indoor vs outdoor

Shanghai Test: Is Indoor air better than outdoor air?

When I got my very first particle counter, I tested sites around Beijing to see whether indoor air was better than outdoor air. It was.

But that test had some limitations. My first particle counter didn’t have a battery, so I had to estimate outdoor particulate in some locations. I also didn’t look into any variables that could give some indoor locations better air than others.

Shanghai Test

Now I’ve got a fancy new Dylos DC1700 battery-power particle counter! Now I can easily take measurements indoors and outdoors. Here’s what it looks like:


I took it to Shanghai and tested 14 times in 11 locations on August 27-29. While I was there, the AQI averaged 158 (70 micrograms). I tested mostly around the French Concession, although I also made it out to Fudan University. None of the locations use air purifiers.


I tested in any type of place I could, and I mean any place. Here’s what I recorded in a public bathroom stall:



I focused on the smaller 0.5 micron particles (which are highly correlated with government PM 2.5 readings). Across the 14 samples, here’s what I found. The red line represents how many particles were in outdoor air.

On average, indoor air had just 69% of the particles of outdoor air.

Why do some places have cleaner air?

This fits with my findings in Beijing–indoor air has less particulate than outdoor air. But next I wanted to see if I could figure out why some places had cleaner air than others. For example, my unpurified apartment air (unpurified because I just got home) had just 30% of particles compared to outdoors, but the public bathroom had 134% of particles compared to outdoors. Woah!

I tested the simplest explanation possible: Were the windows and doors open? Most particulate pollution comes from outside. And in closed rooms, particles will slowly fall to the ground. Thus, indoor air should be better when the windows are doors are closed.

To test that idea, I looked only at places that had doors or windows open:


Yikes! If the doors or windows are open, I was breathing air that was basically as bad as outdoor air (92%). For example, here was how air compared indoors and outdoors in the public bathroom:


But things were much better in places with the doors and windows closed:l

With the doors and windows closed, the air had 57% of the particles of outdoors. For example, here’s the air inside Fudan University’s Economics Institute versus outdoors:z

In places where the doors and windows were closed, the air was always better than outside. The one exception was the Yuanyuan Restaurant, at 115%. Those particles could be coming from the kitchen.

So what?

The conclusion here is simple: We’re usually breathing much less particulate indoors than outdoors–even without a purifier.

Clean Lung Tips

  1. Work out indoors if possible. I work out indoors in a gym rather than outdoors when I’m in China.
  2. Sit inside at cafes or bars (unless people are smoking).

Does that mean indoor air is safe?

Indoor air has less particulate, but remember that “less particulate” does not always mean “safe.” Out of all 14 tests, NONE of the numbers was below the WHO 24-hour PM 2.5 limit:


One Last Exception

Finally, remember that pollutants can sometimes come from indoors. This is usually from the paints and chemicals used in remodeling and new furniture. If your home smells like paint or new furniture, you may be safer opening the windows (or at least using activated carbon).


Does Chinese New Year Affect Inside 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 back on fireworks this year out of a concern for air quality, and that may be true, but you can still see a strong spike in PM 2.5 as Beijingers rang in the year of the horse:


Not all that surprising. But what’s more interesting is that you can see a corresponding increase in the particle counts in my collaborator Gus’s bedroom (blue line):


These indoor counts are without a purifier running, so they demonstrate how quickly outdoor air pollution can find its way indoors and how variable indoor air quality can be in a single room over time. Simply put: the worse the air is outside, the worse it is inside.

Yet the glass half empty can also be half full. When outdoor particulate goes down, indoor particulate can go down in a hurry. Check out what happened after the firework apocalypse ended:


A note for fellow nerds: The indoor particle counts are not precisely on the hours, so the apparent time lag between indoor and outdoor counts may be exaggerated.


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.



Smoking is Worse than Outdoor Air Pollution

A while back, I posted data I collected from places around Beijing showing that indoor air is consistently cleaner than outdoor air. When I analyzed that data, I excluded places that allow smoking, but I’m posting the data here now:

图片 1

Even on days where outside air was bad (AQI ~ 180), the air was even worse in the cafes that allow smoking. This is even more surprising because:

  1. I was not sitting in the smoking section.
  2. The smoke was not very noticeable. (I hate smoking, but the air seemed good enough to me that I sat there and worked.)

My guess is things are much worse in smoky bars and clubs, where the smoke is so thick my clothes smell like smoke the next day.

Conclusion: Indoor air is better than outdoor air in China, but you lose any advantage once people start smoking–even if you’re in the non-smoking section.


Does Air Conditioning Bring in Dirty Outside Air?

A question that I get asked often (and that I have always wondered about) is whether my wall-mounted air conditioner is bringing in dirty air from outside. If so, it’d be safer not to use it, especially on really bad days.

My short answer is no. To explain why, I’ve got three points of evidence:

  1. How air conditioners work. Regular wall-mounted air conditioners in China do have a unit outside connected with tubes to the inside, but that tube is not bringing in outside air. It’s passing coolant, and letting heat escape outside.

So where does the air it’s blowing come from? If you look around your air conditioner, you’ll probably discover that it works like mine: it brings air from the top, runs it over the cooling coils, and blows it out the front. It’s recycling indoor air, not bringing in outdoor air.

  1. Tests of the air coming out of the air conditioner. (See a live test here.) I’ve held my particle counter up into the air coming out of my AC unit, and it’s no different from the ambient room air. I’ve also compared that air to outside air on very dirty days, and the air coming out of the AC is nowhere near as dirty as outside air.

(I did this test when I had just turned on my AC. If the AC were bringing in dirty air and I were to test the exhaust after I had been running the AC for a long time, then my whole room would be dirty, not just the exhaust.)

  1. Tests of the ambient room air before and after turning the AC on. Results? AC makes basically no difference.

Here’s what happened in one test after turning the AC on:


And here is the average effect over 7 different tests in my bedroom.


In each test, I ran my particle counter for 30 minutes to get a baseline. Then I turned on the AC for 30 minutes. Here I’m comparing the numbers just before I turned the AC on and 30 minutes later. As you can see, there’s basically no effect. If anything, PM 2.5 goes down slightly. This could be because of the coarse filter in the AC unit. Or it could be random variation.

Conclusion: If it’s hot outside, don’t sweat it. Use your AC.

Central AC: I should note that these tests are of wall-mounted AC units in China. Central air conditioning may work differently.

For those interested, I’m pasting the detailed data below.



How Safe is Indoor Air?

I recently had a conversation 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: 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.