Air pollution has hit unusually high levels in Londonderry.
The very high levels of particulate matter are linked to the current cold weather conditions and are expected across Northern Ireland until Friday, forecasters said.
The weather conditions can lead to the build-up of local emissions from vehicles and home heating.
A DoE spokesman said: "During periods of high air pollution, the symptoms of people with lung or heart disease may worsen. Healthy people are unlikely to experience any ill effects."
People can help to cut the high pollution levels by avoiding burning solid fuels, avoiding lighting bonfires and reducing car journeys, the DoE said.
"If you must drive, switch off the engine if you expect to be stationary for more than a couple of minutes, and drive smoothly."
Hourly updates on levels of particulate matter (PM10 and PM2.5), sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide are available on the department's website: airqualityni.co.uk.