China's capital city Beijing is notoriously polluted by smog, but today (Dec. 7) the city has issued its first-ever "red alert," meaning the air is so full of poison that people could get sick just from going outside, so schools and businesses are being shut down.

In some places, the smog is so thick that people can only see 200 meters (or about 656 feet) in front of them, according to a report from The Guardian. The same report notes that testing revealed "areas of Beijing had more than 256 micrograms per cubic metre of the poisonous particles. The World Health Organisation (WHO) says anything over 25 micrograms is considered unsafe."

According to The Guardian, "the poisonous smog in Beijing is caused by the burning of coal for industry and heating, and huge amounts of dust from the city’s many construction sites. The problem is being made yet worse by high humidity and low wind."

Here's what the city's iconic Tiananmen Square looks like, with and without/before and after being cloaked in high levels of smog:

The "red-alert" smog warning is expected to stay in place until midday (local Beijing time) on Thursday, when, according to The Guardian, "the weather is expected to change and blow away the smog."

Earlier this year, scientists warned that China's air pollution may also be changing weather patterns and causing more severe storms in the U.S.