In mid-December, smog in Beijing, China, was so thick that the government was forced to issue a "red alert" for the first time, closing schools and limiting use of motor vehicles.
Red alerts are issued when poor air quality is expected for three consecutive days.
Less than two weeks later, a second red alert was issued when smog particles were expected to reach 20 times the level that is considered safe by the World Health Organization.
Pollution in Beijing is a common, everyday issue that millions of people deal with in the bustling city. Beijing's industrial coal burning and high concentration of motor vehicles are significant factors in producing the poor air conditions.
However, other natural factors play a role in enhancing the potentially life-threatening conditions.
Take the city's geography, to start.
"Beijing is backed up right against tall mountains which tend to act as a barrier for any smog to clear out," AccuWeather Senior Meteorologist Jason Nicholls said.
The Xishan and Yanshan mountain ranges act as a natural barrier, stalling air flow.
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Weather conditions in the winter can also lead to enhanced smog levels. Strong high pressure systems lead to temperature inversions, when a dense layer of cold air is trapped under a layer of warm air.
"This traps pollutants near the surface," Nicholls said.
At the time of the record smog levels, a large area of high pressure had built over the region for several days, restricting air flow and allowing pollutants to build.
The thick smog was finally broken up by a front that delivered a fresh air mass to the city.
Valleys in Delhi, India, suffer from similar problems during the winter when air flow becomes stagnant, leading to weeks of low clouds and fog. Though not as severe, Los Angeles is subject to the same problems due to the city's pollution levels and placement.
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