Key facts

• Heat stress is the leading cause of weather-related death, killing around 489,000 people a year
• High temperatures can exacerbate existing health conditions such as asthma, cardiovascular disease and diabetes
• The number of people exposed to high temperatures is growing but the risks of extreme heat can be mitigated

Overview

The UK Met Office defines a heatwave as an extended period of hot weather relative to the expected conditions at that time of year. Heatwaves are increasing in frequency, length and intensity around the world. The World Meteorological Agency reported that 2024 was the hottest year on record and warned that daytime temperatures of more than 40 degrees celsius and even 50 degrees are becoming more common.

Extreme heat brings with it high death rates. In the record-breaking heatwave across Europe in the summer of 2022 – when the heat peaked at 47 degrees celsius in Portugal but even the UK saw temperatures of 40 degrees – there were an estimated 61,700 heat-related deaths. A 44-day heatwave in Russia in 2010 saw an estimated 55,000 excess deaths. Asia and Europe are the areas most likely to see deaths from high temperatures.

Health effects

When the body overheats and cannot regulate its internal temperature the risk of heat exhaustion and heatstroke rises. The body tries to cool down in two ways: by redistributing blood flow to the skin (vasodilation) and sweating. Vasodilation puts strain on the heart while excessive sweating can lead to dehydration and acute kidney injury or failure.

Older people, babies and very young children are more vulnerable because they are less able to regulate their body temperature. Pregnant women and those with underlying health problems – particularly cardiovascular conditions, respiratory illnesses and diabetes – are also more susceptible. Hot weather can increase the risk of heart attack, stroke, lung problems and other diseases.

Who is affected

People living in tropical and subtropical climates are exposed to high temperatures for much of the year while people living in the north experience excessive heat in the summer months. Older people, young children, pregnant women and those with underlying conditions are more at risk from high temperatures.

In August 2025 the World Health Organization warned that millions of people around the world are exposed to extreme heat at work – outdoor workers, manual workers and those in workplaces without adequate cooling are at risk. The report warned that average productivity drops by 2-3 per cent for every degree above 20 degrees celsius.

People living in low quality housing with little access to cooling are also at risk of extreme heat.

Prevention

There are many steps that governments can take to mitigate extreme heat such as better urban planning, improving resilience of essential public services such as hospitals, and providing cooling facilities. However, individuals can also take steps to reduce the risk from high temperatures. This includes:

• Avoid going outside and doing strenuous exercise during the hottest part of the day
• Keep curtains/blinds and windows closed during the day – open windows at night to cool your home
• Only use an electric fan when temperatures are below 40 degrees C – above that it will heat the body
• If using air conditioning set it to 27 degrees celsius and turn on a fan – this will make the room feel about 22 to 23 degrees and will use less electricity than using air conditioning alone at a lower temperature
• Where light and loose fitting clothes
• Take cool showers or baths
• Wet your skin using a damp cloth or spray
• Drink water regularly – 2-3 litres a day
• Avoid alcohol
• Never leave children or animals in parked cars
• Never cover a pram or pushchair with a dry cloth – this makes it hotter. Instead use a wet, thin cloth

Useful links

who.int/fact-sheets/climate-change-heat-and-health

travelhealthpro.org.uk/extreme-heat

heathealth.info/keep-cool/

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