I recently did some research on whether indoor air is as bad as outdoor air. Before that, I had a conversation with a friend in Beijing that went something like this:
Friend: I’m not sure if I can make badminton tomorrow. I have a basketball game in the day.
Me: Oh man, do you play outside?
Friend: Nah, it’s inside.
Me: Oh, phew. Good.
Friend: Wait, why do you say that?
Me: Oh, the air is way worse outside. I used to feel like I had asthma after playing basketball outside.
Friend: Really? No, they’re not that much different. I saw it’s just 20% different.
Seeing as how nerds cannot let matters of fact go, I started using my particle counter to take measurements of inside and outside air at different locations around Beijing. This answer is important: it tells you if it’s any safer to exercise indoors and how much damage you’re doing to your lungs by choosing that seat outdoors at your favorite cafe or restaurant.
So I took measurements in six locations around Beijing, in apartments, cafes, and my gym. I only chose bad days (pollution concentration above the WHO standard of 25), and I avoided days where it rained (because rain can cause quick changes in air quality). Here’s what I found:
On average, indoor air had only 36% of the pollution outdoors.
Things were a little worse for the smaller .5 micron particles, but still much better than outside:
On average, indoor air had only 51% of the .5 micron particulates of outside air. My guess is that the .5 micron data was worse than 2.5 micron data because it’s easier for smaller particles to get into your home and stay suspended in the air.
There is a lot of variation between places. For the 2.5 micron particles, the locations varied from 14% to 58%. Dr. Saint Cyr also found significant variation between two apartments he lived in, 50% to 70%.
Conclusion: Indoor air is relatively safer
In terms of particulate pollution, you’re safer snagging an indoor seat and working out indoors, particularly on bad days (I’ve seen some argue that we are particularly vulnerable when we work out because we breathe more deeply than normal).
But remember that doesn’t mean indoor air is safe, just better than outside. For example, if your air at home had 40% of Beijing’s concentration last night at 11pm (8/15), you would’ve had 64 g/m3 in your home, which is more than twice the WHO standard of 25.
As usual, I’m posting more on my methods and raw data below.