A total of 4,000 tons of pollutants are estimated to be in the air in urban areas of Beijing on serious pollution days, according to a researcher with the Chinese Academy of Sciences quoted by Beijing Morning Post.

The calculation was made by Wang Yuesi, and based on the size of Beijing's urban area and concentration of pollutants, including PM2.5, or particulate matters less than 2.5 microns in diameter.

Many parts of central and eastern China witnessed more than 20 days' of hazy weather in January, according to National Meteorological Center, the worst since 1961.

Beijing and its neighboring areas were the hardest hit, with only five days free of smog weather in Beijing in January, according to the Beijing Meteorological Bureau. Concentration of PM 2.5 was as high as 1,000 micrograms in parts of Beijing on serious pollution days in January.

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