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Due to the weather, pollution was predicted to remain heavy in Beijing until Saturday, said the Beijing heavy air pollution response office. Earlier on Wednesday, the Beijing Meteorological Observatory issued a yellow alert for smog.

It is the first yellow smog alert in Beijing since the beginning of July. Tianjin and Hebei also issued a yellow alert.

Citizens, especially elders and children, were advised to take protective measures. Schools were asked to avoid outdoor activities and reduce physical exercise classes.

Air quality index (AQI) in downtown Beijing exceeded 200 for the past 24 hours, according to data updated at 8 p.m. by the Beijing Municipal Environmental Monitoring Center. An AQI of over 300 is defined as "serious pollution" and an AQI between 201 and 300 is considered "heavy", according to China's standard.

Downtown density 密度 of PM2.5, particles under 2.5 microns in diameter and major air pollutant, at 8 p.m. was more than 300 micrograms per cubic meter., in Beijing.

"Serious smog has appeared in the Beijing, Tianjin and Hebei region," Li Jing, a forecaster at the Beijing Meteorological Bureau. Heavy pollution was reported in Hebei and Henan provinces on Wednesday. Cold air is forecast to blow away the smog on Saturday night, said Li.

Tianjin and Hebei will continue to have heavy pollution until Thursday evening, according to forecasts. The smog also prevented citizens from watching a total lunar eclipse on Wednesday.

Beijing has improved pollution law enforcement and in the first eight months, there were more than 1,000 breaches of the law with fines totalling 23 million yuan issued, up 99 percent and 375 percent respectively year-on-year.