HONG KONG • Severe smog has returned to northern China as heavier traffic returns to the urban areas and factories reopen following the Chinese New Year break.

China's Ministry of Environmental Protection said on Wednesday that the new bout of pollution will engulf the Beijing-Tianjin-Hebei region from yesterday until tomorrow.

The traditional practice of setting off firecrackers on the fifth day of the first month in the lunar calendar, which fell on Wednesday this year, made things worse, reported the China Daily.

Yesterday, the real-time air quality index provided by the US embassy in Beijing recorded the capital's level of PM2.5 particles - the fine pollutants most hazardous to human health - as 114 micrograms per cubic m at 11am.

It also reported that the level of PM2.5 in Tianjin had reached 177 micrograms, according to the South China Morning Post.

The World Health Organisation says exposure to PM2.5 should not exceed 25 micrograms over a 24-hour period.

The levels were higher than those recorded during the earlier part of this week, but much lower than the levels on the first day of Chinese New Year.

The Chinese capital was blanketed in smog last Saturday, with levels of PM2.5 peaking at 547 micrograms per cubic m, according to ministry data. The reading was well beyond the air quality index's upper limit of 500, and double the threshold considered hazardous.

Officials attributed the spike to the barrage of fireworks that residents set off overnight to ring in the Year of the Rooster.

PM2.5 levels in Tianjin and Hebei also exceeded the limit last Saturday. Air quality improved on Sunday as PM2.5 levels fell to moderate levels, reported the South China Morning Post.

The ministry said that many factories and industrial plants, especially large steel mills, near the capital remained in production during the holiday, contributing to the severe pollution over the weekend.