Sometimes I hear people say air pollution is invisible.
But after years of living in Beijing and Delhi, I’ve been amazed at how well we can detect air pollution with our eyes.
But it’s a bit complicated. If you’re not thinking about it, it’s easy to overlook. I discovered that when I spent two years living in heavily polluted Chinese cities like Nanjing and Guangzhou without really noticing the pollution.
How did I play basketball outdoors in Beijing, get dizzy afterwards, and just think, “huh, that’s weird!”
How did I bike through Guangzhou’s hot and humid pollution in 2007 to my English tutoring job and not realize my difficulty breathing and constricted throat was from the pollution?
These days I wear masks and run purifiers and no longer suffer any symptoms. Now my eyes are finely tuned AQI detectors. Here’s what I’ve noticed about how polluted skies look:
1. On bad days, the night sky looks red.
Here’s a picture I took in Beijing on a bad day:
And here’s a nighttime Beijing sky on a good day—ahh, much more normal black sky.
2. Polluted sunlight looks a bit orange. Clean sunlight looks clear and white.
Here are two pictures I took at the same place in my apartment, same season, at roughly same time of day. But the day on the left was a bad day. The day on the right was a good day.
See how the sunlight on a bad day (left) looks more orange? Now in Beijing, I can tell if it’s a bad day just by looking at the color of the sunlight coming through my window.
3. Clouds look fuzzy
When pollution is bad, all those tiny particles scatter sunlight, making things appear fuzzier, less distinct. I founded to host air pollution workshops in China, India, Mongolia and the Philippines and raise awareness about air pollution. To show what air pollution looks like, our team used a timer to take pictures of Beijing’s air for several weeks. Check out the clouds on a good day in Beijing. See how the clouds are pretty distinct from the sky?
Now here’s a worse day (although this is even less than , so most people here would say this is a good day). The clouds here are starting to look fuzzier, less distinct.
Of course, then there are the really bad days.
When I fly back to the US, one of the things I marvel at is how distinct the clouds are. “Look at those crisp edges on that cloud!” I never thought that would be something I’d appreciate.
Bottom line: If you hone your eye, you’ll start seeing how even moderate levels of pollution change the sky around us.
Note: I’m not an expert in air pollution. I’m just a data nerd who got into and after coughing through a Beijing winter. If you’re living in a city with dangerous air, check out my and the specs on before big companies convince you clean air needs to cost $1,000.
Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.