Humans need to maintain and regulate their internal core temperature as near to 37ºC as possible. Any significant deviations away from the ‘normal’ temperature range can produce a decrement in performance, cause illness and ultimately lead to death.
The article will include a description of the physical and physiological factors which affect the thermal balance of the human body. This will cover heat loss/gain mechanisms of convection, conduction, radiation and evaporation including a description of the physiological mechanisms in response to the heat and cold and the effects of physical activity.
A series of International Standards is available that can be used to help in the design and assessment of thermal environments.
There are many factors in the work place which affect the ability of the person to maintain his/her core temperature within ‘normal’ limits such as:
- Extreme environmental temperatures
- Level of physical activity
- Clothing or personal protective equipment (PPE) worn
The core temperature of the body is maintained around a fairly constant temperature by balancing heat loss to the environment and heat production of the body. The heat balance equation describes the amount of heat that is gained or lost by the body (storage) and is described as follows:
Storage = (Heat production) – (Evaporation ± Radiation ± Conduction ± Convection)
Body heat storage either increases or decreases as heat is either gained or lost by physical exchange mechanisms which take place between the environment and the body (evaporation, radiation, conduction and convection). These physical heat losses and gains are moderated by physiological mechanisms such as sweating, shivering, vasodilatation and vasoconstriction of the skin blood vessels. Heat is also produced by the body by metabolism which increases as the level of activity increases 
Heat Production of the Body
The resting metabolic rate (about 40-50Watts/m²) is the amount of energy needed for the basic functioning of the body such as respiration, brain processes and that blood circulation that provides oxygen (O2) and nutrients to the cells. The heat produced by these essential metabolism processes spreads to the surrounding cells by conduction and is distributed round the body by fluids such as the blood.
As soon as the person starts to exercise there is an increase in the metabolic rate. The metabolic rate varies with the type and level of physical activity from low (70-130 Watts/m², eg sitting at a desk writing) to very high (>260 Watts/m², e.g. climbing stairs) . About 80% of the energy produced by the exercising muscles is released as heat. The maximum metabolic rate can be increased by ten fold depending on the individual’s muscular development, physical fitness and training .
Environmental factors and the effects on heat transfer mechanisms
Heat is lost from the skin when water is converted to water vapour. The amount of heat that is lost depends on the humidity of the environment. The higher the air humidity the less evaporation can take place and the lower the amount of heat lost from the body.
The exchange of heat by radiation depends upon the difference in surface temperature between two objects and is not affected by air temperature or air movement. A human can lose heat to the objects in a room, by radiation, which have a lower surface temperature than their bare skin or clothing surface temperature or gain heat from objects that have a higher surface temperature.
When the skin is in contact with a material the amount of heat lost or gained depends on its conductivity. The lower the conductivity of the material the lower the heat exchange. In general clothing fabrics have a low thermal conductivity and metals a high thermal conductivity.
The amount of heat that is lost or gained by convection depends on the difference beween the skin and air temperature and the amount of air movement. Heat is gained by convection when the environmental temperature is greater than skin temperature and lost when the air temperature is less than skin temperature. These gains and losses are increased as air movement increases (See Section 6.2).
If heat storage of the body increases or decreases the body tries to correct the heat balance back to ‘normal’ limits with physiological mechanisms by effecting physical heat losses and gains (Section 2.2). If the body core temperature increases (hyperthermia) the body tries to increase heat loss by sweating and vasodilatation of the skin blood vessels. If body temperature decreases (hypothermia) the body induces shivering to increase heat production and vasoconstriction of the skin blood vessels to reduce heat loss.
The main avenue for heat loss is by sweat evaporation. As the body temperature increases the amount of sweat secreted from the sweat glands is increased, which increases evaporative cooling from the skin surface. For short periods of time in excess of 1 litre/hour of sweat can be produced. The amount of evaporative heat loss is dependent on the moisture content and the temperature of the surrounding air, the higher the humidity the less sweat can be evaporated and the less effective sweating becomes as a cooling mechanism. The higher the skin temperature the greater the amount of heat can be lost by evaporation .
Heat transfer by evaporation, convection, conduction and radiation are all affected by skin temperature. If the core temperature increases skin blood vessels are opened (vasodilated) this will increase blood flow, skin temperature and heat loss from the skin. As the core temperature falls, skin blood vessles are closed (vasoconstricted) reducing blood flow and skin temperature and therefore conserving heat by reducing heat loss. However, in cold climates when the extremity skin temperature falls to about 12˚C there is a sudden vasodilation of the skin blood vessels as the blood vessels become paralysed by the cold (cold induced vasodilatation, CIVD). The sudden increase in blood flow increases blood vessel temperature and allows the blood vessel to contrict again. As this cycle of vasodilatation and vasocontriction is repeated the skin temperature increases and decreases and is described as the ‘hunting’ mechanism. This ‘hunting’ mechanism may protect the extremities from cold injury but it increases body heat losses .
Shivering is a very efficient way of producing heat as no useful work is carried out by the muscles and more of the energy produced is converted to heat. A drop in skin temperature can illicit variable increases in shivering which will become even more violent if core temperature also drops. Shivering can increase metabolic rate by 5 to 6 fold , however, this level is seldom maintained for more than a few minutes and is often followed by periods of rest.
When humans are repeatedly exposed to heat, which increases their core temperature, they become acclimatised to heat. However, no such physiological acclimatisation occurs in response to repeated cold exposures, just behavioural adaptations.
Acclimatisation to heat
The acclimatisation state of a person has a large impact on their response to heat, increasing heat tolerance and reducing the risk of heat stress. The following effects to any given heat stress conditions have been observed in acclimatised individuals :
- Lower core temperature
- Lower heart rate
- Sweating starts at lower core temperature set-point
- Amount of sweat increases and is more dilute
- Improved fluid distribution leading to maintainance of blood pressure
Adaptation to the cold
Though there is little or no evidenece of physiological acclimatisation to the cold the physiological responses such as shivering, vasodilatation and CIVD can cause discomfort . This has lead to humans adapting their behaviour to maintain skin temperature with clothing, increased heat production by physical activity and avoiding cooling by sheltering from the wind and rain .
Clothing reduces the ability of the wearer to lose internal heat from the body by conduction, convection and evaporation. PPE is designed to protect the wearer from the environment, including heat and radiation, and by its very protection can increase the thermal risk to the wearer. More detail on PPE can be found in Personal Protective Equipment, Protective clothing – against chemical and biological hazards and Protective clothing – against physical hazards. The evaporation and heat dissipation through the clothing ensemble depends on many factors including:
- Properties of the clothing
- Air movement
- Environmental conditions
Properties of the clothing
The greater the number and thickness of the layers, the greater the thermal insulation. The thermal insulation of the clothing is produced by trapped still air both in-between the layers and the fibres of the material and the still air bound to the surfaces. Fibres, coating and membranes are used to waterproof the garment to protect the insulation against the ingress of water. The moisture permeability of the fibre or coating used can range from permeable to impermeable. The more impermeable the clothing the lower the amount of evaporative sweat that can be lost from the skin which also reduces heat loss from the body. The build up of moisture within the clothing reduces the thermal insulation and decreases the water vapour permeability by increasing water vapour resistance .
The design of the clothing can affect its thermal properties. Loose or tight clothing does not insulate the body as well as a well fitted garment. Pressure points caused by kneeling can compress insulation layers reducing the thermal protection. Moisture from sweat either escapes from the clothing ensemble through openings in the clothing; evaporates through the material to the environment or condenses within the clothing . The greater the amount of sweat that escapes from the wearer through the clothing ensemble to the environment the greater the body cooling and vice versa.
Air movement within the clothing
Air movement within clothing is increased by wind penetration of the fabrics (dependant on air permeability), movement of the clothing by the wind, openings in the garments (cuffs and fastenings) and by movement of the wearer causing a ‘pumping’ action. As the air movement increases within the ensemble so does the heat loss from the body. These effects all increase the water vapour and heat loss from the clothing ensemble .
Weight of clothing and distribution over the body
Protective clothing increases the metabolic cost of performing a task by adding weight, restricting movement and causing the person to change or alter their movements to compenesate for any problems caused by the clothing. These problems can range from increased bulk and change in the centre of gravity; restricted field of view and reductions in manual dexterity. This ‘hobbling effect’ can increase metabolic rate by as much as 10% . If the clothing is multilayered, the energy cost can increase by another 16%, some of which could be due to the frictional drag between the layers of the clothing and the increased energy requirements of moving in bulky and stiff clothing . The location of the weight of the clothing on the body can also have an effect on the metabolic requirements. If the weight of the clothing is moved from the torso to the hands this can increase the energy requirements by 100% and the added weight is placed on the feet can increase energy requirements by as much as 500% .
It is much easier for humans to protect themselves from cooling and hypothermia (Section 4.2), than from hyperthermia and heat stress. Consequently, the temperature regulation system of the body is more sensitive to body temperature increases than decreases.
If an individual is undertaking a high level of physical activity in a high environmental temperature whilst wearing heavy, insulatative clothing heat gain may exceed heat loss. This may lead to an increase in core temperature and the incidence of heat stress. It is very difficult to predict which individuals will suffer heat stress and which environmental conditions may cause it, as individual responses are varied. These individual responses are influenced by the physical characteristics of the person, such as age, weight, gender and level of physical fitness  .
|Heat illness||Signs and symptoms||Predisposing factors||Underlying physiological disturbance|
|Heat stroke||Core temperature of 40.5˚C and above Faintness, Dizziness Staggering gait Headache Nausea and vomiting Mental confusion Irrational behaviour Coma, Death||Exertion in the heat Lack of physical fitness Obesity Recent alcohol intake Dehydration Individual susceptibility Chronic cardiovascular disease||Reduction or failure of sweating leading to loss of evaporative cooling and an uncontrolled accelerating rise in core temperature|
|Heat exhaustion||Core temperature between 37.5-38.5˚C Headache, Dizziness Fatigue, Hyperirritability and anxiety, Diarrhoea High heart rate Hyperventilation Low blood pressure Nausea and vomiting Heat cramps, Chills||Exertion in the heat Lack of acclimatisation Failure to rehydrate||Dehydration causing a reduction in circulating blood volume. Cardiovascualar strain from the competing demands of the blood flow to both the muscles and skin.|
|Heat rash (prickly heat)||Tiny raised red blister-like spots on affected areas Pricking sensations during heat exposure||Exposure to humid heat causing wet skin||Retension of sweat in the sweat glands|
|Heat cramps||Painful muscle spasm||Depletion of salts after exercise in the heat|
Source: adapted from Parsons, 2003 and Armstrong, 1987 .
The signs and symptoms of heat illnesses (Table 1) are often vague and can be confused with other conditions. They can also vary depending on the work environment, type of exercise, level of hydration and clothing worn by the individual making heat illness very difficult to diagnose. However, high core temperatures are dangerous to the individual and if allowed to rise to 40.6˚C can be fatal in 40% of the population and if it continues to increase above 43˚C, recovery is very unlikely as proteins precipitate and coagulate which is irreversible . Between these temperatures core temperature tends to rise very rapidly as enzyme-dependent metabolic processes speed up, increasing body temperature at a faster and faster rate.
When the body core temperature drops below 35˚C the person is described as hypothermic and if the condition remains untreated it can be fatal. The effects of lowering the core temperature are described in Table 2. A decrease in core temperature (negative heat storage) can be caused by exposure to low air temperatures, wind and rain and also immersion in tepid or cold water. In cold air the wind speed or air movement is a very important factor as it significantly increases the amount of body cooling that takes place in any given air temperature (wind chill index) .
|Core temperature ˚C||Signs and symptoms|
|37||‘Normal’ core temperature|
|36-34||Metabolic rate increases to Maximum shivering Conscious and responsive Blood Pressure (BP) normal|
|35-30||Confusion, apathy and clumsy movements BP difficult to measure Shivering ceases Blurred vision Start to lose consciousness, muscles become rigid|
|29-26||Risk of death below this point as ventricular fibrillation possible Respiration rate decreases, BP and heart rate difficult to measure Consciousness lost|
|25-21||Ventricular fibrillation Pulmonary oedema|
|Below 20||Individuals have recovered from accidental (18˚C) and medical (9.0˚C) hypothermia|
Source: adapted from Parsons, 2003 .
Heat Stress in Cold Environments
Though not widely recognised heat stress can also occur in cold environments especially when the individual is dressed in PPE and also carrying out high levels of physical activity . The clothing may be made of multiple layers of heavy, insulative clothing that cannot be removed or adjusted so the avenues of heat loss are reduced causing heat gain and an increase in core temperature. Sweating into clothing wets the clothing and reduces the thermal insulation and can lead to skin cooling and hypothermia.
When the person is immersed in water, the insulating effects of the clothing is lost and the conductivity of water is greater than air and body heat is lost rapidly. These effects mean that the body’s physiological responses become very important in maintaining core temperature. The range of water temperatures in which individuals can stabilise their core temperature varies from 32 to 12˚C and is dependent on their metabolic response and amount of subcutaneous fat . In contrast to physical activity in air, exercise in cold water (below 25˚C) accelerates hypothermia as heat loss from the exercising muscles to the surrounding water is increased as muscle blood flow increases .
Effects of cold on the extremities
The high surface area of the extremities means that they cool down rapidly when exposed to cold environmental temperatures. Cooling of the hands affects the ability of the person to perform tasks and gloves worn to protect against the cold can make the situation worse. However, it is important to maintain the temperature of the extremities or they can suffer cold injuries. These can be caused by freezing of the tissues (freezing cold injury or frost bite) or exposure to low temperatures of 1-15 ˚C especially in the wet over a long period of time (non-freezing cold injury or immersion foot) .
The increase in sweating that occurs when the body is attempting to maintain heat balance can lead to dehydration if insufficient fluids are consumed. The maintenance of the correct levels of hydration is important as dehydration reduces the amount of sweating, impairing the body’s ability to lose heat.
One of the first signs that dehydration is occurring is thirst, however, this is a very unreliable indicator of the level of hydration. Drinking, though it can quench the thirst, does not instantaneously restore water balance as it takes time to be absorbed from the alimentary canal into the blood and distribute across all the fluid compartments of the body . The other signs and symptoms are shown in Table 3.
Heat acclimatisation increases sweat rate (Section 4.1) but when dressed in clothing or PPE this increase in sweat does not necessarily mean an increase in heat loss, as the sweat produced may not be able to evaporate, due to the low moisture permeability of the clothing (Section 5.1). This means that increased sweat is produced but because the sweat is unable to evaporate, little heat is lost. The increased fluid loss from the body increases dehydration with little thermoregulatory benefit.
Table 3: Signs and symptoms of dehydration
|% Body weight loss||Body water loss for a 70 kg person Litres||Body water loss for a 100 kg person Litres||Symptoms|
|1.0||0.7||1.0||Thirsty when resting Impaired ability to thermoregulate|
|2.0||1.4||2.0||Thirst threshold during exercise Vague discomfort Loss of appetite|
|3.0||2.1||3.0||Decrease in volume of circulating blood Dry mouth Reduction in volume of urine|
|4.0||2.8||4.0||Physical activity becomes harder Flushed skin Impatience and apathy|
|5.0||3.5||5.0||Difficulty in concentrating Headache Impatient and mental tasks harder|
|6.0||4.2||6.0||Thermoregulation severely affected Increased heart rate Risk of heat stroke|
|7.0||4.9||7.0||Collapse likely if combined with heat and exercise|
|8.0||5.4||8.0||Dizziness Laboured breathing during exercise Mental confusion|
|10.0||7.0||10.0||Spastic muscles Inability to balance with eyes closed General incapacity, delirium and wakefulness Swollen tongue|
|11.0||7.7||11.0||Circulatory insufficiency Decreased blood volume Failing renal function|
|12.0||10.5||15.0||Circulatory failure DEATH|
Source: adapted from Parsons, 2003 and Nevola, 1987 . and .
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