Several days of hot weather mean London is facing an inevitable rise in air pollution as the Olympics get underway. The latest forecast shows pollution levels will be at moderate levels over the next 24 hours in London. The situation is expected to remain unchanged over the next few days.
The main concern is ground level ozone. It forms from traffic fumes in sunny weather, and can cause breathing difficulties.
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It will be increasingly humid and thundery with highs of 30C or more today.
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Drink lots of water, stay out of the sun during the middle of the day, and take particular care of vulnerable people.