This summer has seen extreme temperatures across much of the UK.
Much of the health risk is from heat inside people's homes - so how can they be kept cool?
UK homes are designed to keep in warm air.
To let in as much cold air as possible, open the windows overnight or, if concerned about safety, for a couple of hours in the early morning or before bed, when the air temperature is lowest.
Open windows on opposite sides of the home, to let hot air out and cold in - and any loft windows, as hot air rises.
In flats, which may have windows one side only, open doors and use a fan to encourage airflow.
Do not open front doors leading to corridors, as they act as a fire safety measure.
Before temperatures rise, close all windows, external doors, blinds and curtains.
Dr Anna Mavrogianni, who researches sustainable building and urban design at University College London, also advises moving away from windows to avoid direct heat.
Unlike many countries which regularly see high temperatures, most UK homes have no air-conditioning.
To push cool air around the home, you can place a bowl of ice in front of an electric fan.
Avoid cooking for long periods and running electrical appliances that generate heat.
Heat exhaustion can also be brought on by high humidity - moisture in the air - so:
Office buildings or public areas such as sports centres or libraries may be cooler.
Local authorities have identified spaces where the public can go.
The mayor of London's office, for example, has created a map of "cool spaces".
How are you coping with the hot weather? Email HaveYourSay@bbc.co.uk.
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