Asthma sufferers and the elderly have been warned to take care as hot weather brings air pollution and smog.
People are advised to stay in the shade in hot weather
A mini-heatwave forecast for the UK on Friday and early next week may affect people with breathing difficulties, government officials have said.
Meanwhile, scientists are developing a potentially life-saving system to give early warnings of hot weather.
The Liberal Democrats have warned smog could cause up to 100,000 deaths in the next 20 years under current trends.
Smog, which is mostly invisible, is caused by sunlight reacting with pollutants such as ozone in the air.
The high ozone levels are expected to affect London, the South East and East Anglia, the Department for the Environment, Food and Rural Affairs (Defra) said.
Those affected should avoid making car journeys and exercising outdoors in the afternoon, a spokeswoman said.
She went on: "During episodes of air pollution experienced during the summer in the United Kingdom, levels of ozone, nitrogen dioxide and particles may be raised.
"Most people will experience no ill effects. Those suffering from lung diseases, including asthma, particularly if elderly, should be aware that their symptoms might worsen.
"They may need to consider modifying their treatment as they usually do when symptoms increase, consulting their doctor if this is not effective."
Weather experts fear the UK may see increasing levels of smog as the climate hots up.
Predictions for a Nato conference show temperatures could climb by an average 2.5C in southern Europe by 2020, with rainfall decreasing sharply.
Climatologists at the University of Birmingham have started trials of an early warning system for hot weather in five European cities.
The Heat Health Watch Warning System, funded by the European Union, is being tested in London, Paris, Barcelona, Rome and Budapest.
Its developers hope to give health authorities a two-day warning so they can put into action contingency plans to limit the effects of heatwaves on vulnerable people.
Climatologist Dr Glenn McGregor said: "For the first time we are combining climate and weather forecasting with public health science, to give a prediction of the likely health outcomes of periods of moderate to severe health stress."
The Liberal Democrats claim as many as 100,000 extra deaths could be caused by smog over the next two decades.
The party's environment spokesman, Norman Baker, said ozone was already exceeding dangerous levels, causing high air pollution.
He blamed the government for its "failure to take air pollution and climate change seriously".
Mr Baker said: "This is not something that will happen in the distant future, it is already having terrible effects on individual health.
"The spate of unusually hot summers is taking its toll on air quality. If global warming and the rise in vehicle emissions are allowed to continue unchallenged, the situation will only get worse."
Professor Martyn Partridge, an adviser to campaign group Asthma UK, said people with breathing difficulties should take precautions during hot weather.
He said four out of five people would find their asthma symptoms worsened with increased air pollution.