Heavy air pollution, with ozone being the primary contaminant, blanketed the Beijing-Tianjin-Hebei region on Friday.

A weak cold front is expected to arrive during the weekend, improving air quality in the region, the Beijing environmental protection authority said. But some areas will still face moderate levels of pollution, mainly caused by ozone and PM10 particulate matter.

The National Meteorological Center issued a yellow alert on Friday for high temperatures, the least serious in a three-tier warning system, with some northern provinces expected to see temperatures above 35 C and some areas experiencing heat up to 39 C.

As temperature rises, ozone pollution is also likely to increase, experts said.

Ozone, a gas that is a type of oxygen formed with an extra molecule, is created when vehicles, power plants, industrial boilers, refineries and chemical factories emit pollutants that react chemically in strong sunlight.

According to the Chinese Academy of Sciences, intense sunlight and hot weather are ideal conditions for ground-level ozone to form.

When it is 20 to 30 kilometers above the surface, ozone can absorb harmful radiation and protect life on Earth. It also is useful in sanitation and manufacturing. But ozone exposure can cause allergies and respiratory and cardiovascular diseases, according to Lu Mengyao, an engineer at the National Meteorological Center.

"Observations show that ozone pollution has been on the rise across the country since 2013," Zhang Yuanhang, an academician at the Chinese Academy of Engineering, said at an air pollution forum in Langfang, Hebei province, on Thursday.

"In the Pearl River Delta region, ozone has even surpassed PM2.5 as the main pollutant in air pollution," he said.

China recorded only one day of heavy ozone pollution in 2016, but ozone was the primary culprit for around 32 percent of all its bad air days, according to the Ministry of Environmental Protection.

Wang Qifeng, an expert attending Thursday's forum, said ozone problems could be serious enough this summer that there might need to be "some temporary emergency measures to address the rising ozone pollution", according to Science and Technology Daily.

Ozone is one of the six airborne pollutants China regularly monitors; the others are PM2.5 and PM10 particulates, sulfur dioxide, nitrogen dioxide and carbon monoxide.

Still, people can be puzzled when ozone air pollution warnings are issued, since the weather may be clear and sunny at the time.

Ye Dan, a researcher at the environmental office of the Chinese Center for Disease Control and Prevention, said that people should stay indoors when the ozone level is high.

Xinhua contributed to this story.