Beijing suffered its worst air pollution of the year on Monday, with monitoring sites throughout the city reporting extremely hazardous levels of pollutants.

City authorities issued a rare orange alert, the second highest of four danger levels. Schools suspended outdoor activities and polluting factories were required to reduce production.

Visibility was cut to several hundred metres, as buildings receded into thick smog. People complained of a smoky, pungent odour, and many wore tight-fitting face masks.

"I felt like my lungs were blocked," said Xu Pengfei, a security guard at an office building in downtown Beijing. "We have to stand in the open for many hours a day, and the pollution really affects us."

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A woman uses a cloth to cover her face from pollutants Monday after Beijing issued its highest smog alert of the year. (Andy Wong/Associated Press)

The city said the levels of hazardous tiny PM2.5 particles in the air exceeded 600 micrograms per cubic meter at several monitoring sites late Monday afternoon. The U.S. Embassy in Beijing reported 666 micrograms per cubic metre at 8 p.m.

Outside Beijing, the readings were was as high as 976 micrograms in the suburban region of Liulihe.

Several cities in the northern province of Hebei also reported extremely polluted air.

The World Health Organization considers the safe level of PM2.5 particles to be 25 micrograms per cubic meter on a 24-hour average basis.

Face masks don't protect

Liu Juntang, a businessman whose company develops environmental technology, said face masks could not give protection from the tiny pollutants on days of extremely heavy pollution.

"The air pollution is all-encompassing, and it requires both the government and individuals to shoulder the responsibility to clean up the air," Liu said.

Beijing has vowed to clean up its notoriously foul air and had been doing fairly well this year, with generally cleaner air than in 2014.

But the city was hit by unusually cold days and early snow in November, and winter heating was turned on. For the entire month, the capital saw hardly any blue skies and was shrouded in persistent smog.

China Pollution

Buildings are shrouded by heavily polluted haze in Beijing, where authorities issued a rare orange alert, the second highest of four danger levels. (Andy Wong/Associated Press)

The air quality worsened on Friday and continued to deteriorate throughout the weekend, prompting the city government to issue an orange alert on Sunday. Authorities said they didn't issue the highest red alert because they were expecting the air quality to improve by Wednesday.

The last time Beijing issued an orange alert was in February 2014.

Critics say the authorities have stayed away from a red alert because measures that must be activated by the highest warning are too disruptive, such as pulling half of the vehicles off of the roads and suspending schools.