 2 facts govern heat flow within the body and exchange with the environment 1. Body core temperature must remain at 37◦ C 2. 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 fluctuations  Physical activity  Metabolism (has the main impact on heat regulation)

 Metabolically active tissues  Skeletal muscles, organs, fat, bone, connective tissue, nervous tissue  Heat distribution  Circulatory system controls  Blood carries heat throughout the body  Blood vessel constriction and dilation controls heat loss or conservation  Heat exchange with the environment  Lungs  skin

 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 environment  Nature’s solution is to increase the skin temperature above the environment’s  Allows for sweat evaporation  Evaporation 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 environment  Reduces blood flow to the skin  Increase insulation by wearing more layers  Shivering  Heat exchanges with the environment  The 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 temperature  Air humidity  Air/ water movement  Temperatures 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 flow  Anemometers have been replaced by electronic techniques  Measuring radiant temperature  Measuring the temperature via A thermometer placed in a black globe

 Interacting climate factors  Air 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 instruments  Way 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

 Clothing  3 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 distribution  The body produces heat which it must release to prevent overheating  Dissipation works best when the skin temp is higher than the surrounding temp  Circulatory system redistributes blood flow to increase heat flow to the skin  Blood vessel dilation may bring about a 4 fold increase of blood to the skin, greatly increasing heat release

 Sweating  Ability to sweat varies between individuals  Amount 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 functions  Sweat depletes the body of water and electrolytes  Heat distress  1 st 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 heat  Short, maximal muscle exertion is not compromised  High intensity, endurance physical work is severely reduced  Competition 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 redistribution 2. Increase in metabolic rate

 Redistribute blood  Body lowers skin temp to reduce difference with the environment  Constricts vessels near the skin’s surface  Hunting 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 involved  Voluntary: 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 coldness  Experiments 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 strains  Subjective 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 suffers 32ºC = LOC 26ºC heart failure occurs 20ºC vital signs disappear

 Working hard in the cold  Wear appropriate clothing  Climate effects on mental tasks  Lack of objective testing methods  Mental performance deteriorates with temps rising above 25ºC, ºC 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