New South Wales Health has issued a warning for people with respiratory conditions as a toxic gas lingers in Sydney.
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 Tuesday'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 is a respiratory hazard and 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.
Normally, the wind blows pollution away, but the problem is exacerbated on still days.
High levels in western Sydney
Dr Ben Scalley, director of NSW Health's Environmental Health Branch, said ozone levels would be high in western Sydney.
He said the event happened most summers.
"It's important that people with respiratory diseases, such as asthma, follow their action plan and use their relieving medication if necessary," he said.
"If their respiratory symptoms worsen, it's important that they seek medical advice."
The gas serves an important purpose for humans when it occurs in the stratosphere — commonly known as the ozone layer — where it inhibits harmful ultraviolet light from the sun.
Temperatures in the city are forecast to soar on Wednesday, with a top of 38C expected on the coast and 40C in Penrith.
Bureau of Meteorology duty forecaster David Berry, who is asthmatic, said he would be carrying his puffer in his pocket.
"When it's really hot and quite still, we can get a built up of some pollutants, and in this case it's ozone," he said.
"It's from the burning of fuels and having lots of air-cons on and that sort of thing.
"It starts to bank up in the city and the hot weather exacerbates it."ABC