Sydneysiders are being urged to avoid the outdoors on Friday due to poor air quality caused by high ozone levels.

It is anticipated that Friday's hot temperatures and lack of wind will exacerbate air pollution caused by car exhaust and industrial fumes.

Environmental health director at NSW Health Wayne Smith said people should avoid heavy physical activity outside during the day, when ozone levels are at their highest.

"Ozone can irritate the lungs of healthy adults, so it is best to avoid any prolonged outdoor exercise," he said on Friday.

Professor Smith said parents of children with asthma should limit the amount of time they spend outside when air quality is poor.

Ozone levels tend to be lowest in the morning, and reach their peak about 7pm, he said.

"It's best to plan outdoor play this morning while the day is cooler," he said.

AAP