Watch 'Airpocalypse' Smog Overtake Beijing in This Video

Time-lapse footage shows smog moving in and obscuring the city's skyline.

British engineer Chas Pope decided on Monday to video a thick wave of smog he saw rolling into China's capital.

The clip, shot over 20 minutes, condenses into a time-lapse of just 20 seconds and captures smog blocking out sunlight and obscuring buildings and traffic-clogged roads.

Beijing on Tuesday issued the year's first red alert, the most dangerous on the four-color scale (red, orange, yellow and blue). China's National Meteorological Center warned that visibility in certain parts of Beijing could be less than 50 meters.

Pope wrote on Twitter at the time the video was captured that the Real-time Air Quality Index was over 400 - a measure of raw pollutants considered "hazardous" rather than just "very unhealthy."

Recent studies have shown that Beijing's notorious air pollution problems significantly reduce lung function in its 22 million residents, and surprisingly cause metabolic changes that can lead to obesity.