Beijing was among the worst 10 cities in China for air quality in July, the first time this year the capital has dropped so low.
Only 35.5 per cent of days in the month met the national standard, putting the capital at 65th among the country's 74 major cities.
According to the Ministry of Environmental Protection's monthly air quality report on Monday, most of the polluted days registered excessive ozone concentration.
Excessive concentrations of ozone can cause respiratory tract irritation, damage to the body's immune system and affect lung function. Health experts strongly suggest that people stay indoors around noon to avoid the worst of such pollution.
The maximum concentration in eight hours surpassed the national standard by 58.1 per cent, 13.3 per cent points higher than the maximum in June, Luo Yi, head of environmental supervision at the ministry, said.
The ministry report said six cities in Hebei province, which neighbours Beijing, were among the worst 10 in China for severe air pollution in July, of which the city of Xingtai was the most polluted.
Except for the higher ozone concentration, the capital has seen a year-on-year decrease of other major air pollutants. Sulfur dioxide has dropped by 28.6 per cent, and PM2.5－particulate matter with a diameter smaller than 2.5 microns－fell by 31.9 per cent, the monthly report said.
Unlike excessive particulate matter in the air, which makes the days smoggy, excessive ozone concentration can occur under blue skies.
Excessive ozone concentration in the air refers to the ozone in the land surface layer, which is mainly generated after complicated photochemical reactions, said Chen Nianliang, a researcher at the Beijing Municipal Environmental Monitoring Center.
Ozone concentration is affected by the excessive emission of nitrogen oxides and volatile organic compounds, which are the major pollutants from vehicle exhaust, coal burning and emissions from chemical industries, Chen added.
Additionally, strong sunshine and high temperatures speed up photochemical reactions, meaning ozone concentration is at its highest from May to September, he said.
It is difficult to take targeted measures to reduce ozone because of its complicated relationship with the other pollutants, he said.
The ministry said most of the polluted days in major cities in the Yangtze River Delta and Pearl River Delta regions such as Shanghai and Guangzhou were because of excessive ozone concentration.
The inland province of Shaanxi also saw severe ozone pollution in July.