The Lower Hunter's air quality is among the worst in the state with "hazardous" levels of fine particle pollution.
The hazardous air-quality rating has been reached at Newcastle, Wallsend and Beresfield.
Only Armidale in the state's north-west has a higher level of pollution.
The NSW Department of Planning, Industry and Environment says the region's air quality is being affected by smoke from bushfires in northern NSW.
"People with heart of lung disease, other adults, and children should avoid exercising outdoors." the department advises.
"Everyone else should reduce prolonged or heavy exercise.
"If you have symptoms rest and use your reliever medicine. If symptoms persist, seek medical advice."