Beijing residents are bracing for another round of hot and polluted days, starting today, but this time, the main pollutant will be ozone, a key element in forming photochemical smog. China’s Ministry of Environmental Protection said on Thursday that the area around Beijing, Tianjin and Hebei has enjoyed clear skies for some weeks partly due to China’s effort to curb traffic and production ahead of Beijing’s forum on its ambitious trade integration plan, called the Belt and Road Initiative. However, the area could experience middle to heavy air pollution today, with ground-level ozone, an element that is particularly harmful to lungs and vision, being the major pollutant. Ozone is a major component in photochemical smog, one of the most lethal types of air pollution for people living in metropolitan regions. According to the World Health Organisation, the eight-hour average exposure to ozone is limited at 100 micrograms per cubic metre. Inhaling low amounts of ozone can cause headaches, dry mouth and coughing. And when the air’s ozone concentration rises above 240 micrograms per cubic metre, it can exacerbate symptoms of existing health issues such as asthma and heart disease. Smog linked to third of deaths in China, study finds Air quality indexes typically are based on particulate-matter measurements of PM 2.5, meaning fine particles of 2.5 microns or less in diameter, and PM 10, or coarse particles of 10 microns or less in diameter. These particle pollutions can be small enough to enter lungs and harm one’s health. Ozone pollution, however, still is rare in China. It is formed through chemical reactions involving sunlight and pollutants, such as oxides of nitrogen, and volatile organic compounds that can originate from car exhaust, coal burning and industrial emissions, among other things. The ministry said in its statement that an increase of temperature and more intense sunlight since Monday has created conditions conductive to ozone forming in the Beijing-Tianjin-Hebei area. On Monday, Beijing Meteorological Service also issued its first heat-wave yellow alert this year, estimating the maximum temperature across the city to reach above 35 degrees Celsius from Wednesday to Friday. Heavy smog back to haunt Beijing two days after acting mayor Cai Qi vows air pollution crackdown In the daytime on Thursday, Beijing experienced light pollution as its air quality index bounced between 175 and 195 with ozone the dominating pollutant, according to Beijing Municipal Environmental Monitoring Centre. A rise on the index above 50 means air pollution. The air quality could improve slightly next week, when cold air lowers temperature in the region, but residents of some cities still could see smoggy days, carrying major pollutants such as ozone and PM10, according to the ministry.