For the first time ever, Tianjin has issued a red alert for air pollution. The alert will be in effect from Wednesday midnight (3 a.m. on Wednesday in Sydney) to 6 a.m. Thursday (9 a.m. Thursday in Sydney).

Forty cities in north China, including Tianjin and Beijing, have issued alerts for air pollution. A red alert is the most severe weather warning in China, which has a color-coded weather warning system. The most severe is red, followed by orange, yellow and blue. Other cities which have issued a red alert include Langfang, Baoding, Handan and Xingtai.

Beijing issued its first red alert on Dec. 7. Its second red alert is going to end in Tuesday.

Kindergarten, primary schools and middle schools will be closed Wednesday. In addition, public institutions and businesses will have flexible working hours. Construction work and large outdoor activities will be suspended, China Daily reports.

"Affected by increased humidity and temperature inversion, the density of PM2.5 (particles smaller than 2.5 micrometres used to measure air quality) may exceed 500 micrograms per cubic metre on Tuesday," China.org.cn quotes environmental expert Li Yunting from Beijing Municipal Environmental Monitoring Center.

According to BMEMC, Beijing was hit with severe air pollution Tuesday. The pollution level is expected to reach grade six, the highest grade in the pollution measuring system.

Anti-smog masks have become a popular choice for Christmas gifts this year in China, thanks to the high pollution level. Xinhua reports anti-smog masks among scarves and flowers are among the top searches for gifts.

"When I told her I will send her masks as a gift, she thought I was joking," said Shanghai resident Tomiya who bought a box of masks for her friend. "It is more of a demonstration of care for others than a holiday gift," said Zhang Min, the mother of a one-year-old.

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