Introduction
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 lead to reduced performance, illness and ultimately death.
This article includes 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.
Heat Balance Equation
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:
Body Heat Storage = (Body 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[1].
Heat Production of the Body
Metabolic Rate
The resting metabolic rate (about 40-50 Watts/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.
Physical Activity
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, e.g. sitting at a desk writing) to very high (>260 Watts/m², e.g. climbing stairs)[2]. 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[3].
Heat transfer mechanisms
Evaporation
Heat is lost from the body when water on the skin’s surface evaporates into water vapour. The amount of heat that is lost depends on environmental factors such as humidity and air movement. The higher the air humidity the less evaporation can take place and the lower the amount of heat lost from the body. Conversely, increased air speed enhances evaporation by moving humid air away from the skin and replacing it with drier air, allowing more moisture to evaporate.
Radiation
The exchange of heat by radiation occurs through electromagnetic waves and depends upon the difference in surface temperature between two objects and is not affected by air temperature or air movement. Unlike conduction or convection, it does not require a medium to be transferred and can occur in vacuum. A human can lose heat by radiation to surrounding objects, that have a lower surface temperature than their bare skin or clothing surface temperature or gain heat from objects (including the sun) that have a higher surface temperature. This type of heat is quite relevant in workplaces with hot surfaces as foundries. It includes infrared radiation and potentially ultraviolet light that can also cause other health issues (e.g. keratitis, cataracts). Conduction
Conduction is the transfer of heat through direct contact between materials like the skin and a hot surface. 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 most usual risk of this type of heat exchange is burns.
Convection
Convection is the transfer of heat through a fluid, which can be either a liquid or a gas. The amount of body heat that is lost or gained by convection depends on the difference between 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 below). This is a risk in workplaces with furnaces where there can be escapes of hot air.
Physiological mechanisms of thermoregulation
If the body's heat storage increases or decreases the body will attempt to bring the heat balance back to ‘normal’ limits through physiological mechanisms of physical heat losses and gains. 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.
Sweating
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[4].
Blood vessels: vasodilation and vasoconstriction
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 vessels 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,). The sudden increase in blood flow increases blood vessel temperature and allows the blood vessel to constrict again. As this cycle of vasodilatation and vasoconstriction 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[5].
Shivering
Shivering is a very efficient way of producing heat as no useful work is carried out by the muscles and more of the metabolic 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-fold1 , however, this level is seldom maintained for more than a few minutes and is often followed by periods of rest.
Acclimatisation
When humans are repeatedly exposed to heat or cold, they become acclimatised which results in an attenuation of the body response to such exposures. However, acclimatisation is lost if regular exposures are not maintained and there is less evidence of cold acclimatisation.
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[6][7]:
- Lower core temperature
- Lower heart rate
- Sweating starts at lower core temperature set-point
- Number of sweat increases and is more diluted
- Improved fluid distribution leading to the maintenance of blood pressure
Acclimatisation to cold
Acclimatisation to the cold leads to long-term physiological changes, such as improved circulation to extremities or increased metabolic heat production[8][9]. Effects of wearing clothing in hot and cold environments
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 Clothing protecting against selected physical hazards: thermal hazards.
The evaporation and heat dissipation through the clothing ensemble depends on many factors including:
- Properties of the clothing
- Design
- 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[10].
Clothing design
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[11]. The greater the amount of sweat that escapes from the wearer through the clothing ensemble to the environment, the greater cooling effect on the body 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[12].
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 compensate for any problems caused by the clothing. These problems can range from increased bulk and changes in centre of gravity, to reduced field of vision and manual dexterity as well as an increase in metabolic rate[13] [14]. However, advances in textile technology, particularly with regard to thin insulating liners, are improving the ergonomics, comfort and wearability of cold weather garments while maintaining adequate thermal insulation[15].
Health effects of working in hot and cold environments
It is much easier for humans to protect themselves from cooling and hypothermia than from hyperthermia and heat stress. Consequently, the temperature regulation system of the body is more sensitive to body temperature increases than decreases. This has important implications in the context of climate change, as rising average ambient temperatures are expected to increase the frequency and intensity of heat exposure in many workplaces.
Heat stress - heat-related illnesses
If an individual is undertaking a high level of physical activity in a high environmental temperature whilst wearing heavy, insulative 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, pre-existing medical conditions and level of physical fitness[16] .
| Table 1. Classifications of heat illnesses |
| 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. Cardiovascular 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 | Retention 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[17][18][19] [20][21][22]
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[23][24][25]. 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. More on heat stress, risks and prevention measures can be found in Heat at work - guidance for workplaces.
Cold stress
Cold stress occurs when the human body's ability to control its internal core temperature of around 37°C starts to fail. Cold stress includes fatigue and mild to serious health problems such as hypothermia. Early signs are loss of balance, excessive shivering, lack of coordination or slower than normal breathing. A decrease in core temperature (negative heat storage) can be caused by exposure to low air temperatures, wind and rain and also immersion in lukewarm 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.
| Table 2. Signs and symptoms of hypothermia |
| 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 |
| 20 | Heart stops |
| Below 20 | Individuals have recovered from accidental (18˚C) and medical (9.0˚C) hypothermia |
Source: adapted from[26]
More on cold stress, risks and prevention measures can be found in Working in the cold.
