People with health problems are being warned to avoid strenuous activity after forecasts that air pollution will reach high levels in parts of England.
Defra issued warnings as high pollution levels were recorded on Tuesday.
The pollution - a mix of local and European emissions and dust from the Sahara - is forecast in parts of south England, the Midlands and East Anglia.
The elderly and those with lung or heart disease are among those warned against exercising outside.
In February, the European Commission launched legal proceedings against the UK for failing to reduce levels of nitrogen dioxide (NO2) air pollution.
On Wednesday, Defra (the Department for Food, Agriculture & Rural Affairs) admitted it was a "challenge" to meet air pollution targets near busy roads but said air quality had "improved significantly" in recent decades.
Defra has a 10-point scale for measuring air quality - with level one implying a "low" risk of air pollution and 10 warning of "very high" levels.
Levels are determined by the concentration of five pollutants in the air, including NO2, sulphur dioxide (SO2) and ozone.
High levels of air pollution are usually reached about five times a year, Defra said.
On Wednesday, levels were recorded at six - meaning moderate - in East Anglia and south-east England, with pollution readings reaching level five in London and areas of north-east England.
However, forecasters say pollution levels could reach high levels later in the day and on Thursday, before clearing on Friday.
The BBC weather centre said the highest recordings were likely to be in East Anglia and the East Midlands, while moderate to high air pollution levels were also forecast for large parts of southern and central England.
In parts of Wales, areas around Wirral and Merseyside, as well as Devon, levels were likely to be moderate.
On Tuesday, pollution levels hit the maximum of 10 in north-west Norfolk.
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.
"People who use a reliever inhaler should make sure that they carry it with them. If they feel that their conditions are worsening then they should contact their GPs," he said.
Kay Boycott, chief executive of Asthma UK, said the two-thirds of people with asthma who find that air pollution makes their condition worse "will be at an increased risk of an attack".
Advice on the Defra website states that for high levels of pollution "adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms.
"People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion."
It adds that "anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors".
A Defra spokeswoman 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.
"We want to keep improving air quality and have introduced a new five-day forecast service in addition to investing heavily in local and transport initiatives to tackle this issue head-on."
Meanwhile, Maria Arnold, from the environmental law group Client Earth, called for changes to the way pollution warnings are given, saying the public was "generally very poorly warned about these type of events".
"We think the [warning] format needs to become very similar to the warnings for floods and heat waves. It is really important people understand the risks."
Friends of the Earth air pollution campaigner Jenny Bates said air pollution was a "national disgrace".
"There's not much we can do to control dust from the Sahara, but the authorities could and should be doing far more to deal with the UK's contribution to this air pollution episode, particularly from road traffic emissions," she said.
Dr Paul Cosford from Public Health England told BBC Radio 4's Today programme that pollution levels were "clearly a serious issue".
However, he said it was "important we don't get it out of proportion".
He said the "vast majority" of people would suffer no harm from the pollution levels, but said people with health issues should take extra care.
The BBC's environment correspondent Roger Harrabin said the episode could bring further attention to the government's long-term failure to reduce air pollution.