TIANJIN - Roughly 50 cities in northern and eastern China have issued air pollution alerts in the most recent bout of smog.

Eastern Shandong Province issued its first red alert in four cities, after warning that the density of PM2.5 would exceed 400 micrograms per cubic meter for more than 24 hours.

China has a four-tier warning system, with red as the most severe, followed by orange, yellow and blue.

The alert takes effect at 8 a.m. on Thursday, limiting vehicles on the roads while banning fireworks and outdoor barbecues. All construction sites will be closed. People are have been told to reduce outdoor activity and schools are expected to suspend classes.

A blue alert covering the whole province of Anhui, where five cities have had air quality index (AQI) readings over 200 since Tuesday, has also been issued. Another 36 cities and counties in the province have issued their own yellow alerts. The smog is forecast to linger for another two days.

Cities in central China's Hubei Province are also affected by the polluted air mass spreading from the north with the pollution not expected to disperse before the weekend, the provincial weather bureau said.

Hebei, home to six of China's ten "most polluted" cities in November, issued its red alert on Tuesday at noon. Tianjin saw its first red alert the same day, just as Beijing lifted its. The Tianjin alert will last until 6:00 a.m.Thursday.

The Beijing-Tianjin-Hebei region has been through more than 15 days of heavy pollution. Tianjin has choked for more than 25 days.

On Wednesday, the Ministry of Environmental Protection gave a preliminary assessment of the effects of recent emergency measures. According to the ministry, Beijing cut pollutant emissions by 30 percent during its second red alert and neighboring cities emitted 25 percent less compared with ordinary days.