Health Professionals play a vital role in helping people to prepare and stay healthy in the heat.

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Physiology of thermoregulation

In a healthy person, the hypothalamus is responsible for regulating body temperature and keeping it within a range of 36.1 – 37.8 degrees Celsius.

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, a healthy person can lose heat by radiation, conduction and convection. However, when the air is hotter than around 35 degrees Celsius, the body can only lose heat 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 cold packs.

The physiological response of the body to heat includes peripheral vasodilation, which increases 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 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. Young children are also more vulnerable as they produce more body heat, sweat less and have faster rising core temperatures.

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.

Dehydration

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 substantially, 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 

Heat-related illness provides more details on symptoms and emergency treatment.

​Illness​Cause
​Symptoms
Heat rash
​Inflammation of sweat glands​Erythematous papular rash, pruritis, secondary infection
​Heat cramps
​Muscle relaxtion 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

Heat-related illness provides 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

People at higher risk of serious health effects from heat

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

This list should be used as a guide only.

The characteristics of people at higher risk include:

  • physical:
    • being 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 
  • social:
    • living alone or socially isolated
    • having a low socioeconomic status
    • experiencing homelessness
  • occupational or recreational:
    • working in a hot environment (e.g. labourers, gardeners, fire fighters)
    • exercising vigorously in the heat
  • medical:
    • acute illness such as:
      • gastroenteritis or other diarrhoeal diseases
      • infection, especially with a fever
    • chronic illness such as:
      • heart disease
      • high blood pressure
      • diabetes
      • cancer or kidney disease
  • alcohol and other substance use
  • mental illness
  • skin disorders (sunburn, prickly heat, extensive scarring)
  • cystic fibrosis
  • quadriplegia
  • scleroderma
  • congenital impairment of sweating

Heat and medication

Some 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 degrees Celsius 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*

This list should be used as a guide only.

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 and narcotics).

How to help patients prepare for heat waves

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 send your patients to hospital especially if they are socially isolated
  • educate people on how to keep someone healthy during hot weather who care for, support or assist children, older people and other people at increased risk of adverse health effects during a heat wave.
Current as at: Friday 11 December 2020
Contact page owner: Environmental Health