Health authorities have 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 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.
Normally, the wind blows pollution away, but the problem is exacerbated on still days.
Ben Scalley, director of New South Wales Health's Environmental Health Branch, said ozone levels would be high in western Sydney.
The event happened most summers, he said.
"It's important that people with respiratory diseases, such as asthma, follow their action plan and use their relieving medication if necessary," he said.
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 tomorrow, with a top of 38°C expected on the coast and 40°C 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 build 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."