INVISIBLE POLLUTANT: A boy plays in downtown Beijing's Sanlitun area on July 24, when the city suffered medium-level air pollution with high ozone levels (LI XIN)

Chinese urban residents, vexed by the persistent smog that shrouded large parts of the country in winter and spring, can finally breathe a sigh of relief in summertime as the weather begins to change and helps to dissipate the particulate matter responsible.

However, many remain unaware of the mounting threat caused by increasing ozone levels. On hot and sunny days, ozone concentration on the ground is more likely to rise to an unhealthy level.

Chai Fahe, Vice President of the Chinese Research Academy of Environmental Sciences, said that ozone pollution is seasonal, and ozone levels are highest between May and October, with daily ozone level peaks between 2 and 3 p.m.

According to a recent air-quality report released by the Ministry of Environmental Protection (MEP), ozone and PM 2.5, which is fine particulate matter measuring 2.5 microns or less in diameter, were the primary pollutants in June for 74 cities across China and three economically developed zones, namely the Beijing-Tianjin-Hebei region, the Yangtze River Delta and the Pearl River Delta.

The report also shows the areas monitored were polluted 45.2 percent of the first six months of this year. Ozone was found to be the primary pollutant 20.1 percent of the time, while PM 2.5 was the primary pollutant 64.3 percent of the time.

Imminent threat

With rising awareness of particulate matter's health impacts, a growing number of Chinese urban residents have begun regularly checking PM2.5 levels to see whether they need to wear a mask when going outside. But since ground ozone is not as readily visible as the smog caused by particulate matter, the pollution has not caught public attention in the same way as PM 2.5.

Although ozone high up in the stratosphere protects life on Earth from the sun's ultraviolet radiation, exposure to high levels of ozone at ground level can cause a wide range of health problems such as chest pain, coughing, throat irritation and permanent damage to the lungs. Increases in the risk of premature death from heart or lung disease can also be seen, according to experts.

The gas made up of three oxygen atoms is also one of the principal components of photochemical smog, which can reduce visibility.

Ground ozone and PM 2.5 were not included in China's ambient air-quality standards until February 2012, when the State Council released new air-quality standards, which were to be put into effect in different regions at different times before the nationwide implementation scheduled for January 1, 2016.

In 2012, the standards became effective in provincial capitals and municipalities that are directly under the Central Government, as well as key areas such as the Beijing-Tianjin-Hebei region, the Yangtze River Delta and Pearl River Delta.

This year, the standards are implemented in a further 113 cities that are either considered to be key cities for environmental protection or to be model cities for environmental protection. All prefecture-level cities are required to meet the same standards by 2015.