Chinese authorities in Beijing declared on Sunday the "orange level" alert as the air pollution in the city has reached its highest level in 13 months. Residents in Beijing and neighbouring cities have been urged to stay indoors, especially the elderly and children.

The "orange level" alert is the second highest of possible warnings, and requiring factories to cut or shut down  production while large trucks are banned from the road. The visibility in some places reduced to a few hundred metres.

The heavy smog had developed on Friday and would continue until at least Tuesday across the capital and heavily populated areas including the city of Tianjin, Hebei province, the western part of Shanxi province and the north of Henan province, said the Ministry of Environmental Protection of China, as reported on South China Morning Post.

The heavy smog including high levels of PM2.5, the finest pollutant particles smaller than 2.5 micron in diameter that can penetrate deeply into the lungs and cause the greatest harm to public health.

According to The Guardian, the US embassy in Beijing reported the level of PM2.5 in the city reached 391 micrograms per cubic metre on Saturday. The level had raised to 464 micrograms per cubic metre on Saturday afternoon.

The PM2.5 level in Beijing reached about 18 times the level considered safe by the World Health Organisation. WHO considers the safe level of PM2.5 to be 25 micrograms per cubic metre.

While China's AQI standard mentioned the PM2.5 levels between 201 and 300 micrograms per cubic metre as "heavy pollution", and more than 300 as "serious pollution".

Chinese authorities blame coal-powered industries and heating systems as a major culprit for the heavy smog. The dust from construction sites also contribute to the smog which has been increased by humidity and a lack of wind.

Air pollution has been a perennial problem in China's northeast as there are many heavy industries including coal mining, according to BBC. Earlier this month, the PM2.5 level in Shenyang city in China's northeast reached 1,400 micrograms per cubic metre.

Chinese Ministry of Environmental Protection had sent groups of inspectors to Beijing and other cities to check on authorities' efforts in reducing air pollution. The inspectors will check on the discharge of air pollutants by local industries and dust control measures at construction sites as well as the treatment of coal burning pollution.

The local authorities urged to prepare for reducing the serious air pollution levels. The ministry expects the strong wind from the north would blow away air pollutants by Tuesday.