I wear a pollution mask in Beijing, and I’ve always taken it off when I get into the subway. That’s because I’ve always assumed air in the subway is better than outside. But am I right to take it off?
I’ve heard a lot of Beijingers debating whether air in the subway is any better than outside, so I asked Smart Air member Ted to go out and get some real data. Ted took out a portable laser particle counter:
And he took air samples inside and outside of 10 subway stops around the city on two December days:
Here’s what we found comparing particulate outside (blue) to the subway platform (red):
The particle counts varied a lot but, on average, platform air was worse than outdoor air:
That surprised me! I was wrong to take off my mask in the subway.
Inside the Subway Car
Next, we compared air inside the subway car versus outdoor air. Maybe the air is better there because it’s a closed space and they often run the air system?
Again, it varied a lot, but air was worse in the subway car than outside on average:
Conclusion 1: If I need to wear a mask outside, I need to wear it in the subway too.
Conclusion 2: On average, PM2.5 in the subway is at least as bad as outside, sometimes much worse.
Is This A Reliable Finding?
This data is only from two days, so are these findings reliable? A study from the Rock Environment and Energy Institute ran tests in Beijing and also found higher levels of particulate on subways. The findings from these two studies suggest that air is reliably worse in Beijing subways.
So remember to wear your mask on the subway too!
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.