Northern Ireland, London, the South East and Eastern regions are experiencing very high levels of pollutants known as particulate matter, or PMs, which come from sources such as traffic emissions, in particular diesel engines.
High levels of air pollution are currently being monitored in Armagh, Belfast, Londonderry and Newry, according to the Department of Agriculture Environment and Rural Affairs.
The department's monitoring stations indicate that moderate levels are being experienced in some other urban centres across Northern Ireland. Air quality is likely to improve from this evening and into tomorrow as winds strengthen.
The high levels of pollution are believed to be as a result of local pollution sources such as road vehicles and home heating emissions combined with cold, calm weather conditions in which pollutants are not being dispersed.
During periods of high pollution the symptoms of people with lung or heart disease may worsen. Healthy people are unlikely to experience any ill effects.
Hourly updates on levels of particulate matter, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide are available on the Department’s website: http://www.airqualityni.co.uk.
With the high pressure and still conditions continuing, air pollution is also expected to be a problem across much of the UK on Tuesday.
In very high pollution conditions, people are advised to limit exercise outside, while those with lung and heart problems and older people should avoid strenuous activity.
Where there is high air pollution, adults and children with lung problems and adults with heart problems, as well as older people, should reduce the amount of strenuous exercise they do.
High and very high levels of pollution can cause people with asthma to need to use their inhaler more.
In London, one of the areas worst hit by the conditions, Mayor Sadiq Khan has issued pollution alerts at Tube stations, bus stops and roadsides.
The Green Party's Baroness Jones accused the Government of not doing enough to warn people elsewhere in the country of the issue.
And she said: " When air pollution episodes are capable of triggering an extra 300 deaths as well as hundreds of emergency admissions to hospitals around the country, I think that we have to consider emergency measures to discourage driving, encourage a switch away from diesel and promote less polluting alternatives."
Air pollution from sources including factories and vehicles is linked to the early deaths of around 40,000 people a year in the UK, and causes problems such as heart and lung diseases and asthma.