BEIJING, Feb. 4  -- China's Ministry of Environmental Protection (MEP) on Thursday said overall air quality had improved last year, even though only 22 percent of cities monitored met national clear air standards.

According to Luo Yi, MEP head of environmental monitoring, 338 cities were monitored in 2015. Among them, only 73 cities -- mostly in Fujian, Guangdong, Guizhou, Tibet and Yunnan -- had met national standards.

Luo said there were also "signs of improvement."

Density on PM2.5, or particles measuring less than 2.5 microns in diameter, dropped 14.1 percent year on year in 74 key cities in 2015 and readings of PM10 fell 11.4 percent.

Of the 74 cities, Haikou, Huizhou, Lhasa, Xiamen and Zhoushan registered the best air quality, while Baoding, Hengshui, Tangshan and Xingtai -- all in Hebei Province -- along with Zhengzhou in Henan Province were the most polluted.

Nationwide, average PM10, SO2 and NO2 readings dropped by 7.4, 16.1 and 6.3 percent respectively in 2015.