The smoggy weather that has choked many Chinese cities in recent days affected a total area of 1.3 million square kilometers, the Ministry of Environmental Protection (MEP) said Tuesday.

Dense fog enveloped Yantai, China's Shandong Province, Jan. 29, triggering flight cancellations. [Chinanews.com]

The MEP's monitoring on Tuesday morning showed that the hardest-hit regions included the cities of Beijing and Tianjin, the provinces of Hebei, Henan, Shandong, Shanxi and Jiangsu, central China cities of Hefei and Wuhan, and the southwestern city of Chengdu.

The cities of Beijing, Tianjin, Shijiazhuang and Jinan were "gravely polluted"; and air pollution of other cities like Zhengzhou, Wuhan, Xi'an, Hefei, Nanjing, Shenyang and Changchun were rated one level lower, as "seriously polluted", as of 10 a.m. Tuesday.

Beijing has seen its air quality worsen since Monday, with a significant thickening of the PM 2.5, or airborne particles measuring less than 2.5 micrometers in diameter. At 10 a.m. Tuesday, the average concentration of PM 2.5 was tested at 354 micrograms per cubic meter.

It was the fourth round of heavy smog to hit Beijing in four weeks.

The meteorological authorities forecast that the haze will thin out by rain and snow that will begin Wednesday night. However, smog will continue to linger in central and eastern regions of the country on Tuesday afternoon through Wednesday daytime.

Earlier this month, several consecutive days of smoggy weather choked Beijing and other cities in north and east China, as readings for PM 2.5 far exceeded safe levels.