Air pollution: Forecasters hope for cleaner air on Friday
Forecasters have suggested pollution over parts of the UK could be blown away by "clean Atlantic air" on Friday.
BBC Weather's Peter Gibbs said the pollution had "peaked" and "should go down quite rapidly" in the coming days.
Government data showed London and south-east England had the maximum pollution level of 10 on Thursday.
The BBC's Rebecca Morelle said there had been a "horrible" mix of dust blown from the Sahara Desert, industrial pollution from Europe and UK pollution.
- Those with existing lung and heart conditions may find symptoms worsen
- They should avoid doing too much, especially outdoors
- Healthy people may experience minor symptoms such as a sore throat or a tickly cough
- They should avoid strenuous activity in order to reduce such symptoms
Pollution levels in eastern England reached nine on Thursday, with north-west England, Yorkshire, the Humber, the Midlands and south Wales also affected.
But the BBC's Peter Gibbs said south to south-westerly winds on Friday should bring the UK "cleaner Atlantic air".
He said the wind would be "starting to swing around" by 08:00 BST and would continue "picking up the pollution and shoving it away out into the North Sea" for most of the day.
The 10-point scale for measuring air quality is used by the Department for Environment, Food and Rural Affairs (Defra), with level one meaning a "low" risk of air pollution and 10 meaning "very high".
This pollution spell is nothing compared to the "pea-soup" fogs that blighted British cities until the 1960s.
Our air has become cleaner over the past century but at the same time we have learned how even low levels of pollution can pose a long list of health risks.
And although this episode is severe, it is not exceptional.
The Saharan dust makes it more visible than normal but breaches of international guidelines are fairly routine.
Last year one monitoring site in central London recorded 21 separate days when nitrogen dioxide soared above World Health Organisation limits.
One key pollution measure is of tiny particles called PM10, measured by the cubic metre over 24 hours.
Today a few sites in England are enduring levels of about 100. Last month Paris saw PM10 reach twice that amount.
And pity the people of Urumqi in the far west of China. When I was there last November, the air was so bad the airport closed. Today's PM10 level? An eye-watering 710.
Levels are determined by the concentration of five pollutants in the air - ozone, sulphur dioxide (SO2), nitrogen dioxide (NO2) and two types of particulate matter.Asthma risk
High levels of air pollution are usually reached about five times a year.
Defra advised people with lung and heart conditions to avoid strenuous outdoor activity while pollution levels remained high.
It also said people suffering symptoms of pollution - including sore eyes, coughs and sore throats - should cut down the amount they do outside,
Kay Boycott, chief executive of Asthma UK, said two-thirds of the UK's 5.4 million people with asthma "find that air pollution makes their asthma worse" and were at "increased risk of an attack".
The charity said asthma attacks were brought on by different triggers in different people, but urged anyone with the condition to be vigilant and follow basic safety tips in the coming days.
In February, the European Commission launched legal proceedings against the UK for failing to reduce levels of NO2 air pollution.
Friends of the Earth air pollution campaigner Jenny Bates said "thousands" of people in UK were likely to "die prematurely" this year unless the government took urgent action.
"Ministers, local authorities and the London mayor must get much tougher on the causes of air pollution - especially traffic fumes," she said.