29 eerie photos that show just how polluted China's air has become

Although no one knows the exact amount of carbon that China emits each year, it's a well-known fact that the country has some of the most polluted air in the world. This Wednesday, Beijing went under a "red alert" for smog - the highest of four tiers - and a "yellow alert" for fog.

In December, pollution so bad that some called it the "airpocalypse" caused a halt of everyday life in Beijing. Cars couldn't be driven, planes were grounded, and schools had to be temporarily closed.

But Beijing isn't the only city affected. The cold winter months often cause air pollution levels to rise throughout much of China, as more homes are burning coal for heat.

Below, see 29 photos that show China among the smog.

Jack Sommer and Cyrus Engineer contributed reporting to a previous version of this article.