The high air pollution across central and southern England and parts of Wales is due to light easterly winds bringing pollutants from the continent and allowing local pollution to linger close to where it is produced.
Dust blown up from the Sahara may also be a factor, according to Defra. Bernd Heinold, who models Saharan dust clouds at the Leipzig Institute for Tropical Research in Germany, said the dust was often carried into the oceans to the east and west of Africa, but around three times a year it was carried north by strong winds over the Sahara.
If the dust rises high enough in the atmosphere and does not meet a low pressure weather system on the way, the particles can reach the UK before falling to the ground. Winds of more than 20mph are usually needed to sweep sand up into the atmosphere, and over the Saharan desert they have been reaching 40mph in recent weeks.
In high enough densities, the dust clouds can make the sky look bright white, and give sunsets a yellow tinge as the particles refract light from the sun.
The highest levels of pollution in the UK on Wednesday were south of the Wash and east of a line from Cheshire to east Devon.
Health officials said adults and children with lung problems and adults with heart problems should cut down on strenuous exercise, particularly outdoors, especially if they experienced any symptoms. People with asthma were told they may need to use their inhalers more often. Officials advised older people in general to reduce physical exertion until the air pollution clears.
Dr Sotiris Vardoulakis, head of the air pollution and climate change group at Public Health England's Centre for Radiation, Chemical and Environmental Hazards, said: "While most people will not be affected by short-term peaks in air pollution, some individuals, particularly vulnerable groups such as those with existing heart or lung conditions, may experience increased symptoms."
Around one in 50 heart attacks that lead to admissions at London hospitals may be triggered by air pollution, according to a study at St George's hospital in Tooting. Researchers there compared emergency admissions for heart problems with air pollution the day before. Particulates in the air pollution are thought to cause inflammation that exacerbate underlying disease.
The air pollution levels will stay high on Thursday. Most affected will be East Anglia, the Midlands including Lincolnshire, and eastern parts of Wales, through to the Wirral and the north coast of Wales. The north-west coast of England, south-west Scotland and the north-east of Northern Ireland will also see high air pollution. The levels will fall on Friday, as south-westerly winds bring cleaner air to much of the UK.
Frank Kelly, professor of environmental health at King's College London, said: "This air pollution episode follows quickly on the heels of the previous episode which affected most of Europe and led to restricted car use and free public transport in Brussels and Paris.
"Periods of very high pollution in London arise usually because the city's normal pollution emissions are increased further by polluted air arriving on light winds from neighbouring areas such as northern Europe.
"However, whether home-produced or arriving from the continent, the tiny particles we take into our bodies with each breath cause immediate problems for some individuals such as those with asthma and contribute to longer-term problems for most of us in the form of heart disease and stroke."