Work Physiology – Thermal Work Physiology – Thermal ErgonomicsErgonomics

An Introduction to Human Thermal An Introduction to Human Thermal EnvironmentsEnvironments

Ollie Jay Ollie Jay

Email: Ojay@sfu.caEmail: Ojay@sfu.ca

Laboratory for Exercise and Environmental PhysiologyLaboratory for Exercise and Environmental Physiology

ReadingReading

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)

OutlineOutline

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….

ThermoregulationThermoregulation

Humans are “HOMEOTHERMS”

– This means we regulate our deep body (core) temperature around a set-point.. This set-point is 37.0±0.5C 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 Conceptual Heat Balance EquationEquation

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)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)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) 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) 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 (EEvaporation (Esksk))

• 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 (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 Physiological responses to +ve S

• Positive Heat StorageWhen 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 Physiological responses to -ve S

• Negative Heat StorageWhen 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…. 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 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 temperatureAir 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 temperatureRadiant 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 Measuring scales for temperaturetemperature

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 Measuring scales for temperaturetemperature

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 velocityAir 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

HumidityHumidity

• 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 (%)

HumidityHumidity

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 ProductionMetabolic Heat Production

Basic Activity Estimation of metabolic rateLying 45Sitting 58

Standing 65Walking at 2km/h 110Walking at 5km/h 200

• 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

Thermal balance can maintained Thermal balance can maintained under a number of different under a number of different environments……environments……

Metabolic heat production = LOWClothing Insulation = LOW

He looks comfortable so he must bein 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), moderatehumidity () and low air velocity (v)