Prolonged periods of extremely hot weather can have serious health impacts on the more vulnerable groups of society. Health professionals play a key role to protect vulnerable people from potential severe health effects of heat waves.

Health professionals can enable their patients to manage their health appropriately in hot weather and direct them to clinical care if necessary. Health professionals, in particular those in general practice, emergency departments and pharmacies, play an essential role in preventing and managing heat-related illness.

Remember the 4 key messages to keep you and others healthy in the heat

1. Keep yourself cool, 2.Stay hydrated with water, 3. Lookout for each other, 4. Plean ahead for the heat 

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Physiology of thermoregulation

In a healthy person, the hypothalamus is responsible for regulating the normal human body temperature and keeping it within a range of 36.1 – 37.8°C.

The body can lose heat by:

  • conduction (direct contact of a cooler object with the skin)
  • radiation (via infrared rays)
  • convection (through water or air circulating across the skin)
  • evaporation of sweat.

When the temperature of the skin is higher than the air temperature, the body can lose heat by radiation, conduction and convection. A healthy person can lose heat via these mechanisms with air temperatures up to around 35°C. However, when the air is hotter than the skin, the only means by which the body can lose heat is through sweating (evaporation). Sweating and heat loss can be impaired by humidity, excess fat, skin disorders and excessive layers of clothing. Heat loss can be improved with wind or fanning, applying cool water or a cool object (cold packs).

The physiological response of the body to heat includes peripheral vasodilation, which increases the blood flow to the skin. This results in large quantities of warm blood from the core of the body being carried to the skin in order to facilitate heat loss through radiation, convection and conduction. Peripheral vasodilation requires an increase in cardiac output. People with chronic medical conditions that affect the ability of peripheral vasodilation or who cannot increase their cardiac output accordingly will be most at risk during heat waves.

The body’s heat regulation system can be impaired in the elderly, the chronically ill and by certain medications. Also, young children are more vulnerable as they produce more body heat, sweat less and have faster rising core temperatures.

People at higher risk of serious health effects

Everyone needs to take care in hot weather but some people are at higher risk of heat illness.

Characteristics of people at higher risk of serious health effects

Individual characteristics

  • Over the age of 75
  • Infants and young children
  • Overweight or obese
  • Pregnant or breastfeeding
  • Low cardiovascular fitness
  • Poor mobility
  • Cognitive impairment
  • On certain regular medications (see list below)

Social characteristics

  • living alone or socially isolated
  • low socioeconomic status
  • homelessness

Occupation and recreational activities

  • working in hot environment (e.g. labourers, gardeners, fire fighters)
  • exercising vigorously in the heat

Chronic diseases

  • heart disease
  • high blood pressure
  • diabetes
  • cancer or kidney disease
  • alcohol and other substance use
  • mental illness

Conditions that affect sweating

  • heart disease
  • dehydration (e.g. conditions causing diarrhoea)
  • extremes of age
  • skin disorders (sunburn, prickly heat, extensive scarring)
  • cystic fibrosis
  • quadriplegia
  • scleroderma
  • congenital impairment of sweating
  • people taking medications with anti-cholinergic effects

Acute illness

  • dehydration (e.g. gastroenteritis, diarrhoea from other causes)
  • infection, especially with a fever

Please note this list is not comprehensive and is to be used as a guide only.

Heat-related illnesses

Some illnesses or conditions can occur as a direct result of excessive heat, such as heat rash, cramps, exhaustion, heat stroke and exertional heat stroke. However, exacerbation of chronic conditions contribute to the majority of heat-related morbidity and mortality.


Even mild dehydration leads to an increased risk of injury, heat stress illness and poorer performance of complex tasks. Cardiac work is increased by mild to moderate dehydration and leads to reduced fluid available for sweating. A person can sweat up to 15 litres per day and it is important to note that thirst does not match fluids lost by sweating, even when fluids are freely taken.

Brief overview of heat related illnesses

Illness Cause Symptoms
Heat rash Inflammation of the sweat glands Erythematous papular rash, pruritis, secondary infection
Heat cramps Muscle relaxation is affected by loss of salt in sweat Muscle spasms in the abdomen, arms or legs
Heat exhaustion Dehydration leading to poor blood flow affecting the brain and heart Flushed or pale complexion and sweating, tachycardia, muscle cramps, weakness, dizziness, headache, nausea, vomiting, syncope
Heat stroke Core temperature rise leading to widespread organ injury
Same symptoms as for heat exhaustion, plus hyperthermia, shock, arrhythmia, dry skin with no sweating (skin may be damp from earlier sweat), altered mental state, ataxic gait, convulsions, unconsciousness, death
Exertional heat stroke Core temperature rise precipitated by intense or prolonged exercise in hot weather As for heat stroke, plus rhabdomyolysis and renal failure

Please see heat-related illness for more details on symptoms and emergency treatment.

Other illnesses exacerbated by heat

The majority of heat-related morbidity and mortality is due to exacerbation of existing chronic conditions.

Conditions that contribute to the most common causes of death during a heat wave include:

  • cardiac events
  • asthma or other respiratory illness
  • kidney disease
  • diabetes
  • nervous system diseases
  • cancer.

Dehydration and subsequent medication toxicity may exacerbate:

  • altered mental state
  • kidney stones
  • cardiovascular impairment
  • falls.

Heat and medication

A number of prescribed medications can increase the risk of heat-related illness. Also, some medications can be less effective or more toxic when exposed to and stored in high temperatures. Most medications need to be stored below 25°C or in the fridge if indicated. This applies particularly to antibiotics, adrenergic drugs, insulin, analgesics and sedatives. It is important for health professionals to discuss the correct use and storage of medications with people who take regular prescription medications as part of their care plan for hot weather.

Physiological effects of medications*

  • Interference with sweating, caused by:
    • anticholinergics, e.g. tricyclics antidepressants and benztropine
    • beta-blockers
    • antihistamines
    • phenothiazines
    • vasoconstrictors
  • Interference with thermoregulation, caused by:
    • antipsychotics or neuroleptics (e.g. risperidone, clozapine, olanzapine)
    • serotoninergic agonists
    • stimulants (amphetamine, cocaine)
    • thyroxin
  • Decreased thirst, caused by:
    • butyrophenone e.g. haloperidol and droperidol,
    • angiotensin-converting enzyme (ACE) inhibitors
  • Dehydration or electrolyte imbalance, caused by:
    • diuretics (especially loop diuretics)
    • any drug causing diarrhoea or vomiting (colchicines, antibiotics, codeine)
    • alcohol
  • Reduced renal function, caused by:
    • NSAIDS
    • sulphonamides
    • indinavir
    • cyclosporin
  • Aggravation of heat illness by worsening hypotension, caused by:
    • vasodilators e.g. nitrates (GTN) and calcium channel blockers
    • anti-hypertensives
  • Levels of drug affected by dehydration (possible toxicity for drugs with a narrow therapeutic index), caused by:
    • digoxin
    • lithium
    • warfarin
    • antiepileptics
    • biguanides (e.g. Metformin)
    • statins
  • Altered state of alertness, caused by any drugs which alter the state of alertness (e.g. alcohol, benzodiazepine, narcotics and many more).

*This list is not comprehensive and should only be used as a guide.

How health professionals can prepare for heat waves

  • Review and improve your knowledge of:
    • mechanisms of thermoregulation
    • risk factors for heat-related illness
    • conditions that may reduce the body’s ability to adapt to heat exposure
    • signs and symptoms of heat-related illnesses, diagnosis and treatment
    • early signs of heat stroke as it is a medical emergency
    • potential side effects of medications in hot weather.
  • Identify your patients at risk and:
    • educate them about how to stay healthy in hot weather (ways to reduce heat exposure, advice about fluid intake and medication)
    • provide them with the urine colour chart if appropriate
    • adjust their dose of medications if necessary
    • inform them that the efficacy of drugs can be affected by storage at high temperatures
    • regularly review their medications and fluid intake, especially older patients and people with advanced cardiac diseases
    • have a lower threshold to admit your patients to hospital especially if they are socially isolated
    • educate people who care for, support or assist children, older people and other people at increased risk of adverse health effects during a heat wave (the booklet How to keep someone healthy during hot weather can be helpful).
  • Review and adapt your practice for hot weather by:

A note on drinking recommendations

It is important that people drink enough fluids during hot weather, even if they are not feeling thirsty. Each older person and vulnerable patient should receive personalised drinking recommendations. This is particularly important for patients with a decreased perception of thirst or for patients with a restricted fluid intake.

A good way to find out if someone is dehydrated is by checking their urine colour. If the urine is pale, they are drinking enough. Please refer to the urine colour chart under resources and provide to your patients if applicable.

Page Updated: Thursday 19 December 2019
Contact page owner: Environmental Health