A smog warning has been issued for the Easter weekend, amid fears the combination of hot weather and pollution could cause health and breathing problems.
The Department for Environment, Food and Rural Affairs issued the warning, covering the whole of England and Wales, ahead of what is expected to be the hottest Easter since 1984.
It's the first smog alert in two years, the last being on 17 July 2009, but smog warnings are normally annual events.
Vulnerable people like asthmatics, heart patients, and the elderly are advised to be particularly vigilant.
Officials also advised people who suffer from a shortage of breath on hot sunny days to avoid strenuous outdoor activity, especially in the afternoon.
Children with asthma can play outside normally, but might need to increase their use of medicines such as inhalers.
Officials are concerned that pollution could settle on Britain from southern Europe, and urged drivers to leave their cars at home and in particular to avoid short journeys.
Smog is created when vehicle exhaust gases such as nitrogen dioxide react with the air in strong sunlight to form ozone.
While ozone in the stratosphere is benign and protects people from ultra violet rays, at ground level it is toxic and dangerous to health. It can lead to headaches, a sensation of burning eyes, shortness of breath, wheezing and coughing, and can also cause lung inflammation and an increased risk of heart attack.
Environmental campaigners have argued that smog levels in London in particular are routinely higher than what is acceptable under EU pollution laws.
Ed Dearnley, policy officer with Environmental Protection UK, a charity researching air pollution, told the Telegraph: 'Most air pollution in cities is due to traffic.
'Defra has a higher threshold than other bodies when it comes to sending out smog alerts. We believe it is too high especially for vulnerable groups such as asthma sufferers.'
Monitoring in Marylebone Road, central London, showed that the number of days on which polluting particles known as PM10s were elevated was higher than the EU maximum. Britain has been given until June to meet standards before it risks being fined.
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