China air pollution
Vehicles travel on the Guomao bridge (bottom L) as the Central Business District (CBD) area is seen amid heavy smog after the city issued its first ever "red alert" for air pollution, in Beijing, China, December 8, 2015. 40 cities in China have issued alerts for air pollution.Reuters

As many 40 cities across China have issued alerts for air pollution, with Beijing and four others issuing red alerts, the most severe on a four-tier warning system warning for air quality reaching hazardous levels. 

All the cities are located in smog-hit northern China, where industrial emissions and coal burning are the biggest pollution-causing factors. It is not clear if all 40 cities issued alerts on the same day. 

Of these, 13 cities have issued orange alerts, 17 issued yellow alerts and five issued the least severe blue alerts on the colour-coded scale, Xinhua reported citing the air pollution emergency management headquarters of Beijing. 

Beijing, which issued a red alert for the first time ever this month, Baoding, Handan, Langfang and Xingtai issued red alerts, with PM2.5 density reaching 330 micrograms per cubic metre in the capital till 2 pm local time on Tuesday.

PM2.5 refers to pollution causing fine particles less than 2.5 microns (0.0025mm) in diameter which are extremely dangerous to health as they can cause respiratory troubles. According to the World Health Organisation, 25 micrograms per cubic metre is the safety benchmark. 

Tianjin was among the cities to issue an orange alert, but it will scale up the alert to red on Wednesday, which will force closure of schools and only allow odd or even-numbered cars on road.

The thick smog is likely to stay on for a few more days in northern China, as per reports. 

China, the biggest polluter in the world, said it plans to cut emissions form coal-based power plants by half in the next five years. 

The Asian Development Bank is set to give the country a $300m loan to rein in pollution and improve air quality.