A combination of hot still weather and pollution is being blamed for an excess of harmful ozone in the city's western suburbs.
New South Wales Health rated today's air quality as "poor" and said people with respiratory conditions, such as asthma, should take care.
Ozone is a colourless but pungent gas that can damage the lungs and cause chest pain, coughing and throat irritation when inhaled.
The gas is formed when nitrogen oxides from things like car exhausts and air conditioner fumes react with oxygen in the air on hot days.
