NSW Health has warned about respiratory conditions amid toxic gas in Sydney.

A hot, still weather and air pollution has been blamed for creating excess harmful ozone in Sydney’s western suburbs.

New South Wales Health rated air quality as “poor” this week, warning that people with respiratory conditions like asthma should take care.

Ozone is an invisible yet pungent gas that in high concentrations can damage the lungs, causing chest pain, coughing and throat irritation.

The gas is created by nitrogen oxides from car exhausts and other sources react with oxygen on hot days.

Normally, wind carries enough pollution away for this not to be a problem, but still air can create the perfect environment for a reaction.

“It's important that people with respiratory diseases, such as asthma, follow their action plan and use their relieving medication if necessary,” director of NSW Health's Environmental Health Branch,Dr Ben Scalley said.

“If their respiratory symptoms worsen, it's important that they seek medical advice.”