Due to the weather, pollution was predicted to remain heavy in Beijing until Saturday, the Beijing heavy air pollution response office said today.

The Beijing Meteorological Observatory yesterday issued a yellow alert for smog, state-run Xinhua news agency reported.

Besides the commencement of winter, officials say the pollution is caused by ramped-up production by industrial units in and around the city which were opened yesterday after a week long National Day holiday.

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 this morning obtained 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 of PM2.5, particles under 2.5 microns in diameter and major air pollutant, at 8pm. was more than 300 micrograms per cubic meter, in Beijing.

"Serious smog has appeared in the Beijing, Tianjin and Hebei region," said 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 this evening, according to forecast.

The smog also prevented citizens from watching a total lunar eclipse yesterday. Beijing officials claim to have improved pollution law enforcement.

In the first eight months, there were more than 1,000 breaches of the law with fines totalling 23 million yuan (about USD 3.8 million) issued, up 99 per cent and 375 per cent respectively year-on-year.