Ground-level ozone triggered on sunny, still days in areas of high air pollution will have Sydney residents with asthma and other respiratory conditions on alert.

On Tuesday afternoon, Sydney's ozone levels are predicted to rise above national standards, reaching levels that are likely to affect health, the NSW Office of Environment and Heritage has warned.

Children with asthma are more susceptible to the effects of ozone pollution and parents should limit the time they play outside, NSW Health director of environmental health Ben Scalley said.

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

"Asthma sufferers need to follow their asthma action plan and take their relieving medication where necessary. If symptoms get worse, asthma sufferers need to seek medical advice."

The ozone level national standards thresholds are a one-hour average of 100 parts per hundred million (pphm) and a four-hour average of 80 pphm.

The high temperatures forecast particularly across the Sydney basin could push levels to between 105 and 110 pphm on Tuesday and Wednesday.

Sydney's air quality was "poor", according to the OEH's Air Quality Index on Tuesday, which rates air quality as very good, good fair, poor, very poor and hazardous on a scale of zero to 250, with 250 being the worst.

Ozone is composed of the basic oxygen molecule with an additional oxygen atom, making it an unstable, highly reactive gas.

It forms a protective barrier in the Earth's upper atmosphere, filtering out damaging UV radiation from the sun. But when sunlight and extreme heat on still days combine with hydrocarbon and nitrogen oxide from car exhausts and other air pollutants to form ozone in the atmosphere at ground level, it can have significant negative impacts on health.

It's the blankets of smog that sits over the city on very sunny, very still Sydney days.

Cities such as Sydney that have long sunny periods, little wind and high temperatures are most likely to experience ground-level ozone.

Motor vehicle exhaust accounts for as much as 50 per cent of the organic chemicals that form ozone. Other sources are oil refining, printing, petrochemicals, lawn mowing, aviation, bushfires and burning off.

Western Sydney would bear the brunt of high ozone levels, director of climate and atmospheric science at the Office of Environment and Heritage Matthew Riley said.

"Emissions across the Sydney basin get pushed towards the west and the mountains on the other side trap the emissions," Mr Riley said.

The level of ozone and air pollution on Wednesday will depend on the wind and cloud coverage. The Bureau of Meteorology is predicting a strong north-westerly that could blow emissions out to sea, lowering the ozone level.

Cloud coverage would also disrupt much of the sunlight that triggers the ozone-forming chemical reaction in the atmosphere.

A spike in asthma-related hospital admissions sometimes occurs during days of high ozone levels, Dr Scalley said, though a causal link is difficult to confirm.

But there is no doubt an increase in ozone levels and air pollution increases the health risks for people with respiratory problems, he said.

More than one in 10 Australians, and one in six children, have asthma, accounting for 2.4 million people nationally. Some 5 to 10 per cent of cases are severe.

Earlier this month, doctors and health experts criticised a government review of vehicle emissions and air pollution for under-reporting the health risks, saying Australian fuel quality and emissions standards were "appalling".

The review acknowledged 1483 premature deaths in 2012 were due to outdoor air pollution, with about half of these linked to road transport pollution.

Environmental scientists suggest the premature death rate associated with air pollution is as high as 3000 deaths a year in Australia.

Asthma Foundation CEO Michele Goldman urged people to be alert to any changes or worsening of asthma symptoms, to stay indoors and carry their relievers wihthem at all times while the warning remained in place.

"We would highly recommend people have an up-to-date action plan from the GP, and to take their preventer medication regularly to minimise the effects when they are exposed to poor air quality," Ms Goldman said.