British authorities on Wednesday warned people with heart or lung conditions to avoid exertion as a combination of European emissions and Sahara dust created a "perfect storm" of pollution that blanketed the country in smog.
The environment department said air pollution in some areas reached the top rung on its 10-point scale.
The department said the smog was caused by pollution from Britain and industrialized areas of the continent — trapped in place because of light winds — mixing with dust blown up from a storm in the Sahara desert.
Many motorists across England awoke this week to find cars covered in a film of red dust left by overnight rain.
Paul Cosford of Public Health England told the BBC that people with heart or respiratory problems should "reduce the amount of strenuous exercise outdoors over the next few days."
Experts said Britain's smog was the result of an unusual combination of factors. Helen Dacre, a meteorologist at the University of Reading, said conditions had conspired "to create a 'perfect storm' for air pollution."
"Toxic gases, such as nitrogen dioxide and ozone, as well as fine dust particles in the air blown in from the Sahara and from burning fossil fuels, all contribute to cause problems for people with heart, lung and breathing problems, such as asthma," she said.