electric fields
Frequency range
f / Hz
Note:
10
100
Frequency [Hz]
≤f ≤
<f ≤
<f ≤
Peak external electric field strength
Whole body exposure
[V/m]
600
3000
100 · 103
(a,b)
5000
3 · 106 /f
1000
(a)
Value refers to the spatial maximum
(b)
The peak values given in this table can be exceeded if compliance with the
exposure limit values given in tables 4.1 or 4.3 is shown
Lower action level for occupational exposures of the whole body to external
static and time-varying electric fields
Fig. 4.6 and table 4.8 show the lower action level for the external electric field strength. All values
refer to the spatial maximum and are given as the peak external electric field strength.
28
Compliance with the lower action level for occupational exposures to external static and timevarying electric fields avoids most indirect effects, e.g. hair movement, micro shocks and touch or
grasp currents.
4.2.2.2
Magnetic fields
Whole body
Peak magnetic flux density [T]
1
100 m
10 m
1m
100 µ
<0.01
Figure 4.7:
0.1
1
10
100
Frequency [Hz]
1k
10 k
100 k
Lower action level for occupational exposures of the whole body to static
and time-varying magnetic fields
Fig. 4.7 and table 4.9 show the lower action level for the static and time-varying magnetic fields.
All values refer to the spatial maximum and are given as the peak magnetic flux density.
For static and extremely low frequency (f < 1 Hz) magnetic fields the lower action level for the
magnetic flux density of 30 mT ensures that projectile (translational) or rotational risks from
ferromagnetic objects in these magnetic fields, as well as effects like vertigo and nausea will not
occur. For higher frequencies (f > 20 Hz) the lower action level for magnetic fields also ensures
that other adverse or annoying direct and indirect effects are safely avoided.
Workers with active implanted medical devices, e.g. pacemakers or cardioverter defibrillators,
should not be exposed to static magnetic fields with flux densities in excess of 0.5 mT at the
location of the implant. Additional information is given in section 5.6.
29
Frequency range
f / Hz
0
0.55
25
1000
3000
Note:
Table 4.9:
≤f
≤f
<f
<f
<f
≤
≤
≤
≤
≤
Peak magnetic flux density
[T]
(a,b,c,d)
30 · 10−3
16.5 · 10−3 /f
660 · 10−6
660 · 10−3 /f
220 · 10−6
0.55
25
1000
3000
100 · 103
(a)
Value refers to the spatial maximum
(b)
The peak values given in this table can be exceeded if compliance with the
exposure limit values given in tables 4.2 or 4.3 is shown
(c)
Workers with active implanted medical devices, e.g. pacemakers or cardioverter defibrillators, should not be exposed to static magnetic fields with
flux densities in excess of 0.5 mT at the location of the implant – see also
section 5.6
(d)
The values for the magnetic field strength H can be calculated from the
values of the magnetic flux density B by using the formula H = B/µ0 with
µ0 = 4π · 10−7 T·m
A
Lower action level for occupational exposures of the whole body to static
and time-varying magnetic fields
30
5
5.1
Special exposure situations
Simultaneous exposure to electric and magnetic fields
Workplaces with simultaneous whole body exposures to both external electric and magnetic fields
exceeding the upper action levels are rarely found in the work environment.
Referring to eqn. 2.2, the contribution of the external electric field component to the electric field
strength in the tissue is very small in general. Furthermore, for most cases, the external electric
and magnetic field component have different points of interaction within the tissue and are not
additive with regard to the electric field strength in the tissue.
Therefore it is sufficient to show compliance with the lower or upper action levels for both the
electric and magnetic field component separately.
If both the external electric and magnetic field components exceed the upper action levels, or if
compliance with the exposure limit values is shown directly, both the external electric and magnetic
field component should be used in order to calculate the electric field strength in the tissue correctly.
5.2
Simultaneous exposure to multiple field sources operating with the
same frequency
This exposure situation is covered by measuring the combined peak electric and magnetic field
strength or magnetic flux density of all simultaneously used field sources at the workplace. The
measurement time must be sufficiently long to cover the worst-case exposure scenario, especially if
the operation of the different field sources is not continuous or they are operated under changing
conditions, e.g. loads, cycles, settings, parameters.
Alternatively it is possible to take measurements of the peak electric and magnetic field strength
or magnetic flux density for the worst case exposure condition of each field source independently
and sum up the results before comparing them with the lower or upper action values, respectively.
Both procedures, especially the second one, introduce an overestimation of the exposure situation.
However, they are easy to apply and in most cases sufficient to show the compliance of a workplace
with the lower or upper actions levels.
The correct procedure would require to perform a vector addition for the external electric or
magnetic field vectors, respectively, of all field sources for each point within the dimensions of the
human body and compare the worst case result with the lower or upper action values. However,
the use of this solution is limited to numerical calculations of the electric field strength in the tissue
using anatomical body models.
5.3
5.3.1
Simultaneous exposure to multiple frequency fields
Summation formulae
When it comes to the assessment of simultaneous exposures to multiple frequency fields current
safety standards often refer to summation formulae [51, 53, 57, 63]. Both [51] and [57] state, that
’it is important to determine whether, in situations of simultaneous exposure to fields of different
frequencies, these exposures are additive in their effects’. However, neither of these documents
provides any guidance whether or not this is the case for a certain exposure scenario. As already
shown in [44] there is no additivity associated with simultaneous exposures to multiple frequency
fields for exposure situations at workplaces in general.
The use of summation formulae [51, 53, 63] or the weighted filter approach [57], which relies on the
same mathematical principle, for the assessment of simultaneous exposures to multiple frequency
fields introduces by default a large overestimation of the exposure situation at the workplace.
Because both assessment methods are easy to apply, they can only be used to show compliance
31
of an exposure situation with the reference or action levels, respectively. However, if the exposure
situation is deemed non-compliant when using these methods, this need not be the case at all. For
those situations a more physiological based assessment method must be used.
5.3.2
Assessment of fields with arbitrary temporal behaviour
The procedure outlined in the following section can be used for the assessment for all kinds of fields
independently from their temporal course. These assessment procedures are especially useful for,
but not limited to, the assessment of non-sinusoidal or pulsed fields.
As already pointed out in section 3 and summarized in section 3.2.6, the area of interaction with
excitable tissue is dependent, among other parameters, on both the direction and the value of the
vector of the electric field strength in the tissue.
The signum function sgn(x) is defined as:

 +1
0
sgn(x) =

−1
x>0
for x = 0
x<0
(5.1)
With this function the effective duration of a pulse or stimulus can be defined as the timeframe τP
where the signum function of the electric field strength in the tissue Ei is constant but different
from zero: Either sgn(Ei ) > 0 or sgn(Ei ) < 0.
According to eqn. 2.2 the electrical field strength in the tissue Ei is proportional to the external
electric field strength E0
Ei ∼ E0
(5.2)
proportional to the contact current Ic as given by Ohm’s law
Ei ∼ Ic
and also proportional to the time derivative
eqn. 2.4
d
dt
(5.3)
of the external magnetic field B as is shown by
dB
(5.4)
dt
and therefore allows that this concept can be extended to both external electric and magnetic fields
and contact currents, if necessary. However, within this document only pulsed magnetic fields will
be covered.
Ei ∼
Magnetic fields with exponential waveforms require some special consideration because the time
derivative of an exponential function reaches zero only for infinite time durations. Therefore the
effective duration of a magnetic field with an exponential waveform is defined as the timeframe τP
where it rises between zero and (1 − e−π/2 ) of its peak value or where it decays from its peak value
to a value of e−π/2 of its peak value.
If the values for τPi differ significantly over time or are different for rising and falling slopes a
conservative approach is to base all further assessment on the smallest value for all τPi :
τP,min = min (τPi )
(5.5)
In every case the frequency fP can be calculated as:
fP =
1
2 · τP,min
(5.6)
For an arbitrary time function of a magnetic field both the maximum and the mean rate of change
of the magnetic flux density need to be limited. However, for sinusoidal, triangular, trapezoid
32
and exponential waveforms it is sufficient to show compliance with the mean rate of change of the
magnetic flux density only. The rationale for this assessment method can be found in [21, 42].
The maximum allowable rate of change of the magnetic flux density for a pulse with duration
τP,min can be calculated as:
dB π
= ω · B̂ = 2π · fP · B̂ =
· B̂
(5.7)
dt τP,min
max
Whereas the mean rate of change of the magnetic flux density for a pulse with duration τP,min can
be calculated as:
dB B̂
=
= 2 · fP · B̂
(5.8)
dt τ
P,min
mean
Compliance with the values for lower and upper action levels for magnetic fields with frequency fP
given in tables 4.9 and 4.7 also ensures compliance with eqn. 5.8. This means that for all durations
τP,min the absolute value of the change in the magnetic flux density ∆B = |B(t + τP,min ) − B(t)|
must be lower than the peak value B̂ listed in tables 4.7 and 4.9 for the lower and upper action
levels for magnetic fields with frequency fP .
For sinusoidal, triangular, trapezoid and exponential waveforms it is sufficient to check compliance
with eqn. 5.8. However, for arbitrary waveforms compliance with eqn. 5.7 must be checked, too.
If needed, the admissible values for the maximum and mean rate of change of the magnetic flux
density can be calculated for the peak values of the magnetic flux density for the lower and upper
action levels listed in tables 4.7 and 4.9 by using eqn. 5.7 and eqn. 5.8.
5.3.3
Harmonic content
In general, only a limited number of harmonics and, with rising ordinal numbers of the harmonics,
i.e. higher frequencies, a decay in the harmonic amplitudes is usually present in electric power
systems.
[44] shows that any harmonic content shortens the effective duration of such a stimulus, which,
according to Lapicque’s law, renders it less effective for stimulation. As already pointed out in
sections 3.2.2 and 3.2.6 hardly any additivity of the different spectral components exists under
practical exposure conditions.
In these cases it is sufficient to separately show compliance for each spectral component with the
lower or upper action levels. If this compliance check fails, a more sophisticated method – see
section 5.3.2 – for exposure assessment should be used.
5.4
Localized exposure
The worst-case exposure conditions used to derive the lower and upper action levels from the
corresponding exposure limit values assume a homogeneous exposure of the whole body or the
head and trunk to an electric or magnetic field, respectively.
Especially magnetic field sources with small dimensions in the comparison to the human body or
parts of it, which are used in close proximity to the workers’ body lead to highly localized exposure
conditions. Because the magnetically induced electric field strength in the tissue is mainly confined
to the geometrical dimensions of the source itself, according to Faraday’s law stated in eqn. 2.4
this leads to a smaller value of the surface integral for a given magnetic flux density. Together
with a given temporal derivative, e.g. frequency, this in turn leads to a lower electric field strength
in the tissue.
This means, that for localized exposures is it possible to use higher values for the external electric
field strength or magnetic flux density than those given by the lower and upper action levels,
33
respectively. Because a large number of parameters affect these permissible values, they have to be
calculated on a case by case basis. However, for a given field source, e.g. device, cable, tool, at a
workplace, it is possible to assume worst-case exposure conditions again and give simple expressions
or even certain numbers for the external electric field strength or the magnetic flux density.
5.5
Movement in static magnetic fields
dB
dB ds
=
·
dt
ds dt
(5.9)
Eqn. 5.9 links the temporal derivative of a magnetic field to its spatial derivative, i.e. spatial
gradient, and a velocity. For maximum effect the way element ds, i.e. the direction of movement,
needs to be mutually perpendicular to the magnetic field vector.
With eqn. 5.9 and the results from section 5.3.2 it is possible to show if a movement with a given
velocity v in a static magnetic field with a given spatial gradient dB/ds of the magnetic field is
compliant with the lower and upper action levels, respectively. However, in the work environment,
neither the velocity of the movement nor the spatial gradient of the magnetic field will be constant
for long times or over large spatial areas [69]. Therefore it is necessary to break down the whole
path of movement into small distances for which both a constant velocity and a spatial gradient
of the magnetic field could be assumed. Only those parts of the whole path need to be analyzed
where either the velocity or the spatial gradient of the magnetic field or both reach a maximum.
Measurements of electric fields induced by typical human body movements such as walking or
turning in the fringe magnetic field, e.g. of a whole body 3 T scanner gave 0.15 V/m for the upper
abdomen, 0.077 V/m for head and 0.015 V/m for tongue [56].
5.6
Interference with active implanted medical devices (AIMD)
Static and time-varying electric and magnetic fields can influence the proper function of active
implanted medical devices, e.g. pacemakers, implanted cardioverter-defibrillators (ICD) and insulin
infusion pumps. The possibility of such interference depends on type, strength, frequency and
polarization of the field(s) and furthermore on the sensitivity of the AIMD and can impair the
well-being of the worker or can even have life-threatening consequences [11, 98].
Because such an interference may occur even if the lower action levels are not exceeded, special
attention must be given to all workplaces where workers with a AIMD are present in the workforce.
Whether or not a worker with a AIMD is fit for his or her job must be determined on a case by
case basis taking into account the exposure situation at the workplace, the type and location of
the implant, its individual programming and, if applicable, the type and routing of the electrodes.
Additional information and guidance for the assessment process is given in [11, 20, 28].
Static and extremely low frequency magnetic fields can trigger a reed switch inside the AIMD,
which disables certain functions of the implant or causes it to change its mode of operation. This
can be safely avoided if workers with a AIMD are not exposed to static and extremely low frequency
magnetic fields with flux densities exceeding 0.5 mT.
The proper function of a AIMD may be impeded by time varying electric and magnetic fields
interfering with either the device circuitry directly or the measurement and detection of body
signals and parameters, e.g. electrocardiogram (ECG) or blood sugar level. As already pointed
out the interference threshold is dependent on many parameters and can only be determined on
an individual basis.
[28] lists formulae which can be used to calculate peak electric and magnetic field strength for time
varying electric and magnetic fields for a given implant and its individual parameters. Compliance
of a exposure situation with these calculated peak electric and magnetic field strength safely avoids
any interference of these fields with the proper function of the AIMD.
34
5.7
Projectile risk
Ferromagnetic materials, including so-called ’non-magnetic stainless steel’, can become dangerous
objects if exposed to strong static magnetic fields. Depending on their magnetic susceptibility
and their shape, the resulting translational forces and torques can range from negligible to lethal
values. Current literature often refers to this effect as the so-called ’projectile risk’ [3, 21, 56, 99].
However, the magnetic flux densities where these effects are deemed to occur, differ significantly
and range from 3 mT [3], ’in the order of several millitesla’ [56] to ’more than 67.9 mT’ [21].
Implanted devices like aneurysm clips, metal surgical clips or stents, metallic dental implants or
even tattoos and permanent makeup with magnetite or iron oxide based colors can be affected
by rotational or translational forces too, when being exposed to strong static magnetic fields with
sometimes life threatening consequences.
A quantitative solution for the translational and rotational forces on a ferromagnetic object being
placed in a static magnetic field can be obtained by solving Maxwell’s equations for static magnetic
fields in a Cartesian coordinate system [30]. By restricting the shape of the ferromagnetic object
from a general ellipsoid to a rotational symmetrical ellipsoid object and further to the shape of
a sphere, the number of independent principal axes can be reduced from three to one. Further
simplifications can be reached by placing the ferromagnetic sphere in the static magnetic field at
points located along the central axis of a cylindrical (superconducting) magnet. The unit vectors
~ex , ~ey and ~ez form a right-handed coordinate system, with ~ez pointing to the inside of the magnet
and the origin of the coordinate system being placed on the central axis of the magnet.
As is true for magnets commonly used in MRI, the only non-zero spatial magnetic component at
a location with coordinates x = 0 and y = 0 is Bz .
The translational force on a ferromagnetic sphere is given by:
Fz ≈
with
V
µ0
Bz ·
∂Bz
∂z
∂Bz
3V
· Bz
µ0
∂z
(5.10)
being the volume of the sphere
permeability of free space; µ0 = 4π · 10−7 AN2 = 4π · 10−7 T·m
A
product of the z-component of magnetic flux density and its spatial gradient
in the z-axis
According to eqn. 5.10 a translational force on the ferromagnetic sphere exists only, if the magnetic
z
flux density-spatial gradient-product Bz ∂B
∂z is different from zero. This means, that far away from
∂Bz
z
the magnet (Bz → 0 and ∂z → 0) and in the homogeneous region of the field ( ∂B
∂z → 0), usually
inside the magnet, no translational force exists and therefore no so-called projectile risk could
occur. The maximum translational force is to be expected near the opening to the bore for most
z
magnets, where the product Bz ∂B
∂z reaches its maximum.
In order for the sphere to be accelerated it is necessary to overcome at least the sliding friction
force Fsf :
!
Fz = Fsf
(5.11)
Fsf can be calculated as:
Fsf = µsf · δ · V · g
with
µsf
δ
g
(5.12)
sliding friction coefficient; for steel on steel µsf = 0.06
kg
mass density; for steel δ ≈ 8000 m
3
N
standard gravity; at sea-level g = 9.80665 kg
Solving
3V
∂Bz
· Bz
= µsf · δ · V · g
µ0
∂z
35
(5.13)
z
for the magnetic flux density-spatial gradient-product Bz ∂B
∂z , the result becomes independent of
the volume V of the sphere and the exact magnetic quantities of its material, as long as it is a
ferromagnetic substance with a magnetic volume susceptibility χmv 1:
Bz
∂Bz
µ0
= µsf · δ · g ·
∂z
3
(5.14)
For non-magnetic materials χmv 1 the result is still independent of the volume V of the sphere,
but depends on the exact magnetic quantities of the material of the sphere:
Bz
∂Bz
µ0
= µsf · δ · g ·
∂z
χmv
(5.15)
As given by eqn. 5.15 so-called ’non-magnetic’ materials require a much higher magnetic flux
density-spatial gradient-product than ferromagnetics, in order to overcome the initial friction force.
So, for a worst case assumption, it is safe to focus on ferromagnetic materials with a high susceptibility value.
Eqn. 5.14 gives for a ferromagnetic steel sphere:
Bz
∂Bz
T2
≈ 2 · 10−3
∂z
m
(5.16)
Most unshielded superconducting cylindrical magnets
usedin MRI, independent of their absolute
z
magnetic field strength, have a ratio max Bz2 / max ∂B
in the range of 1.8 . . . 2 m−1 [45, 97]
∂z
which can be derived from characteristic manufacturer data.
This magnetic field characteristic (spatial magnetic gradient) together with eqn. 5.16 gives a minimum magnetic flux density Bz ≈ 60mT needed to overcome the initial frictional force, which in
turn makes it possible that the sphere is accelerated in the magnetic field and a so-called ’projectile
risk ’ can occur. This result is in line with the value given in [21].
z
In general, shielded magnets have a smaller ratio max Bz2 / max ∂B
than unshielded magnets.
∂z
Because of the higher spatial gradients this leads to a lower minimum magnetic flux density which
could constitute a so-called ’projectile risk ’. Current data for shielded systems suggests minimum
magnetic flux densities in the central axis of a superconducting cylindrical magnet in the range
from 30 . . . 40 mT necessary for a projectile risk to occur.
For non-spherical objects not only a translational force can exist, but a torque as well. Needle
shaped rotational ellipsoids try to turn their long axis parallel to the direction of the field. The
magnitude of the torque is proportional to Bz2 , so the maximum torque is to be expected in the
center of the magnet and can be higher than the maximum translational force.
36
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43
Annex
A
Quantities, variables, abbreviations and SI-units
Quantity
Magnetic flux density
Electric field strength
Current
Voltage, potential
Force
Frequency
Permittivity
Permeability
Permeability of free space
Conductivity
Charge
Capacity
Resistance
Distance
Diameter
Time
Volume
Velocity
Standard gravity
Mass density
Sliding friction coefficient
Symbol or
Abbreviation
B
E
I
u, U , ϕ, Φ
F
f
ε
µ
µ0
κ
q
C
R
s, d
D
T , t, τ
V
v
g
δ
µsf
Constant
π
Unit vector
~e
Unit
(Value)
Tesla (T)
Volt per meter (V/m)
Ampere (A)
Volt (V)
Newton (N)
Hertz (Hz)
Farad per meter (F/m)
Henry per meter (H/m)
Henry per meter (H/m)
(µ0 = 4 · π · 10−7 H/m)
Siemens per meter (S/m)
Coulomb (C)
Farad (F)
Ohm (Ω)
Meter (m)
Meter (m)
Second (s)
Cubic meter (m3 )
Meter per second (m/s)
Newton per kilogram (N/kg)
(at sea level: g ≈ 9.80665 N/kg)
Kilogram per cubic meter (kg/m3 )
(for steel: δ ≈ 8000 kg/m3 )
—
(for steel on steel: µsf = 0.06)
—
π ≈ 3.14159
–
44
B
Tissue data
Tissue data necessary for numerical calculations using anatomical body models, e.g. Visible Human,
were extracted from the body tissues database established by Gabriel et. al. [32, 33, 34, 35].
Table B.1 contains some sample tissue data used for calculations in this report. The full data set
is listed in the body tissue database.
Mean tissue conductivities of the whole body or parts of the body in the low frequency range listed
in table B.1 are obtained by integrating the individual tissue properties over the whole body or
parts of the body using an anatomical body model.
Mean tissue conductivity [S/m]
Table B.1:
Frequency
Whole body
Head
Torso
50 Hz
10 kHz
100 kHz
0.216
0.276
0.288
0.254
0.285
0.300
0.233
0.256
0.332
Mean tissue conductivity in the low frequency range for the whole body and
parts of the body
45
Affiliations
Börner, F.
Institute for Occupational Safety and Health of the German Social Accident Insurance, Sankt Augustin, Germany.
Brüggemeyer, H.
Lower Saxony Water Management, Coastal Defence and Nature Conservation Agency, Hildesheim,
Germany.
Eggert, S.
Federal Institute for Occupational Safety and Health, Berlin, Germany. (retired)
Fischer, M.
German Social Accident Insurance Institution for the Energy, Textile, Electrical and Media Products Sectors, Cologne, Germany.
Heinrich, H.
2h–engineering & –research, Hausen, Germany.
Hentschel, K.
Federal Institute for Occupational Safety and Health, Berlin, Germany.
Neuschulz, H.
Federal Institute for Occupational Safety and Health, Berlin, Germany.
Acknowledgments
The research presented in this report was supported and funded by the German Federal Ministry
of Labour and Social Affairs.
46