Residents of Somerset and Dorset are being warned that they may suffer breathing problems with top-of-the-scale air pollution forecast in some parts the country.
The South West, West Country, Midlands and South Wales will be worst affected with levels in some parts hitting ten out of ten.
Defra, the environment department, advises those in affected areas to reduce the amount of strenuous outdoor exercise they do, especially if they start to suffer from a cough or sore throat.
Adults and children with lung problems, heart problems and pensioners should avoid vigorous activity altogether, while asthmatics are being warned that they could suffer flare-ups.
In many other parts of the country, levels could still be high enough to cause problems even among healthy people.
The problem is being caused by a combination of factors, including light winds, as well as the Saharan sand that has been raining down over the country in the past few days, leaving distinctive red dust on cars and skylights.
Sometimes known as ‘blood rain’, it occurs when strong winds in North Africa sweep up desert sand.
The particles stay in the clouds before being deposited during showers – and if the winds are blowing in the right direction, the UK can be on the receiving end.
Met Office forecaster Emma Sharples said: ‘This does happen - it has happened before.
'But you need the combination of elements - the sandstorm in the Saharan region, the wind from the South/South-East, and the right sort of rain.
‘You need a light rain, not too much - just enough to bring it down and then when it dries out it leaves that residue on cars. It's probably because the rain is not that heavy that it does not get brought down and washed off straight away. Of course it isn't dangerous.
‘The most it's going to be is an inconvenience to people who have it on their clean cars.’
A spokeswoman for Defra, which has revamped its pollution forecasting service, said: ‘The high level of air pollution this week is due to a combination of local emissions, light winds, pollution from the continent and dust blown over from the Sahara.’
High levels of pollution are expected to continue tomorrow, before the air clears on Friday.
VIDEO: Met Office video showing movement of Saharan Dust...