Work Physiology – Thermal Ergonomics An Introduction to Human Thermal Environments Ollie Jay 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 + E SK ) + 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 E SK = 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) 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) HOT COLD

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 (E sk ) 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 (E sk ) 1 gm sweat = 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 + E SK ) + S Increase in M due to SCBA apparatus Decrease in C due to protective clothing Decrease in E sk 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 (t a ) –“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 (t r ) –“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 ( ) = 36.9ºC

Measuring scales for temperature Absolute temperature scale: –degrees Kelvin (K) –increments the same as ºC –0K is absolute zero = ºC K = C e.g boiling point of water = 100ºC therefore: K = = K

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 (E sk ) ‘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 ( P a ) to the saturated water vapour pressure ( P sa ) –given in percentage (%)

Humidity Relative Humidity  = P a x 100 P sa Partial vapour pressure (P a ) is the pressure exterted by the water vapour in the air Saturated vapour pressure (P sa ) is the vapour pressure at which no more water can be held air temperature water content limit P sa Antoine’s equation: P sa = exp ( 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/m 2 –difficult to measure (e.g. calorimetry) Estimation tables

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 (E sk ) convection (C) radiation (R) conduction (K) Due to moderate - high air temperature (Ta) and mean radiant (Tr), moderate humidity (  ) and low air velocity (v)