The Chinese government issued the second-ever "red alert" for deadly air pollution in the capital city of Beijing on Friday, putting the city's 22.5 million people on notice that the air contains an unhealthy amount of pollution. The alert triggered sharp restrictions on vehicle traffic, with half of the city's cars forced off the road each day, and forced authorities to close city schools.

The alert is the second red alert in one week. Prior to December 8, no red alert had ever been issued for astronomically high levels of pollutants that can penetrate human lungs and cause heart attacks, lung disease and respiratory distress, among other deadly ailments.

The smog, caused by excessive pollutants from surging amounts of vehicles in and around Beijing as well as coal-burning power plants and polluting industrial facilities, combined with a stagnant airmass that is trapping pollutants in place, is expected to smother the Beijing region through Tuesday. The smog is also affecting surrounding areas.

The level of PM2.5, which are the smallest and deadliest airborne particles, are expected to top 500, which is more than 20 times the safe level, according to the World Health Organization (WHO).

Beijing smog on December 18, as seen by a NASA satellite. The milky, hazy appearance shows areas with poor air quality. Image: NASA

Visibility in some parts of Beijing is expected to fall to less than 500 meters (1,600 feet) on Tuesday, when the smog may peak, aided by calm winds.

Smog red alerts are triggered when PM2.5 levels exceed 300, and are forecast to remain at or above that level for more than 72 hours. The current warning system has four tiers and was launched two years ago, but no red alerts had been issued until last week, when the pollution was mentioned by the Chinese delegation to the U.N. Climate Summit in Paris.

China is the world's top emitter of greenhouse gases, with coal-fired power plants as the main culprit for both problems. The smog in Beijing, which also affects a broad swath of the country outside the city, is increasingly seen by the Chinese government as a threat to its very stability, since it is endangering the lives of tens of millions in Beijing and other cities and is a visible manifestation of the government's management of rapid economic growth.

It has been estimated that anywhere from 350,000 to more than 1 million people die prematurely each year in China due to air pollution.

Xie Zhenhua, the lead Chinese climate negotiator, told reporters in Paris on December 8 that the smog is a product of industrialization, and is a key impetus for China's efforts to shift away from fossil fuels.

"It’s fair to say to solve environmental issue and work to address climate change is connected, as our President Xi pointed out, to address climate change and protect the environment is not something imposed by outsiders but is something we need to do ourselves," Xie said through a translator.

Xie compared Beijing's smog issues with previous problems that plagued cities like London and Los Angeles. “It’s normal that industrial countries who is undergoing industrialization will be suffering from pollution problems," he said.

“That’s why we are stressing the importance of low-carbon… the transition to a green climate economy.”

China has pledged to peak it global warming emissions by the year 2030, and to dramatically scale up its use of clean, renewable energy sources like solar and wind power, in a bid to do its part in the fight against global warming while also tackling its air pollution woes.

Additional reporting by the Associated Press