People are being warned of the possible health risks after officials issued a smog warning for many parts of England.
Air pollution combined with warm weather creates the smog
A combination of air pollution and warm weather is creating high ozone levels across London, the south-east, East Anglia and central England.
Conditions are likely to persist until Saturday, the government said in its first such warning of 2005.
It said people sensitive to air pollution, such as those with heart and lung conditions, should heed the alert.
Ben Bradshaw, minister for local environmental quality, said people should "take sensible precautions, such as avoiding exertion outdoors on hot afternoons" to protect their health.
"We can all help to reduce the current high levels of pollutants by avoiding unnecessary car journeys by walking, cycling and taking public transport instead," he added.
People who are particularly sensitive to air pollution may begin to notice an effect on their breathing in the coming days.
Asthma sufferers are not necessarily more sensitive but, if affected, are advised to use their inhaler.
Summer smog is produced by the sun reacting with substances such as car fumes and solvents, producing ground level ozone.
Smog can also contain elevated levels of nitrogen dioxide and breathable dust.
Mr Bradshaw added: "Air pollution is something that can affect human health.
"During the 2003 heatwave, it was estimated... that up to 800 premature deaths may have occurred as a result of the poor air quality.
"Fortunately, such episodes are becoming less frequent and severe due to large reductions in pollution from vehicles and industry following tighter regulation in the last few decades."
Health advice is available on a Defra freephone helpline on 0800 556677.