Beijing issued a blue alert for air pollution, the lowest emergency response, on Thursday afternoon amid the lingering smog that has covered the capital since Wednesday.

Pollution is expected to peak on Friday at the "severe" level, the second-worst, and then ease on Saturday, when rain is forecast, the city's Environmental Protection Bureau said.

Since September, residents have seen several days with heavy smog. A yellow alert was triggered on Oct 2, a notch up from blue. Major air pollutant readings soared last month, according to data from the Ministry of Environmental Protection on Thursday.