High levels of air pollution are being monitored by Department of Environment officials in Belfast and Londonderry.
Moderate levels are also being recorded in towns across Northern Ireland.
The department said “elevated levels” of pollution are likely to continue for as long as the cold and calm weather conditions persist.
The levels are believed to be as a result of local pollution sources like cars and home heating emissions.
The calm weather conditions are exacerbating the problem, because pollutants are not being widely dispersed by wind.
During periods of high pollution, people with lung or heart disease may find their symptoms worsening.
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.
There is also an SMS service, Air Aware, which members of the public can subscribe to if they want to receive direct notification of high air pollution episodes.