Hot & Cold Stress

Radiant heat exchange

• Heat absorbed & released through electromagnetic (infrared) radiation

• For typical indoor temperature, this is nearly constant

• Affects: temperature & emissivitity characteristics & surface areas participating in energy exchange

Convective heat exchange

• Heat exchange between the skin & air or fluids through motion of mediating fluid

• Affects: temperature of source & receiving body (lower surrounding, higher heat loss), surface areas participating in heat exchange, & flow characteristics of mediating fluid (air velocity & turbulence)

Conductive heat exchange

• Heat exchange between the skin & solids through contact

• Not a major contributor to overall heat balance in most situations

• Affects: head conduction characteristics of sources & receiving body, temperature of source & receiving body, & surface area participating in exchange

Evaporation

• Heat loss through the evaporation of water (sweat) on the skin & other surfaces of the body (respiratory system)

• ONLY heat loss

• Affects: relative humidity of body surface & surrounding air, surface areas participating in energy exchange, & flow characteristics of surrounding air

Heat balance

• 37℃ or 99℉

• Plus or minus 2℃ can affect body functions & task performance

• Plus or minus 6℃ is usually fatal

Cold stress

• Skin temp 15℃-20℃: reduced manual dexterity

  • 8℃-10℃: loss of tactile sensitivity & motor function

  • 0℃: destruction of the cells (frostbite)

• Core temp below 36℃: decrease in vigilance

  • 36℃-32℃: loss of most conscious function (confusion)

  • 32℃: loss of consciousness

  • 26℃: heart failure

Hypothermia

• Maybegin to shiver & stomp feet to generate heat; as symptoms worsen, shivering will stop

• Lose coordination & fumble with items, becoem confused & disoriented

• May be unable to walk or stand, pupils become dilated, breathing slows, & loss of consciousness can occur (death if not treated)

Frostbite

• Reddened skin develops gray/white patches

• Numbness in affected part, feels firm or hard, & blisters may occur

Trench foot

• Prolonged exposure to wet & cold temperatures

• Can occur at temperatures as high as 60℉ if the feet are constantly wet

• Non-freezing injury occurs because wet feet lose heat 25x faster than dry feet

Heat fatigue

Decrease in productivity, coordination, alertness, & vigilance due to poor aclimitization

Heat rash

Red painful rash in affected area due to continuous exposure to head (& sweat production) leading to plugging of sweat glands

Heat cramps

Painful spasms of skeletal muscles due to loss of salt

Heat exhaustion

Extereme weakness or fatigue, nausea, headache, etc., due to loss of water and/or electrolytes

Heat syncope

• Fainting while upright & immobile in head

• Due to pooling of the blood in dilated vessels

• Usually associated with lack of acclimation

Heat stroke

• Core temperature above 40℃, convulsions, loss of consciousness, & possible brain damage or death

• Due to a loss of thermo-regulatory function in the brain

Individual differences in acclimatization

• Physical fitness (more fit are more tolerant)

• Age (older is more sluggish)

• Body fat (insulation)

• Drugs & alchohol (physiological effects)

• Nonheat disorders (generative disorders of cardiovascular system & other diseases)

Acclimatization

Continuous or repeated exposure to hot or cold conditions brings a gradual adjustment of body functions to the conditions

Heat acclimatization

• Improved blood flow, increased sweat production, better distribution, reduced electrolytes & minerals present in sweat, & increased stroke volume & decreased heart rate

• Decreased core temperature for a given workload

Cold acclimatization

• Not as well measured

• Some increase in blood flow to extremities (decreased reflex constriction of blood vessels)

• Loss of acclimatizations occurs in ~1 to 2 weeks

Dry-bulb temperature

• Standard ambient air temperature measured with an ordinary thermometer shielded from sunlight

• Measurement of air temperature using a thermometer

Natural wet-bulb temperature

• Measures the cooling effect caused by water evaporation, which depends on humidity & air movement

• Measurement of humidity & expected heat exchange due to evaporation (air flow as well)

Humidity

• Amount of water in the air

• Limited by the temperature (hot allows higher)

• Expressed in absolute terms (total mass of water vapor) or as relative humidity (percentage of saturation)

Globe temperature

• Evaluates radiant heat absorbed from direct sunlight & surrounding surfaces

• Measurement of heat transferred by radiation

Air flow temperature (wind speed)

Measurement of air movement across the body, which increases sweat evaporation & convective cooling

Wet bulb globe temperature (WBGT)

• Accounts for air temperature, humidity, wind speed, sun angle, & solar radiation/radiant head from pavement, making it the gold standard

• Dry, wet, & globe temperatures

• Thermometer, humidity, & radiation

Heat stress indices

• Dry bulb, wet bulb, gload, & air flow

• Air temperature, thermometer, humidity, radiation, & air flow

Administrative controls for thermal stress

• Rest in cool places

• Rotation from hot to cool

• Protective clothing

• Water & electrolyte intake

• Training in symptom recognition

• Acclimatization (4-10 days)

Engineering controls for thermal stress

• Shield from R sources

• Increase air velocity

• Decrease environmental temperature

• Decrease air capor pressure (dehumidify)

• Redesign to minimize energy expenditure

• Body ventilation systems & cooling garments