High levels of air pollution have been recorded in Londonderry and Newry because of the calm and cold weather.
People with lung or heart disease may experience a worsening of symptoms, the Department of Environment has said.
The problems are expected to continue until Wednesday when winds are expected to increase. Moderate levels are also being recorded in Strabane and Armagh.
Vehicles and burning solid fuels contribute to increased levels of emissions like carbon monoxide.
A DoE statement said: "This localised air pollution is likely to be associated with the build-up of emissions from vehicles and home heating during the calm, cold weather conditions experienced across Northern Ireland over the weekend and it is forecast to continue until Wednesday when winds are expected to increase."
When pollution is high, sensitive people may notice their symptoms worsen and may need to act to reduce the symptoms or their exposure to the pollution.
Asthmatics will find that their symptoms will ease through use of their relief inhaler in the usual way advised by their doctor.