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Heat-related illness

Medical Professionals

Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find the Sun and sunburn article more useful, or one of our other health articles.

An increase in body temperature can cause symptomatic illness. Temperature alone does not define the type of heat-related illness, but does so in combination with the associated symptoms and signs. This usually comes about as a result of conditions of:

  • High ambient temperature.

  • High relative humidity.

  • Physical exertion.

There is a spectrum of heat-related illness ranging from heat cramps, through heat exhaustion to heatstroke.

Because heat-related illness is largely avoidable, the most crucial point of intervention concerns the use of appropriate prevention strategies by susceptible individuals and their carers. Knowledge of effective prevention and first-aid treatment, besides an awareness of potential side-effects of prescription drugs during hot weather, is crucial for physicians and pharmacists and other healthcare professionals.

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The incidence is relatively low in the UK and causes around 2,000 deaths each year, mainly in the elderly. However, the incidence of heat-related deaths has been predicted to rise by around 257% by the 2050s. This increase has been partly driven by projected population growth and ageing and also by climate change.1

The incidence may rise when customs or beliefs cause groups of people to be exposed to the heat for long periods of time such as Hajj, the annual Muslim pilgrimage in Saudi Arabia. Heat-related illness and mortality increase significantly during heat waves.2

Body heat loss is controlled by peripheral centres in the skin and organs and the central nervous system via the hypothalamus, with a greater cooling response to temperature elevation via central sensors. A temperature gradient exists between the body core and skin, which promotes heat dissipation when the core is higher than the surface.

When the core temperature increases during exercise and the skin temperature also rises as a result of the environment or internal heat production, heat dissipation is reduced. Likewise, when the body's metabolic heat production is more than heat transfer, core temperature rises and heat-related illness can occur.

Risk factors3

  • Environmental - hot and humid. Absence of shelter/shade, lack of access to water.

  • Age - infants and elderly (particularly if bed-bound/unable to self-care).

  • Impaired cognition

  • Physical - obesity, dehydration, being unacclimatised to heat, unusual exertion, inappropriate clothing, sleep deprivation, sunburn, sweat gland dysfunction.

  • Medical conditions - alcoholism, anorexia, cardiac disease, cystic fibrosis, dehydration, delirium tremens, dermatological conditions with decreased sweating, diabetes insipidus, epilepsy, poorly controlled diabetes mellitus, febrile illness, gastroenteritis, previous heat-related illness, hypokalaemia, Parkinson's disease, spinal injuries, and thyrotoxicosis.

  • Medicines - anticholinergics, alpha-adrenergics, antihistamines, tricyclic antidepressants, selective serotonin reuptake inhibitors (SSRIs), diuretics, phenothiazines, neuroleptics, beta-blockers, calcium-channel blockers, aspirin, clopidogrel, laxatives and lithium.

  • Illicit drugs - cocaine, amfetamines, ecstasy, LSD, phencyclidine (PCP), alcohol.

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The physiological effects of exposure to heat can be directly heat-related (heat cramps, heat exhaustion and heatstroke) or can contribute to a worsening of respiratory and cardiovascular diseases, electrolyte disorders and kidney problems. Heat-related illnesses occur on a continuum from mild symptoms to fatalities.

Heat cramps

  • Body temperature often elevated, usually <40°C.

  • Intense thirst with muscle cramps and tachycardia.

  • Sweating and heat dissipation mechanisms preserved.

  • Normal alertness and higher functions with no neurological problems.

Heat exhaustion

Signs of heat-related illness in an individual often begin with heat exhaustion, which, if left untreated, might progress to heatstroke.

  • Central nervous system (CNS) function is usually largely preserved, but those experiencing heat exhaustion may experience mild confusion, irritability, anxiety and poor co-ordination.

  • Heat dissipation is still functioning and temperature is usually <41°C.

  • Patients may experience nausea, oliguria, weakness, headache, thirst, occasionally syncope, sinus tachycardia, and orthostatic hypotension.

  • They often complain of being hot and appear flushed and sweaty.


  • This is a combination of hyperthermia (classically defined as a core body temperature of at least 40.6°C),4 often with loss of the capacity to dissipate heat, and CNS impairment.

  • Divided into classic heatstroke (due to passive exposure to severe environmental heat) and exertional heatstroke (due to strenuous physical exercise).

  • Loss of ability to sweat is often a late and ominous sign.

  • Hyperventilation is almost invariable, with hypotension and shock occurring commonly.

  • If the condition progresses to a more severe form (core temperature >41.5°C) it can cause widespread damage, in particular to the brain, liver, kidney and muscle.

  • The thermoregulatory centre may fail so that the patient actually feels cold with dry, vasoconstricted skin, leading to a vicious cycle.

  • Heatstroke involves coagulopathies and cytokines, and can result in systemic inflammatory response syndrome and multiple organ dysfunction.5

Differential diagnosis

  • The history of exposure to an adverse environment ± physical exertion usually confirms the diagnosis, but sepsis and alternative causes of fever (particularly malaria if relevant area/travel history) should be considered as a cause or precipitant.

  • In patients taking phenothiazines or other antipsychotics, consider neuroleptic malignant syndrome. Those on SSRIs or other serotonergic medication may be suffering from serotonin syndrome.

  • If there is a history of recent inhalational anaesthetic, consider malignant hyperpyrexia.

  • Recreational drug toxicity, particularly involving cocaine, amfetamines and ecstasy, is an increasingly common cause of hyperthermia.

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  • Monitor temperature regularly, using a rectal thermometer.

  • U&E.

  • Glucose.

  • Urate - may predict acute renal failure.

  • LFTs.

  • Creatine kinase - may indicate rhabdomyolysis.

  • FBC.

  • Clotting profile.

  • Arterial blood gases.

  • Urinalysis - may show myoglobinuria.

  • ECG.

  • CXR - to check for aspiration/pulmonary oedema.

The Wilderness Medical Society has developed a set of evidence-based guidelines for the recognition, prevention, and treatment of heat-related illness. The basic premise is rapidly lowering the core temperature to about 39°C (to avoid overshooting and rebound hyperthermia) remains the primary goal.

Patients with heatstroke should be taken to a medical facility capable of critical care management of patients with multiple organ failure.

Management should begin (as always) with resuscitation ABC:

  • Airway protection may be necessary, as coma, fits and vomiting are common. Intubate, but avoid using suxamethonium.

  • Breathing should be checked and supported, as necessary.

  • Circulatory support is usually given with IV fluids as 0.9% NaCl or 5% dextrose:

    • Avoid K+ containing fluids.

    • Gradually reduce concentration of Na+ if hypernatraemic.

    • If inotropes are required, try to use those with less alpha activity - eg, dopamine.

  • Rapid cooling - aiming for <41°C as soon as possible. Target temperature no lower than 39°C.

    • Cold-water immersion is the superior method for rapidly lowering core body temperature below the critical levels normally found for those with exertional heatstroke.

    • Wetting and fanning the skin can be undertaken for classic heatstroke.

    • Evaporative and convective cooling may be augmented with the addition of ice packs over the entire body to promote conductive cooling.

    • Strip the patient, spray with tepid water and use gentle fanning (cools at ~0.3°C/minute).

    • Apply ice packs to the patient's neck, axillae and groins (cools at ~0.1°C/minute).

    • Cold intravenous fluids may be given for adjunctive cooling.

    • Gastric/rectal/peritoneal lavage and cooled cardiopulmonary bypass/haemodialysis can also be used in some cases.

    • Antipyretics and dantrolene should be avoided.

  • Benzodiazepines and non-depolarising muscle relaxants - should be used to control shivering and fits:

    • Neuroleptics may be used to treat excessive shivering associated with cooling.

  • Catheterisation - should be considered to monitor urine output.

  • Complications - should be treated as they arise - see below.

  • ICU - consider transfer to intensive care if the patient does not improve despite aggressive treatment or if they have signs of organ failure.



With rapid cooling, sufficient rehydration and careful management of complications survival rates for heatstroke approach 85-90% in most modern centres. However, many patients experience permanent neurological impairments or death despite these efforts.7

Poor prognostic indicators include:

  • Coagulopathy.

  • Lactic acidosis (in the absence of severe physical exertion).

  • Core temperature >42.2°C.

  • Coma lasting >4 hours.

  • Acute kidney injury.

  • Hyperkalaemia.

  • Very high transaminase level.

  • Prolonged period of hyperthermia.


Education of the public about the risks of heat is extremely important. Adverse effects can often be avoided by:

  • Adequate hydration.

  • Avoiding exercising in the heat.

  • Acclimatisation, ie the process of repeated or increasing exposure (for example, over 1-2 weeks and with daily exercise in heat) of initially 30-60 minutes increasing to about 100 minutes at a time. During acclimatisation the body becomes more efficient in work production as well as heat dissipation through various mechanisms, including a number of changes to sweat rate, volume and composition.

  • Pre-cooling either by cold water immersion and the application of cooling garments or by ingesting cooled drinks has become a popular strategy for those who are going to exercise in hot environments. It has been shown to be effective for lowering pre-exercise core temperature, increasing heat storage capacity and improving exercise performance in the heat.8

Further reading and references

  1. Hajat S, Vardoulakis S, Heaviside C, et al; Climate change effects on human health: projections of temperature-related mortality for the UK during the 2020s, 2050s and 2080s. J Epidemiol Community Health. 2014 Jul;68(7):641-8. doi: 10.1136/jech-2013-202449. Epub 2014 Feb 3.
  2. Rein EB, Filtvedt M, Raeder JC, et al; Preventing hyperthermia: a cross-over study comparing two negative pressure devices during continuous passive heat stress. J Med Eng Technol. 2014 Jan;38(1):37-41. doi: 10.3109/03091902.2013.859756.
  3. Heat Stroke in Adults; BMJ Best Practice, September 2022
  4. Hajat S, O'Connor M, Kosatsky T; Health effects of hot weather: from awareness of risk factors to effective health Lancet. 2010 Mar 6;375(9717):856-63. Epub 2010 Feb 12.
  5. Krau SD; Heat-related illness: a hot topic in critical care. Crit Care Nurs Clin North Am. 2013 Jun;25(2):251-62. doi: 10.1016/j.ccell.2013.02.012. Epub 2013 Mar 25.
  6. Lipman GS, Gaudio FG, Eifling KP, et al; Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2019 Update. Wilderness Environ Med. 2019 Dec;30(4S):S33-S46. doi: 10.1016/j.wem.2018.10.004. Epub 2019 Jun 17.
  7. Leon LR, Helwig BG; Heat stroke: role of the systemic inflammatory response. J Appl Physiol (1985). 2010 Dec;109(6):1980-8. doi: 10.1152/japplphysiol.00301.2010. Epub 2010 Jun 3.
  8. Siegel R, Laursen PB; Keeping your cool: possible mechanisms for enhanced exercise performance in the heat with internal cooling methods. Sports Med. 2012 Feb 1;42(2):89-98. doi: 10.2165/11596870-000000000-00000.

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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