2 facts govern heat flow within the body and exchange with the environment1. Body core temperature must remain at

37◦ C2. Heat flows from hot to cold matter

Need to properly engineer environments to maintain the core body temperature using the physics principle that heat moves from hot to cold matter

Human body has a complex control system to maintain a stable body temperature

Small deviations affects cellular structures, enzymes, impairs physical work and mental capacities Normal minor fluctuationsPhysical activityMetabolism (has the main impact on heat

regulation)

Metabolically active tissuesSkeletal muscles, organs, fat, bone,

connective tissue, nervous tissue Heat distribution

Circulatory system controlsBlood carries heat throughout the body Blood vessel constriction and dilation

controls heat loss or conservation Heat exchange with the environment

Lungsskin

Temperatures of the core tissues must be kept constant

Temperature control takes place at the external tissues

Core and shell temperature difference

Normally 4◦C at rest Cold environment 20 ◦C or

more

Hot skin in a hot environmentNature’s solution is to increase the skin

temperature above the environment’sAllows for sweat evaporationEvaporation of 1 cm³ of water requires

2440J (580 calories)The body must reduce its internal heat

production when it cannot disperse enough energy

Cold skin in a cold environmentReduces blood flow to the skin Increase insulation by wearing more layersShivering

Heat exchanges with the environmentThe body’s heat control systems must interact

with the physical components of the environment Convection Conduction Radiation Evaporation

Heat exchange by convection or conductance Convection – transfer of heat energy to air or

water that is flowing over the skin Conduction – transfer of heat energy to a solid

mass As the temperature of the 2 contact surfaces

becomes equal, the energy exchange ceases Dependant on:

Body surface available for heat exchange Body surface temperature Temperature of the medium in contact with the skin Heat conduction coefficient

Material with heat coefficients less than skin feel warm to touch; greater heat coefficients feel cool

Heat exchange is facilitated if the medium moves quickly along the skin’s surface

Free convection – normal transfer of heat due to temperature gradient

Induced convection – heat transfer due to a forced action Figure (8.1)

Heat exchange by radiation Doesn’t depend on the temperature of air

between 2 surfaces; instead it depends on: The body surface The temperature of the emitting surface Emission coefficient of the emitting surface Absorption coefficient of the receiving surface Temperature of the receiving surface

Heat transfer from the window to the body (Figures 8.2 and 8.3)

Wavelengths radiated from the human body are in the infrared range

Emission coefficient = 1; absorption coefficient depends on skin color

Heat exchange by evaporation Occurs in only one direction Heat loss depends on:

Volume of sweat evaporated Vapor pressure of the skin The humidity of the surrounding air Vapor pressure of the surrounding air

Can only occur if surrounding air is less humid Air movement across skin helps to increase

evaporation by replacing the humidified air with dry air

Can also occur in a cold environment when physical work is performed and via lungs

Evaporation is more effective than convection and radiation

4 physical factors define the thermal environment:Air/water temperatureAir humidity Air/ water movementTemperatures of surfaces

The combination of these factors determines the physical conditions of the climate, its effects on us and how we perceive it

Measuring temperature Dry bulb thermometer filled with alcohol

Measuring humidity Hygrometer – electrical conductivity changes with

changing humidity Psychrometer – consists of a DB and WB

thermometer; the WB is more cooled by the evaporation

Absolute humidity – when further increase would lead to water droplets Depends on air temperature and barometric

pressure Relative humidity – actual vapor content in relation

to absolute

Hygrometer

Measuring air flowAnemometers have been replaced by

electronic techniques Measuring radiant temperature

Measuring the temperature viaA thermometer placed in a black globe

Interacting climate factorsAir temp affects body temp, air humidity

determines our ability to sweat, air movement across surfaces, radiation depends on the temps of the 2 surfaces

The combinations of the different factors can create different effects on how we perceive temperature

Effective temperature chart – expresses the combined effects of environmental factors by measuring air temp, velocity and humidity

Combining instrumentsWay to assess current climate by using an

instrument which weighs the different factors and calculates a single index

Wet bulb globe temperature (WBGT) weighs the effects of several climate parameters in warm environments: WB is concerned with the humidity GT is concerned with radiant temp DB is concerned with dry air temp

Microclimate vs macroclimate Suitable Microclimate depends on:

Age Elderly are less active, have a decreased muscle

mass/tone, sweat at higher skin temperatures Surface to volume ratio Fat to lean body mass ratio

Personal thermal comfort Work performed, type and intensity determine Hard physical work is more comfortable in a

cool climate than a warm climate

Clothing3 major traits

1. Insulation – the measure of resistance to heat exchange by convection, conductance and radiation

Impedes energy exchange Light colors minimizes heat gain by radiation Dark colors absorb sun’s radiated heat Reduces risk of injury when contacting hot or

cold objects CLO units – the defined insulating value of

clothing Air bubbles between clothing layers increase

insulation

2. Permeability – measure of how clothing permits movement of water vapor through fabric

Important in hot climates to allow evaporation from the skin to allow cooling

Important in cold climates to prevent clammy feeling when water is trapped under clothes

3. Ventilation – measure of the ability of air to move through clothing

Good in warm climates to promote evaporation Undesirable in cold climates

Clothing also determines SA of exposed skin

Fingers, toes, head and neck need special protection in cold environments

Wind chill – the air temp felt on exposed skin due to wind; type of convective heat loss Cooling is more pronounced with higher wind

speeds

Acclimatization –adjustment of an individual’s mind and body to changed environmental conditions In hot conditions:

Blood flow increases at the skin, sweating increases, stroke volume increases

Achieved within 1-2 weeks Cold conditions

True physiological acclimatization is questionable because most adaptation occurs through increased clothing

However, blood flow is directed towards the core of the body

Seasonal changes

Blood distributionThe body produces heat which it must

release to prevent overheatingDissipation works best when the skin temp

is higher than the surrounding tempCirculatory system redistributes blood flow

to increase heat flow to the skinBlood vessel dilation may bring about a 4

fold increase of blood to the skin, greatly increasing heat release

Sweating Ability to sweat varies between individualsAmount of sweat developed and evaporated

depends on: Clothing Environment Work requirements Individual acclimatization

Reducing physical effort Final necessary action of the body if core

temp is above an acceptable level Body ceases muscular efforts

Signs of heat strain Increase of circulatory activities – increase in

CO from increased HR and a decrease in BP High sweat rate

Normal = 50 cm³/hr During normal work = 1L/hr Sweat drips off the skin when generation is at 1/3

max

Drink water!Dehydration of 1-2% body weight can affect

ability to perform bodily functionsSweat depletes the body of water and

electrolytes Heat distress

1st reaction = skin eruptions, prickly heat associated with clogged sweat ducts

Heat cramps due to lack of Na+

Heat distress Heat exhaustion

Dehydration and overload of the circulatory system

Fatigue, headache, dizziness, nausea, giddiness, excessive sweating

Heat syncope Failure of the circulatory system Fainting

Heat stroke Overload of the circulatory and sweat systems Hot dry skin, increased core temp and mental

confusion Medical emergency

Working hard in the heatShort, maximal muscle exertion is not

compromisedHigh intensity, endurance physical work is

severely reducedCompetition between the cardiovascular

functions of heat dissipation and blood supply to the muscles

Heat dissipation wins and the person cannot perform the physical labor

Few natural defenses against the cold 2 major ways to regulate temp:

1. Blood flow redistribution2. Increase in metabolic rate

Redistribute bloodBody lowers skin temp to reduce difference

with the environmentConstricts vessels near the skin’s surfaceHunting reflex – cold induced automatic

vasodilation Wear gloves, caps

Blood displacement to the core may endanger peripheral tissues

Increase metabolism Involuntary: shivering

Begins in the neck Muscle units firing at different frequencies of

repetition, out of phase with each other No mechanical work is done so total activity is

devoted to heat production Increases metabolism up to 4 x RMR Shivering becomes violent when large muscle

units become involvedVoluntary: dynamic muscular work

Increases metabolism to 10 x RMR

Goose bumps Retains a layer of warm stationary air close

to the skin Insulates and reduces energy loss by the skin

How cold does it feel? Cold perception

Surface receptors Body core sensors

Sensation intensity increases below 35.5ºC, is strongest at 20ºC and decreases at lower temps

Hard to separate feelings of pain from cold

How cold does it feel?Conditions of exposure influence perceived

coldnessExperiments show that subjects have a hard

time determining how cold they really are Overshoot sensation

When the temperature plunges, temperature receptors respond to both the decrease in temperature and the rate of change

Very cold water accentuates this phenomenon

Signs of cold strainsSubjective sensation of cold is an unreliable

indicator of core and surface temp If vasoconstriction and metabolism aren’t

enough, skin will suffer first while the body core is protected for as long as possible Frostbite

Important temps Joint temps below 24ºC and nerve temps below

20ºC reduce fine motor skills Dexterity is reduced with finger temps below 15ºC Tactile sensitivity is decreased below 10ºC Touch and pressure receptors fail below 5ºC =

numb

Signs of cold strains Important temps (cont.)

8ºC PN velocity = 0= nerve block; physical impairment Apathy hypothermia Core temps

35ºC CNS coordination suffers32ºC = LOC26ºC heart failure occurs20ºC vital signs disappear

Working hard in the coldWear appropriate clothing

Climate effects on mental tasksLack of objective testing methodsMental performance deteriorates with

temps rising above 25ºC, 30 - 35ºC if acclimated

Motor skills are affected by temps above 30ºCWBGT; mental is not affected until 40ºCWBGT

Comfortable climate conditions 21-27ºC in a warm climate; 18-24ºC in a cool

climate Relative humidity between 30 – 70% Air temp between floor and head level should

differ by less than 6ºC Differences between body and surfaces <10ºC Velocity of cool air < 1.5 m/s

Comfortable climate design must consider: Work performed Acclimatization Clothing Psychological inclination

The body must maintain a core temp of 37ºC

Heat energy may be gained or lost through: Convection Conduction Radiation Evaporation (one way transfer from body to

environment) In a hot environment, the body tries to keep

the skin hot; in cold the body tries to keep the skin cold

Wear the proper clothing!

Thermal environment is determined by combinations of:Air humidity (evaporation)Air temp (convection and evaporation)Air movement (convection and evaporation)Temperature of solids touching the body

(conduction)Temperature of surfaces adjacent to the

body (radiation) Combined effects of the physical factors

are expressed in a climate index, WBGT