Beijing has issued its first ever red alert, the highest of its four-colour warning system for air pollution. As of 3pm (7am GMT) on 8 December, the readings of PM 2.5 (particulate matter with a diameter of 2.5 micrometers) soared above 300. Beijing's environmental protection bureau defines heavy pollution as anything above 200. At levels higher than 300, residents are encouraged to remain indoors. A grey soupy haze shrouded Beijing's landmarks. Factory suspensions and several other restrictions will seek to reduce the dust and other particulate matter in the city of 22.5m people.

Beijing smog
Beijing smog
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Tiananmen Square is seen in heavy pollution on 1 December 2015, and under a clear sky on 2 December 2015
Beijing smog
Beijing smog
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The Forbidden City is seen in heavy pollution on 1 December 2015, and under a clear sky on 2 December 2015
Beijing smog
A building and a large screen are seen through thick smog in BeijingDamir Sagolj/Reuters
Beijing smog
The China Central Television building and the Central Business District area are seen amid heavy smog after the city issued its first ever red alert for air pollutionReuters
Beijing smog
Vehicles travel on the Guomao bridge in the Central Business District of BeijingReuters
Beijing smog
Residential buildings are seen on a hazy day in BeijingJason Lee/Reuters
Beijing smog
A bird flies over the grounds of the Temple of Heaven amid heavy air pollution in BeijingWang Zhao/AFP
China: Beijing issues first ever red smog alertIBTimes UK

Most of the smog is blamed on coal-fired power stations, along with vehicle emissions, construction and factory work. The Beijing city government has ordered all outdoor construction work to stop on red alert days, and urged schools to close. The notice, issued after days of heavy smog last week, also puts traffic restrictions on certain types of vehicles. Use of a car is restricted to every other day depending on its licence plate, and officials planned to deploy extra subway trains and buses to compensate. Some businesses closed and others said employees could work from home.

Beijing smog
Primary school students exercise inside a classroom as outdoor activities are banned due to heavy smogReuters
Beijing smog
A primary school student wearing a mask walks along a street after his school was closed due to the first ever pollution red alertJason Lee/Reuters
Beijing smog
Vehicles travel on a bridge amid heavy smog in BeijingReuters
Beijing smog
A man walks along an overpass during the evening rush hour amid the heavy smog in BeijingJason Lee/Reuters
Beijing smog
A man walks along an overpass amid heavy air pollution in BeijingWang Zhao/AFP
Beijing smog
Morning commuters wait in line at the Tiantongyuan subway station on a smoggy dayReuters
Beijing smog
A Chinese-produced electric car is driven a street on a heavily polluted day in BeijingChai Hin/AFP

Face masks have become a routine cold-weather accessory, along with hats and gloves, with many making a fashion statement while filtering out the pollution. Masks with patterns, textures, logos, messages and cutesy animals bring some colour to the grey haze. Air purifying machines have become an increasingly common household necessity in a city that even on normal days is one of the world's most polluted.

Beijing smog
A women wearing a mask to protect from extreme smog visits the Tiananmen GateDamir Sagolj/Reuters
Beijing smog
Women wearing masks protecting them from smog take pictures of themselves at the Tiananmen Gate in BeijingDamir Sagolj/Reuters
Beijing smog
People wearing masks perform during their daily exercise amid the heavy smog in BeijingJason Lee/Reuters
Beijing smog
A man wearing a mask uses his mobile phone amid heavy smogKim Kyung-Hoon/Reuters
Beijing smog
The Forbidden City is seen in heavy pollution on 1 December 2015 and under clear skies a day laterKevin Frayer/Getty Images

Although Beijing has seen smog at much worse levels, the latest bout of pollution was the first to trigger a red alert under a two-year-old system that requires a forecast of at least 72 hours of consecutive high pollution. The looming smog underscores the challenge facing the government as it battles pollution caused by the coal-burning power industry, and will raise questions at the Paris talks about its ability to clean up its economy and environment.