The Office of Environment and Heritage dropped the air quality level in Sydney's southwest, east and northwest to "hazardous" on Monday morning as smoke from hazard reduction burns settled across the city.

Air quality in Sydney's east improved to "very poor" by Monday evening and the northwest recorded "fair" air quality.

The levels in Sydney's southwest remained at "hazardous".

People with heart or lung disease, the elderly and children have been urged to avoid exercising outdoors in such conditions.

The NSW Rural Fire Service conducted hazard reduction burns at Sutherland, Wollondilly and Hawkesbury over the weekend with smoke likely to settle across the Southern Highlands, Goulburn, Blue Mountains and Wollongong overnight.

"It's a good time for us to be burning. The warmer daytime temperatures and the cooler night time temperatures are optimum," a RFS spokesman told AAP on Monday.

Smoke will remain over Sydney on Tuesday morning but it isn't expected to be as thick as it was on Monday, he added.

Fire crews are planning more hazard reduction burns later in the week.

Sydney's air quality is forecast to be "fair" on Tuesday.

Australian Associated Press