People wear masks in Xi'an, Shaanxi province, on Tuesday, the fifth consecutive day of serious air pollution. Provided To China Daily

Air quality in China improved in the first 11 months of the year, although it deteriorated in November, the national environmental authority said on Tuesday.

In the first 11 months, 80.5 percent of the days had "good" air quality, a year-on-year increase of 2.6 percentage points, but the readings of PM2.5 and PM10 - particulate matter with diameters of 2.5 and 10 microns respectively - increased in November, said Luo Yi, head of the Ministry of Environmental Protection's department of environmental monitoring.

Air quality is considered "good" when the comprehensive Air Quality Index is 100 or below.

The AQI is based on the concentrations of six airborne pollutants - PM2.5 and PM10, ozone, sulfur dioxide, nitrogen dioxide and carbon monoxide.

Average readings of PM2.5 increased by 7 percent and those of PM10 by 20 percent last month due to heavy smog.

In November, the 13 cities in the Beijing-Tianjin-Hebei region had 36.9 percent of days with good air quality, a year-on-year decrease of 16.3 percentage points, while PM10 increased by 24.3 percentage points compared with the level last year, the ministry's monthly report on air quality among 74 major domestic cities said on Tuesday.

In the trilateral region, six cities were listed in the bottom 10 for severe air pollution last month. Shijiazhuang was the worst, followed by Baoding, Xingtai, Tangshan, Handan and Hengshui, it added.

Last month, three bouts of severe smog covered the Beijing-Tianjin-Hebei region and northeastern regions where the air pollution readings were off the charts, reaching alarmingly high levels.

Some northwestern cities, such as Lanzhou in Gansu province, were hit by sandstorms three times, which worsened the pollution, the statement said.

In the latest round of smog in northern regions from Saturday to Monday, emergency responses addressing smog were issued by 42 cities, among which Langfang in Hebei province and Linfen in Shanxi province issued the highest red alert.

The ministry has required authorities to punish polluting companies more strictly.

By Zheng Jinran