Earlier this week, the Government released summer smog warnings but rain dispelled any real problem. Yesterday, however, experts said that they expected ozone levels to increase to potentially "high" levels over the next few days.
"We've had rain this week which washes ozone out of the atmosphere," said Michael Dukes, from the Press Association Weather Centre. "But we are having very sunny weather now and I expect the ozone levels to build up. It happens gradually if the days stay sunny without the weather breaking. I expect ozone levels will rise everywhere in the UK, apart from Northern Ireland, to moderate and possibly high levels in some areas."
A spokeswoman for the Meteorological Office said: "We are taking the risk of smog seriously because it's so still at the moment."
Mr Dukes said: "Obviously the wind is a factor as it mixes pollution with cleaner air and there are low levels of wind expected over the next few days."
He added that, contrary to public perceptions, an increase in ozone will choke up the countryside more than the cities. "I'm sure people think of pollution as being an urban problem but the other pollutants in cities get rid of ozone fairly quickly."
Asthma sufferers and those with respiratory problems will need to be careful when outside, the Department of Environment, Transport and the Regions said. "Most people will experience no ill effects," a spokeswoman said, "but those suffering from lung disease - including asthma - should be aware their symptoms may worsen."
The elderly should be particularly careful and anyone who has noticed their breathing being affected by hot weather in the past should also avoid strenuous activity, she added.
The environment minister, Michael Meacher, said that everybody needed to "do their bit" to reduce air pollutants. A spokeswoman from his department said that as cars were the major source of ozone in cities, anyone who left their motor at home would help reduce the smog. "People need to ask themselves before a journey - do I really need to use the car?" she asked. "And if they are stuck in a traffic jam they should turn the engine off."
She added that not using solvent-based paints would also help and people should refrain from burning solid fuels.