Download What is the risk of noise-induced hearing loss at 80, 853 90 dB(A

Occup. Med. Vol. 50, No. 4, pp. 274-275, 2000
Copyright © 2000 Lippincott Williams & Wilkins for SOM
Printed in Great Britain. All rights reserved
0962-7480/00
FOCUS ON
PRACTICE
What is the risk of noise-induced
hearing loss at 80, 853 90 dB(A)
and above?
M. E. Lutinan
Institute of Sound and Vibration Research, University of Southampton, UK
Regulations regarding risk from exposure to noise at work entail action at levels of
85 dB(A) and above. At 80 dB(A) there is no material risk in the vast majority of
individuals. At 85 dB(A) there is a marginal risk with susceptible individuals accruing a
significant hearing impairment from a lifetime of exposure. At 90 dB(A) and above the
risk becomes material, with the majority of individuals accruing a significant hearing
impairment.
Key words: Age; hearing loss; ISO 1999; medicolegal; noise-induced.
Occup. Med. Vol. 50, 274-275, 2000
Claims for damages from negligent exposure to noise at
work constitute the largest category of settlements under
employer liability in terms of both numbers of claims and
total payment. This and recent reports surrounding the
huge liability faced by the Irish army for hearing damage
to soldiers are reminders of the need for adequate risk
assessment in noisy workplaces. This is a requirement
under the Noise at Work regulations that are underpinned by the Health and Safety at Work Act and have
been effective since 1990. Similar regulations apply
throughout the European Union. Previously there was
well-publicized guidance from the Health and Safety
Executive in the form of the Code of Practice for the
Exposure of Employed Persons to Noise, published in
1972.
The Noise at Work regulations require the employer
to perform a survey if the noise level may be above
85 dB(A). If the survey indicates that the noise exceeds
this threshold level, certain actions are required including
reduction of the noise as far as is reasonably practical,
marking noise hazard zones and making hearing protection available to those exposed. This is referred to as the
first action level. If the level exceeds 90 dB(A), the
second action level, steps must be taken to protect the
hearing of exposed employees by reducing exposure
times or reducing the level of noise reaching their ears,
for example with personal hearing protection such as
earplugs or earmuffs. Hearing checks must also be made
available to employees who wish them.
The reasons for these Action Levels arise from studies
that have examined the relationship between noise
Correspondence to: Prof. M. E. Lutman, Institute of Sound and Vibration
Research, University of Southampton, Highfield, Southampton SO17 1BJ,
UK. Tel: (+44) 1703 592798; fax: (+44) 1703 593190;
e-mail: [email protected]
exposure and hearing loss. The consensus of such
studies is that the risk of damage from noise exposure
can be quantified in terms of the cumulative A-weighted
noise energy reaching the ear. A-weighting involves
applying a filter to adjust frequencies according to the
sensitivity of the human ear and the noise dose is
accumulated over the working day. Risk accumulates
over a working lifetime in a somewhat similar manner.
From a combination of the daily exposure and the
working duration it is possible to predict the typical
noise-induced hearing loss from any noise exposure,
although susceptibility varies amongst individuals and
there is a wide spread around the median expected loss.
International standard ISO 1999 gives formulae that
allow expected hearing loss in men and women to be
calculated for any noise exposure at any percentile of the
distribution of susceptibility.
Noise-induced hearing loss typically shows the greatest shift at the audiometric frequency of 4 kHz and the
formulae in ISO 1999 reflect this. The examples in Table
1 indicate the median threshold shift expected at 4 kHz
for men who have been exposed to noise at various levels
throughout a 45-year working lifetime. These threshold
shifts are the median to be expected in a population of
male employees and will therefore be exceeded by half of
a typical sample. Therefore, from a perspective of
prevention, a percentile nearer to the more susceptible
Table 1. Median threshold shifts at 4 kHz as a function of noise level
Noise level (dB)
Threshold shift (dB)
80
85
90
95
2
7
15
27
M.E. Lutman: What is the risk of noise-induced hearing loss at 80, 85, 90 dB(A) and above?
extreme may be more helpful. The examples in Table 2
indicate the threshold shift expected at 4 kHz for men
exposed to noise at various levels throughout a 45-year
working lifetime at the 5th percentile (i.e. exceeded by
only 5% of the population).
Reference to Tables 1 and 2 indicates that the risk is
negligible at 80 dB(A). At 85 dB(A) there is a marginal
risk, but at 90 dB(A) and above the risk becomes
material. Assessment of these risks should recognize that
there will also be material shifts associated with age.
When examining individuals who have been exposed to
noise and who are shown to have a high-frequency
sensorineural hearing impairment, it is important to
realize that such hearing impairment is common in the
general population without noise exposure. In many
employees there will be an element of age-associated
hearing loss. The examples in Table 3 indicate the
median threshold shift expected at 4 kHz for men at
various ages.
If the measured impairment is greater that that shown
in Table 2, after allowance for age, there is probably a
third cause for the hearing impairment in addition to
noise and age. Unfortunately, there are no available tests
275
Table 2. Extreme (5th percentile) threshold shifts at 4 kHz as a
function of noise level
Noise level (dB)
Threshold shift (dB)
80
85
90
95
2
9
20
35
Table 3. Median threshold shifts at 4 kHz in men, due to age
Age (years)
Threshold shift (dB)
35
45
50
65
5
12
22
35
that can be used to identify the cause post hoc. From a
preventive point of view, as long as daily noise exposures
do not exceed 85 dB(A), the risk of hearing loss is
minimal.