BEIJING • A miasma of hazardous smog blanketed Beijing and the rest of northern China for the fifth day, while the capital city lifted its highest pollution warning on expectation the smog will ease.

The smog continued to rise earlier yesterday, though a new forecast said it would start to improve later in the day. By 10am, the concentration of PM2.5 - the tiniest particles that pose the greatest health risks - was 327 micrograms per cubic metre at Tiananmen Square. The overall air quality index (AQI) was 404, indicating "severe" pollution.

The cancellation of the red alert meant traffic restrictions were lifted and schools reopened despite the lingering smog. That highlighted the challenge government officials face in both forecasting the pollution - the red alert is imposed when the air quality index is forecast to rise above 200 for three days - and cleaning it up without disrupting the lives of Beijing's 20 million residents.

The red alert will be lifted when the daily air quality index drops below the "heavy" pollution level, said Mr Dong Liansai, climate and energy campaigner at Greenpeace East Asia. The alert "is different from what the public feels every hour and this might need to be improved", said Mr Dong.

Air quality would improve from north to south between yesterday evening and tomorrow, the Ministry of Environmental Protection said in a statement on Monday.

Two other areas in northern China - Tianjin and Hebei - have issued red alerts for the first time even as the capital lifted its own warning.

The city of Tianjin issued its first red alert on Monday night. The alert, which goes into effect when the AQI rises above 500 for at least a day, was to last from yesterday to 6am today, said a government statement.

In Hebei, the cities of Baoding, Langfang, Handan and Xingtai were also under a red alert. "The warning system of pollution is just a temporary way to reduce smog," said Mr Dong. The nation should curb coal use, the biggest cause of smog, to tackle the issue in the long run, he said.

Shanghai also warned residents to stay indoors because of the smog yesterday. The air in China's financial centre was described as "heavily polluted", the second-worst on a six-grade scale, as of 10am, the Shanghai Environmental Monitoring Centre said on its website.

BLOOMBERG, CHINA DAILY/ ASIA NEWS NETWORK