Smog rolls into Beijing in this incredible time-lapse video of China's capital.
The city appears clean with blue skies over the high-rise buildings at the start of the "Airpocalypse" footage.
But within 20 minutes, a thick cloud of dark air appears to sweep across the city like a tidal wave.
British expat Chas Pope shared the 12-second video on Twitter - but also mentioned it was the first time he had witnessed this in 13 years.
China issued a five-day pollution “red alert” in Beijing in December, forcing the closure of schools and ordering vehicles off the roads.
Red alerts are issued when the Air Quality Index (AQI) is forecast to exceed 200 for more than four days in succession, 300 for more than two days or 500 for at least 24 hours.
The country's government declared a "war on pollution" in 2014 but promised to continue its 7.5% growth in industry, much of it coal-based.