(CNN) - NASA astronaut Scott Kelly tweeted an amazing picture of the sky over a part of Asia on Thursday.
Kelly took the photo while on the International Space Station. He immediately noticed something different about the sky to the left of the picture.
Kelly said the haze is actually air pollution, and it exists across large parts of Asia.
Beijing recently announced its second-ever "red alert" for air pollution.
According to independent research group Berkeley Earth, air pollution is killing an average of 4,000 people each day in China.
The group said it's as if "every man, woman and child in China smoked one and a half cigarettes every hour."