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.
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".
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.
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.