Air pollution in parts of England has reached high levels in the past 24 hours, Defra has said.
People with health problems have been warned to take particular care because of the pollution – a mix of local emissions and dust from the Sahara.
Greater London, rural areas of south-east England and East Anglia's towns and cities experienced the high levels.
People with lung or heart disease are among those warned against exercising outside in the affected areas.
The elderly could also be particularly affected by high levels of air pollution.
Defra (the Department for Environment, Food & Rural Affairs) has a 10-point scale for measuring air quality – with level one implying a "low" risk of air pollution and 10 for "very high" levels.
On Wednesday, levels were recorded at eight – meaning high – in rural parts of south-east England and towns and cities near busier roads in East Anglia.
In London, air pollution levels were recorded at level seven, which is also in the "high" category.
Moderate levels of pollution were recorded in the Midlands, Yorkshire and Humberside, and north-west England and Merseyside.
Defra said the air pollution on Wednesday was not as high as it had been in recent days. Level 10 air pollution was recorded in north-west Norfolk on Tuesday, as well as in the East Midlands, Yorkshire and Humberside on Monday.
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 said.
It is predicted that high levels will be recorded on Thursday in East Anglia, the Midlands, parts of north and east Wales, areas of north-west England and south-west Scotland.
The pollution is expected to clear in most places by Friday.
BBC Weather presenter Tomasz Schafernaker said Atlantic winds would blow away the pollution, improving air quality.
Dr Keith Prowse, honorary medical adviser to the British Lung Foundation, warned higher pollution levels could have a "significant impact" on people with respiratory conditions.
Written By: BBC News
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