BEIJING - Most urban districts of the Chinese capital and other parts of northern China have been hit by smog which sent the air pollution index to "hazardous" levels in cities like Baoding and Shijiazhuang on Monday (Oct 15).

It is the third day in a row that the region has been blanketed with smog.

The pollution is expected to reach its peak on Monday (Oct 15), and some parts of south-east Beijing will see heavy air pollution, reported Beijing Youth Daily on Sunday evening, citing the Beijing Environmental Monitoring Centre.

The environmental watchdog said the pollution will clear on Tuesday.

Greenpeace East Asia tweeted that Monday is the third consecutive day northern China is hit by a blanket of smog.

China has a six-level air quality index (AQI) system.

The air quality is classified as excellent if AQI levels are below 50, good (51 to 100), lightly polluted (101 to 150), moderately polluted (151 to 200), heavily polluted (201 to 300), and severely polluted for AQI levels above 300.

According to the centre's official Weibo account, the index in south-east Beijing reached 141 by 8am on Sunday. North-west and north-east Beijing reached almost 130.

The index hit "very healthy level" of 208 at noon on Monday.

Mr Ma Jun, director of the Institute for Public and Environmental Affairs, said this is the first time in three months that Beijing has been hit by large-scale air pollution.

"The smog won't dissolve until Tuesday, and Beijing residents should use protection, such as masks and air purifiers," he was quoted as saying by China Daily.