There are extreme temperatures across most of the UK.
Much of the health risk is from heat inside people's homes - so how can they be kept cool?
1. Let in cool air
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.
2. Encourage airflow
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 the front-door and use a fan to encourage airflow.
3. Keep out hot air and direct sunlight
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.
4. Use a fan
Unlike in countries with frequent high temperatures, most UK homes have no air-conditioning.
To push cool air around the home, place a bowl of ice in front of an electric fan.
5. Limit hot activities
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:
* take shorter and colder showers
* wipe excess water from surfaces
* move indoor plants outside
* What does hot weather do to the body?
6. Find an alternative space
Office buildings or public areas such as sports centres or libraries may be cooler.
Local authorities have identified spaces where the public can keep cool.
The mayor of London's office, for example, has created a map.