Pedestrians cross the road in Zhongguancun, Beijing, during a smoggy day in December.[Photo/China Daily]

BEIJING - Beijing is to raise the thresholds for issuing its highest air pollution warnings, the state news agency Xinhua reported on Sunday, two months after smog triggered the city's first ever "red alerts".

In future, the highest alert will only be issued when the daily average air quality index (AQI) is forecast to exceed 500 for a day, 300 for two days in a row or 200 for four days, Beijing's environmental protection bureau said.

At present, a red alert is issued when the AQI is forecast to exceed 200 for at least three days.

Schools were closed and outdoor construction work was suspended when thick smog prompted Beijing to issued its first red alert last December. Beijing has issued two red alerts for smog since the start of this winter.

The new criteria take effect at the end of March and are designed to standardise pollution alerts across Beijing, the neighbouring port city of Tianjin and four cities in the surrounding Hebei province, i.e. Tangshan, Baoding, Langfang and Cangzhou. It would then expand to a larger area.

Liu Wei, deputy head of the emergency response office at the environmental protection bureau, said this would in effect tighten the pollution controls for the outlying regions.

At present?in Tianjin and Hebei, a red alert is triggered only when AQI exceeds 500 for at least one day.?Tianjin and Hebei also issued their first air pollution red alerts in December.

Environmental officials admit that China is unlikely to meet the air quality standards set by the state until at least 2030.

Beijing last month announced it would close 2,500 small highly-polluting firms this year. It also plans to develop a network of ventilation "corridors" to help disperse smog.