# How Much Does Outside Air Pollution Affect Indoor Air?

I’ve always wondered: how much does outdoor air pollution affect the air in my home? Even if I keep my windows closed, if the air outside gets really polluted, does air in my home get a lot worse?

To answer that question, my Smart Air collaborator Anna has been taking particle counts in her Beijing apartment everyday for the last two months and comparing the counts to outside pollution. Here’s how she does it:

When she gets home from work (and before she turns on her DIY filter), she uses a particle counter to measure how much particulate matter is in her room and compares it to outside air pollution from the US Embassy Twitter feed. Her windows are always closed, and her home is near Chaoyangmen, which is in central Beijing.

Here’s the data from 41 days:

Inside and outside pollution correlate at = .71, which is quite strong. Compare that to the = .89 correlation between air outside my home and the US Embassy numbers.

With this data, we can actually start to predict how polluted the air is inside based on how polluted it is outside. To do that, I removed a few outlying datapoints and plotted a regression line:

So when does indoor air pollution get dangerous?

To answer that question, we first have to define “dangerous.” I use the WHO limit of concentration of 25 micrograms per meter cubed.

Then we need to convert the 0.5 micron measurements from my Dylos machine into official concentration numbers. That’s tough, but we can get a rough estimate based on my prior tests and from a formula Dylos published to convert to micrograms (0.5 microns – 2.5 microns)/100. My data shows that the WHO limit of 25 micrograms is equivalent to about 4,000 on the Dylos; according to the Dylos formula, that’s around 2,500. Using either standard, the graph above shows that the air in Anna’s home is frequently over the WHO pollution limit.

If we round those numbers up to a very conservative 5,000, we can reach a rough conclusion:

Rough rule: If outside concentration is above 40 (AQI 112), the indoor counts are very likely to be above the 5,000 limit.

Conclusion: Outdoor air pollution strongly affects indoor air pollution, and indoor air is often more polluted than the WHO limit–even with the windows closed.

Now, remember this data is from one apartment in Beijing. The numbers probably vary a lot between apartments that have better or worse seals around the windows. The numbers probably also vary between downtown and farther out in the suburbs. That said, this data convinces me that indoor air in China frequently has dangerous levels of particulate.

As always, I’m posting the data for fellow nerds below.

Here’s the raw data:

I’ve highlighted the outlying datapoints in orange. “Concentration” and “AQI” are measurements of outside air from the US Embassy.

All tests were in Anna’s bedroom, which is 15 meters squared. Her apartment is near Chaoyangmen, central Beijing. The outside AQI and concentration come from the US Embassy Twitter feed from the closest hourly timepoint to when Anna took the measurement of indoor air. The US Embassy measures air pollution from their location near Liangmaqiao, in northeastern Beijing.

The windows were always closed, so this represents the effect of outside air on inside air without opening the windows. Anna’s apartment is not particularly modern, so I’m guessing the seal around the windows isn’t great. Numbers might be lower if we tested in a more modern apartment.