Before-and-after photos of China's air show just how terrible its air pollution is

China’s air is notoriously polluted, full of grey smog that covers the sky. 

The health problems associated with that pollution are the reason for 1.6 million deaths a year, or about 4,000 people a day.

But for a couple of weeks, the sky was bright blue. 

In honour of a military parade to celebrate the 70th anniversary of the end of World War II, Beijing managed to clean up its air. They have done this before, notably in advance of the 2008 Summer Olympics.

Here’s how they pulled it off.

But in just one day, the sky made a drastic change back to its grey hue. On September 3, the air quality was at a healthy level of 17 on the air quality index (a measure of how many harmful pollutants, including PM 2.5 are in the air). One day later it jumped up to 158, which is considered an unhealthy level of air pollutants.

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