Indoor Air vs Outdoor Air: Which is Dirtier?

In places with heavy air pollution, going outside can often be daunting. Countries likes China, India and Indonesia see outdoor air pollution levels over 10 times the World Health Organisation safe limit. But are indoor PM2.5 levels any better ? I’ve tested indoor and outdoor locations around Beijing to see …

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Is Air Pollution a Problem in Mongolia?

Air pollution or grasslands – which is more likely to come to mind when you think of Mongolia? Air purifiers and air pollution masks certainly aren’t top on the packing list when people are heading off to Mongolia, but should they? According to The Guardian, Ulan Bator is the world’s …

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Tackling New Frontiers of Air Pollution

Tackling New Frontiers of Air PollutionWatch this video on YouTubeSubscribe to Smart Air on YouTube   Today, I’m proud to announce a brand-new, revolutionary product for a problem that has been ignored–been swept under the rug for too long. Finally, a solution to a problem we all face.

What Time of Day Is PM2.5 the Lowest?

My intuition has always been that air pollution is lowest at night because there are fewer cars on the road and fewer factories humming. Apparently I’m not alone: 139 voted for their guess about what time of day has the lowest PM2.5, and night time came in first: I also …

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Is Guangzhou’s Air Safe?

Beijing’s air pollution is the most famous in China, but that can make people in some other cities think their air is good–at least, not as bad as Beijing’s. But is the air in other cities safe? In 2013, Shanghai had newsworthy air pollution that convinced many people in Shanghai …

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How Long Do Smart Air HEPAs Last? At Least 90 Days

The most frequent question people ask me these days is: how long does the HEPA last? This question is important because replacement HEPAs are the biggest long-term cost of clean air. IQ Air charges $370 for its filters. So if you have to replace the HEPA every two weeks, the DIY might …

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Chinese New Year fireworks pollution

Do Chinese New Year Fireworks Pollute Indoor Air?

I’ve posted data before showing that outdoor air quality is strongly correlated with indoor particle counts (r = .71), but Chinese New Year gives nerds like me a great chance to see what happens when we get a momentary shock to air quality. The media made a big deal about people cutting …

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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.

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PM2.5 Particle Size Virus

The Really Small Stuff

An open-data DIY air purifier test found that even simple, homemade purifiers can reduce particulate, but can they capture the smallest particles? This question is important because it’s the smallest particles that can penetrate deep into our lungs and even enter our bloodstream. To test how effective the DIY purifier …

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Smart Air low cost purifiers

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