What does extreme heat do to our bodies?
Periods of hot weather can affect anyone, but some individuals are at greater risk of serious harm. Experts advise that vulnerable groups, such as older adults, babies, and young children, take extra precautions.
As the body temperature rises, blood vessels dilate, which lowers blood pressure and forces the heart to work harder to circulate blood. This physiological response can lead to mild symptoms like itchy heat rash or swollen feet.
Simultaneously, sweating causes loss of fluids and salt, disrupting the body's fluid and electrolyte balance. Combined with lowered blood pressure, this can result in heat exhaustion. Symptoms include feeling faint or dizzy, excessive sweating, clammy skin, nausea or vomiting, and muscle cramps.
If blood pressure drops excessively, the risk of heart attacks increases.
Why do bodies react this way to heat?
The human body maintains a core temperature of approximately 37°C, regardless of external conditions such as snowstorms or heatwaves. This temperature is optimal for bodily functions.
When ambient temperatures rise, the body struggles to maintain this core temperature. To dissipate heat, it dilates blood vessels near the skin surface and initiates sweating. The evaporation of sweat significantly enhances heat loss from the skin.
How can I stay safe in the heat?
The UK Health Security Agency (UKHSA) recommends vigilance for individuals who may have difficulty staying cool, including older adults, those with underlying health conditions, and people living alone.
Recommended measures include:
- Keeping indoor spaces cool by closing curtains on sun-facing rooms
- Drinking plenty of fluids while limiting alcohol intake
- Avoiding sun exposure between 11:00 and 15:00 when ultraviolet rays are strongest
- Staying in the shade, applying sunscreen with a high SPF and UVA rating, and wearing wide-brimmed hats
- Avoiding physical exercise during the hottest parts of the day
- Carrying water when traveling
Individuals considering cooling off in rivers or open water should heed local warnings and be aware of potential hazards.
It is critical that no one, especially babies, young children, or animals, be left unattended in locked vehicles.
What should I do if I think someone has heat exhaustion or heatstroke?
If you suspect someone is experiencing heat exhaustion, the NHS advises the following steps:
- Have the person lie down and elevate their feet slightly
- Encourage them to drink plenty of water; sports or rehydration drinks are also acceptable
- Cool their skin by spraying or sponging with cool water and fanning them; applying cold packs to the armpits or neck can also help
If the person recovers within 30 minutes, heat exhaustion is typically not serious. However, failure to recover within this timeframe indicates a risk of heatstroke, a medical emergency requiring immediate attention by calling 999.
Heatstroke symptoms may include cessation of sweating despite high body temperature, which can exceed 40°C, seizures, or loss of consciousness.
Who is most at risk from hot weather?
Older adults and individuals with chronic conditions such as heart disease may have reduced capacity to manage the physiological stress of heat.
Diabetes can accelerate water loss and affect blood vessels and sweating ability, increasing vulnerability.
Children, those with limited mobility, and individuals with neurological conditions like dementia may be less aware of heat risks or unable to respond appropriately.
People experiencing homelessness are particularly exposed to sun and heat, while residents of top-floor flats may face elevated indoor temperatures.
Do some drugs increase the risk from hot weather?
Certain medications can heighten heat-related risks, but the NHS advises continuing prescribed treatments while focusing on hydration and cooling strategies.
Diuretics, or "water pills," increase water excretion and can exacerbate dehydration and electrolyte imbalances during hot weather. These are commonly prescribed for conditions such as heart failure.
Antihypertensives, which lower blood pressure, may compound the blood vessel dilation caused by heat, potentially leading to dangerously low blood pressure.
Some medications for epilepsy and Parkinson's disease can inhibit sweating, impairing the body's cooling mechanisms.
Other drugs, including lithium and statins, may accumulate to harmful levels if fluid loss is excessive.
Does heat kill?
During 2022, which recorded the highest temperature in England at 40.3°C, there were an estimated 2,985 excess deaths attributed to heat, the highest number recorded in any year.
Estimated heat-associated deaths numbered 1,311 in summer 2024 and 1,504 in summer 2025.
Most heat-related excess deaths result from heart attacks and strokes caused by the body's efforts to maintain temperature stability.
The increased mortality rate typically begins when temperatures exceed 25°C to 26°C.
Evidence indicates that heat-related deaths are more frequent during spring or early summer heatwaves rather than peak summer, possibly due to behavioral adaptations as the season progresses.
Data from previous heatwaves show that mortality increases rapidly, often within the first 24 hours of a heatwave.
Additional resources
For information on sunscreen application, water safety during hot weather, sleeping comfortably in heat, cooling homes, and guidelines on walking dogs in high temperatures, please consult relevant health advisories.
