Download Human skin contact with cold materials: Pain and Thermal sensation

Work Physiology – Thermal
Ergonomics
An Introduction to Human Thermal
Environments
Ollie Jay
Email: [email protected]
Laboratory for Exercise and Environmental Physiology
Reading
Course Reading:
“Exercise Physiology” – G.A.Brooks
Chapter 22
Other Recommended Reading if you like:
“Life at the extremes – The science of survival” – Frances
Ashcroft (2001)
“Survival of the fittest” – Mike Stroud (2004)
Outline
Human Thermal Environments
– How the body interacts with a given thermal environment
(hot, moderate or cold)
– Human heat balance equation
• How does it work?
• Avenues of heat exchange
• Thermoregulatory responses
– Human-Environment Interaction
• Six parameters
– What happens when the body cannot maintain thermal
balance?
• Hypothermia, hyperthermia… A matter of survival…..
• Thermal comfort … A matter of productivity….
Thermoregulation
Humans are “HOMEOTHERMS”
– This means we regulate our deep body (core)
temperature around a set-point.. This set-point
is 37.0±0.5C depending upon time of day,
metabolism, menstrual cycle for females etc…
– It is a DYNAMIC equilibrium, meaning that we
fluctuate around this set-point in order to
maintain HEAT BALANCE, depending upon the
environmental conditions and the way in which
we interact with them
Conceptual Heat Balance Equation
Conceptual heat balance equation
(M – W) = (K + C + R + ESK) + S
Where: M = rate of metabolic heat production
W = rate of mechanical work (effectively = 0)
K = rate of conductive heat loss
C = rate of convective heat loss from the skin
R = rate of radiative heat loss from the skin
ESK = rate of evaporative heat loss from the skin
S = rate of body heat storage
Metabolic Heat Production (M-W)
• All reactions in the body at the cellular level require energy
• Most reactions are actually quite inefficient and therefore
produce vast amounts of heat as a by-product
• Only a negligible amount of this heat is transferred into
mechanical work (W) and it is therefore assumed to be
equal to zero
• The body must balance this remaining heat produced
within the body with the environment in order to maintain
heat balance
– If the environment is too cold extra metabolic heat is produced
– If the environment is too warm the body must dissipate this heat
Conduction (K)
HOT
COLD
• Heat transfer by conduction
is the transfer of heat
through direct contact with a
solid material
• Usually this avenue of heat
transfer is negligible for the
means of whole body
thermoregulation
• This is most important when
considering exposure of the
extremities under extreme
hot or cold environments
(burns/frostbite)
Convection (C)
• Heat transfer by convection is the physical movement of
air or fluid past the body, which serves to carry heat
• The surface temperature of the body is usually greater
than that of the surrounding air. The layer of air in contact
with the skin and clothing is warmed
– the air can be moved by a draught -”forced”
– or the buoyancy of the warmer air - “natural”
Responsible for 70-80% of heat loss in the cold
Near to 100% when immersed in water
Radiation (R)
• Heat transfer by the means of
electromagnetic radiation
• Largest source of radiant heat is the sun
with a surface temperature of 5500ºC and is
93 million miles away
Evaporation (Esk)
• Heat transfer due to sweating
• Liquid - vapour change produces latent heat, this
is lost through evaporation of sweat at the skin
surface
• It is the evaporation of sweat NOT the production
sweat that cools the body
Responsible for 70+% of heat loss in the heat
Heat Storage (S)
• Heat Storage
– Negative = decrease in core temperature
• Continued decrease leads to HYPO thermia
– Positive = increase in core temperature
• Continued increase leads to HYPER thermia
• Measuring core temperature
– Experimentally the most effective is esophageal
– Others include:
• Rectal (large time lag)
• Tympanic / Aural (not accurate)
• Oral (difficult to measure during exercise)
Physiological responses to +ve S
• Positive Heat Storage
When core temperature increases above set-point (37°C), anterior
hypothalmus elicits physiological cooling mechanisms
– Sweating
•
•
•
•
Increases heat loss via evaporation (Esk)
1 gm sweat = 2411.3 Joules (0.58kcal)
Ecrine glands (Forehead, back, palms) - cooling
Apocrine glands (axillary and pubic regions) – odours
– Vasodilatation
• Dilation of the vascular smooth muscle cells allows a greater
peripheral blood flow, facilitating greater heat dissipation from
body core via convection (C) and radiation (R)
Physiological responses to -ve S
• Negative Heat Storage
When core temperature increases above set-point (37°C), anterior
hypothalmus elicits physiological warming mechanisms
– Shivering
• Increases metabolic heat production (M) by up to 5 times
• Onset of shivering is determined by skin temperature
– Vasoconstriction
• Constriction of vascular smooth muscle cells reducing peripheral
blood flow and heat losses via convection (C) and radiation (R)
– Piloerection
• Hairs “stand on end” in order to trap still air layer against skin
• Arrector pili muscles attached to the hair follicle involuntarily
contract
Problem of the fire-fighter….
(M – W) = (K + C + R + ESK) + S
Increase in M due to SCBA apparatus
Decrease in C due to protective
clothing
Decrease in Esk due to vapour
impermeable clothing
Decrease in R due to environment.. In
fact when in the building the decrease
in R will be such that the value will be
negative i.e. HEAT GAIN
Positive
Human – Environment Interaction
Six fundamental parameters that define how a
human will respond to a given thermal environment
Four Environmental Parameters
–
–
–
–
Air temperature
Radiant temperature
Air movement
Humidity
Personal Parameters
– Activity
– Clothing Insulation
Air temperature
Molecular level:
– average kinetic energy (heat) in a body
Air temperature (ta)
– “the temperature of the air surrounding the
human body which is representative of that
aspect of the surroundings which determines
heat flow between the human body and the air”
Measured
– using an in-glass thermometer (mercury, alcohol
etc.)
Radiant temperature
Molecular level:
– produced by the vibration of molecules
– part of the electromagnetic spectrum
Mean radiant temperature (tr)
– “the temperature of of uniform enclosure with
which a small black sphere at the test point
would have the same radiation exchange as it
does with the real environment”
Measured
– black globe thermometer
Measuring scales for temperature
Practical working scale:
– degrees Celsius (ºC)and Fahrenheit (ºF)
– increments different: 180F = 100C
– 0ºC = 32ºF
F = 9/5 C + 32
C = 5/9 (F-32)
e.g body temperature of 98.4 ºF gives
C = 5/9 (98.4 - 32) = 36.9ºC
Measuring scales for temperature
Absolute temperature scale:
– degrees Kelvin (K)
– increments the same as ºC
– 0K is absolute zero = -273.15 ºC
K = C + 273.15
e.g boiling point of water = 100ºC
therefore: K = 100 + 273.15 = 373.15K
Air velocity
• Air movement across the body can influence
heat flow to and from the body (R)
Air velocity (v)
– will affect the rate at which warm air or vapour is
‘taken’ away from the body, thus affecting body
temperature
– measured in m/s (metres per second)
Measured
– Kata thermometer
– Hot-wire anemometer
Humidity
• Human body exchanges heat with the environment by
vapour transfer (Esk)
• ‘Driving force’ is the differences in humidity (partial
vapour pressures)
Relative humidity ()
– the ratio of the prevailing partial vapour pressure of
the water vapour in the air (Pa) to the saturated water
vapour pressure (Psa)
– given in percentage (%)
Humidity
Relative Humidity
 = Pa x 100
Psa
• Partial vapour pressure (Pa) is the pressure exterted by
the water vapour in the air
• Saturated vapour pressure (Psa) is the vapour pressure at
which no more water can be held
air temperature
water content limit
Psa
Antoine’s equation: Psa = exp 18.956 - 4030.18
(t = air temperature in C)
t + 235
Metabolic Heat Production
• heat generated within the cells of the body
• increases with activity
Metabolic rate (M)
– some heat expended due to external work (M-W)
– measured in W/m2
– difficult to measure (e.g. calorimetry)
Estimation tables
Basic Activity
Estimation of metabolic rate
Lying
45
Sitting
58
Standing
65
Walking at 2km/h
110
Walking at 5km/h
200
Thermal balance can maintained under a
number of different environments……
Metabolic heat production = LOW
Clothing Insulation = LOW
He looks comfortable so he must be
in heat balance?? i.e. S = 0
Therefore minimal heat loss through
• evaporation (Esk)
• convection (C)
• radiation (R)
• conduction (K)
Due to moderate - high air temperature
(Ta) and mean radiant (Tr), moderate
humidity () and low air velocity (v)