A traffic policeman is seen on duty wearing a mask on the Chang'an street in Beijing, capital of China, Nov. 30, 2015. (Xinhua/Luo Xiaoguang)

Environmental watchdogs strengthened inspections to reduce pollutant emissions as the air quality deteriorated on Monday to the most severe readings in the country's northern regions, including the worst this year in Beijing.

Beijing required over 2,100 major companies in polluting industries to suspend their production. It also ordered all construction sites to stop work as well to cut emissions, after issuing an orange alert, the second-highest response to air pollution, on Sunday.

The Ministry of Environmental Protection has sent several inspection teams to supervise the implementation, saying on Monday that at least 11 construction sites and cement-making plants continued to work despite the restriction.

For example, the construction site for a government subsidized housing project continued its dust-generating dirt and rock work, the ministry said.

Ignoring the suspension order can result in punishment including hefty fines. The inspection teams will strengthen their monitoring on Tuesday, to further cut emissions to control the deterioration of air quality.

The air quality index in many cities, including Beijing and Langfang and Bao-ding in Hebei province, reached the top reading of 500 on Monday, meaning air pollution had reached the most severe level, according to the cities' environmental monitoring stations.

The concentration of PM2.5－particulate matter with diameter of less than 2.5 microns that can penetrate the lungs and harm health reached 945 micrograms per cubic meter in some monitoring stations in southern Beijing as of 6 pm, according to the capital's environmental monitoring center. The readings far exceeded the national standard of 75 micrograms per cubic meter.

Liu Xin, who is from Chongqing and went to Beijing for business on Saturday, canceled going out for dinner with friends due to the severe smog, saying she would rather be safe and stay indoors.

Heavy smog has persisted in the northern region since Friday and might continue until Wednesday, the ministry said.

By Wednesday, strong cold air moving from west to east is forecast to blow away the air pollutants that have lingered in the north since Friday, said Fang Chong, chief weather forecaster of the Central Meteorological Station.