## 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.

## Q&A: Do ACs Bring in More Dirty Air?

tmthyliu 提问: Just built a DIY air filter, pretty excited to have clean air in the house! I was wondering though, how does this work when the air conditioner is on? Does it make a difference at all? I don’t know how ACs work but it seems they pump more … Read more

## How Safe Is Indoor Air?

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.

### The Test

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.