Air pollution prompts warning for Sydney

People with asthma and other respiratory conditions are being urged to take caution with high air pollution levels predicted during Sydney's heatwave weekend.

NSW Health has issued the warning, saying high ozone levels are expected on Saturday, leading to poor air quality.

"Parents are advised to limit outside play for children with asthma," NSW Health's Ben Scalley said in a statement.

"Ozone levels reach their peak about 7 in the evening and tend to be lowest in the morning, so it's best to plan outdoor play in the morning when the day is cooler."

Ozone pollution is caused by car exhaust and industrial fumes and gets worse on hot, still days, Dr Scalley said.

The state is set to swelter on Saturday, with Sydney forecast to reach 39C on the coast and hit 46C in the west.

Severe fire danger has prompted a statewide fire ban for the weekend.