Parts of the UK face soaring levels of air pollution on Friday, prompting heath warnings for vulnerable people.
Very high levels of pollution are expected in some areas of south east England, as still, warm conditions create a build-up of local pollutants from sources such as traffic fumes, combined with pollution from the continent and Saharan dust from the south.
Much of the South East and eastern England will see high levels of pollution, although the problem is expected to be short-lived, with Atlantic winds dispersing the polluted air in the early hours of Saturday, the Environment Department (Defra) said.
In areas experiencing very high levels of air pollution, adults and children with lung problems, adults with heart problems and older people are advised to avoid strenuous activity. Words: PA.
People are also advised to reduce physical exertion, particularly outside, and asthma sufferers may need to use their reliever inhaler more often.
Where pollution levels are high, adults with heart or lung problems and children with lung problems should reduce strenuous physical activity, older people should reduce physical exertion and anyone suffering from sore eyes, a cough or sore throat should cut down on activity, particularly outdoors.
A Defra spokeswoman said: "Locally generated air pollution, combined with pollution from the continent and Saharan dust, could cause high or very high levels on Friday.
"This is expected to clear on Saturday and pollution levels will return to low throughout the morning."
The latest air pollution forecasts and health advice can be found here.
Dr Penny Woods, chief executive of the British Lung Foundation, said: "This latest high air pollution episode, coming so soon after the last, is troubling, and could again put people living with respiratory conditions at risk of worsening symptoms."
She advised people who find their health affected to take simple steps including avoiding busy roads and refraining from strenuous exercise, while those who carry a reliever inhaler should always have it with them.
"That said, people living with lung conditions shouldn't have to take such steps simply to avoid being made ill by the air they breathe," she said.
"With periods of high pollution often resulting from a combination of domestically-produced pollution and that coming from mainland Europe, we need urgent action from government to clean up the air we breathe, across all government departments at home and at a pan-European level."
A case brought by environmental lawyers ClientEarth against the Government over levels of air pollution that breach European Union rules is shortly to return to the Supreme Court.
Philip Insall, director of health for cycling charity Sustrans said: "The poor state of air pollution in Britain is both criminal and a national embarrassment.
"We know that local air pollution is causing at least 29,000 premature deaths a year and we know that it is primarily due to emissions from motor traffic. It's clear that we can only address this by reducing reliance on motor vehicles.
"The next Government will need to get a grip on air pollution. That will mean serious, dedicated investment and an effective programme of action to help more people out of their cars and choosing walking and cycling for short journeys."
And Keith Taylor, Green MEP for South East England, said: "Air pollution is a deadly serious problem as it causes serious heart and lung problems and is responsible for tens of thousands of deaths every year in the UK.
"Whoever forms the next Government one of the first things they must do is to urgently address this public health crisis that currently only seems to be getting worse."