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Heat-related conditions


Hyperthermia results from an elevated body temperature caused by external or internal heat stress and/or heat loading. This leads to failure of the body’s thermoregulatory systems and can be categorised into the following six conditions:

  • Heat exhaustion
  • Heat faint
  • Heat oedema
  • Heat rash (prickly heat)
  • Heat cramp
  • Heat stroke

Heat exhaustion

Heat exhaustion is a mild response to exposure to hot environments. It results from a combination of thermal and cardiovascular strain, where the body is unable to maintain normal function owing to loss of fluids and salts. Symptoms include:

  • A feeling of being unwell
  • Tiredness/irritability
  • Headaches
  • Dizziness
  • Nausea
  • Vomiting
  • Breathing difficulties (shallow, rapid respiration)
  • Rapid pulse, which may be bounding or weak
  • Extreme thirst and mouth dryness
  • Muscle cramps, particularly affecting the stomach and legs
  • Poor control over movements/stumbling/weakness

This leads to two forms of heat exhaustion: salt depletion and, more likely in firefighting situations, water depletion. Heat exhaustion usually responds positively to prompt treatment but may predispose the firefighter to heat stroke.

Heat faint (syncope)

Heat faint is caused by a reduction in blood flow to the brain. It is more likely to occur in non-acclimatised people during early exposure to heat. Symptoms include:

  • Headache
  • Dizziness
  • Weakness
  • Tunnel vision/blurring of vision
  • Pale, sweaty skin
  • Weak, slow pulse
  • Loss of consciousness

Heat oedema

Oedema usually occurs among those not acclimatised to heat. It is unlikely to be encountered in the sporadic exposure conditions of firefighting. Symptoms include swelling of the hands, feet and ankles

Heat rash (prickly heat)

Prickly heat appears in red papules on the skin, usually in areas where clothing is restrictive. It gives rise to a prickling sensation, particularly as sweating increases. It occurs in skin that is persistently wetted by unevaporated sweat, possibly because the sweat ducts become blocked.

The papules may become infected unless they are treated. Heat rash is not dangerous. However, it may result in patchy areas of skin that are temporarily unable to produce sweat, which may adversely affect evaporative heat loss and thermoregulation. Prickly heat has been shown to decrease tolerance to heat and to reduce work capacity. Sweating capacity has been to shown to recover within three to four weeks. If heat rash is likely, the individual should be referred for a medical opinion.

Heat cramp

Heat cramps (painful muscle spasms) may occur in individuals working in the heat. These are caused by salt deficiency, when salt is lost during severe sweating and large amounts of water are taken in without replacing the salt. The condition may have a delayed onset and is most likely in people who are unacclimatised to hot work or have a low dietary salt intake. Cramps usually occur in the muscles principally used during work (limbs) or stomach. Adequate salt intake with food should prevent this occurring.

Heat stroke

If the total heat load (environmental conditions and metabolic heat generation) is such that sufficient body heat cannot be lost to the environment, core temperature will rise. If this continues, body temperature may exceed its controllable limits.

In wet humid conditions, sweating may reduce due to the sweat glands swelling and blocking sweat glands. Although normally associated with humid external environments, the humid microclimate inside a firefighter's clothing can also create the conditions for heat stroke. Alternatively, sweating may cease because of depletion of body water. This decrease in sweating promotes a further, often rapid, rise in core temperature to beyond 38-39°C, where collapse may occur, and to above 41°C (rectal temperature) where heat stroke may occur.

With heat stroke, there is a major disruption of the central nervous function. At body temperatures above about 40°C, a person's mental functions are disturbed and sweating often stops. Normal temperature control mechanisms are lost and a further rapid temperature rise occurs.

The condition can have a sudden onset with no warning, or may be preceded by headache, dizziness, confusion, faintness, restlessness or vomiting (symptoms of heat exhaustion). The change from normal aches or tiredness to serious symptoms may not be obvious to the casual observer.

The transition from moderately elevated body temperature to heat stroke can be very rapid. For this reason, no one should work alone or unsupervised in potential heat stress conditions. If work performance deteriorates, this is usually a reliable indication that significant physiological strain has already occurred. This is a life-threatening emergency.

Symptoms may include:

  • Headache
  • Dizziness
  • Muscle weakness/cramps
  • Being hot, dry and flushed with a high pulse and a core temperature probably in excess of 41°C
  • Failure of central nervous thermoregulation and sweating
  • Mental confusion
  • Convulsions
  • Unconsciousness


After a period of heat exposure, a significant amount of heat will be trapped in the body and clothing of the firefighter. Their highly insulated clothing will now act to retain that heat, preventing its dissipation into the environment. The simple act of unfastening a tunic can help to speed up the cooling process. In some circumstances, without this, body temperature can continue to rise as heat in working muscles continues to be distributed around the body.

Where appropriate, and following a dynamic risk assessment and review, ‘dressing down’ of personal protective equipment to allow body heat to vent and reduce the resulting physiological burden, should be a normal procedure pre- and post-deployment

Where a heat related condition is identified or likely, the individual(s) should dress down and drink cool water. If possible, warm, not cold, water should be sprayed onto the affected individual(s) and they should be fanned to aid the process of cooling by evaporation.

Managing hyperthermia by carrying out the following steps should be considered:

  • Moving the casualty to a cool (shaded) environment
  • Cooling: this can be rapid but monitor temperature to prevent hypothermia
  • Removing heavy clothing

If heat stroke is likely, urgent medical assistance must be sought, stating clearly that this is a likely case of heat stroke. The individual should then be treated following the guidance contained within this section, and the incident should be reported according to service policy.

Faints (syncope) can be helped if the person lies down with their legs raised above the level of their head.

Special consideration should be paid to head cooling. Wrist cooling with cool water using plunge buckets will also aid core temperature cooling but acts more slowly than the methods identified above.

Rehydration reverses most of the adverse effects of heat exposure, which stem from dehydration as the body loses copious quantities of sweat in an attempt to regulate its temperature. Fluid replacement is therefore an important aspect of restoring the thermal and physiological equilibrium.

Similarly, although a few mouthfuls of water may be enough to remove the immediate sensation of thirst, it is not sufficient to restore thermal balance. Adequate fluid replacement is particularly important for those who may be exposed to elevated temperatures for longer periods.

Only still water should be permitted for rehydration purposes. Cool (10-15°C) rather than cold water is preferable. The direct cooling effect of any fluid is minimal and, if a drink is too cold, it may cause local vasoconstriction of the blood vessels in the stomach resulting in a slower rate of absorption.

Despite the importance of fluid, firefighters should be discouraged from drinking copious quantities too rapidly. Rapid absorption of large volumes of water can result in excessive dilution of blood ions (salts), with adverse effects.

If in any doubt, seek medical advice.

Other factors to consider

Heat intolerance can be caused and exacerbated by a number of factors including medication, diet and general wellbeing. These will influence a person’s ability to deal with heat-related conditions.

For further information regarding heat and cold related conditions see National Operational Guidance: Operations  – Physiological Stress