The Department of Environment said Newry is the worst affected area, while a moderate level of pollutants has been monitored in Armagh, Belfast, Londonderry and North Down.
Moderate levels of nitrogen dioxide were recorded in the Ballymena and Dungiven areas.
The department said the problem could be linked to the current cold, dry weather conditions, which can lead to a build-up of emissions from vehicles and home heating.
It forecast moderate levels of pollution for the rest of the week in milder weather, with the possibility these could rise to high if the cold weather returns for the weekend.
High pollution can cause the symptoms of people with lung or heart disease to worsen, and anyone who finds themselves affected is advised to reduce the amount of time they spend outdoors or avoid busy, congested streets - healthy people are unlikely to suffer ill effects.