Source Pollution

10 Facts About Air Pollution

We get many questions about air pollution in our office, and understandably so. It’s a topic that isn’t well understood or well-reported about in certain parts of the countries in which we work. In some cases, it is difficult to distinguish research-backed findings from common beliefs. To contribute to collective learning, here is a quick list of top 10 facts about air pollution.

  1. Air pollution is made up of chemicals, particulates, and biological materials. Common  components include, but are not limited to: nitrogen, sulfur, carbon monoxide, carbon dioxide, dust, and ash.
  2. Air pollution is caused by both human and natural contributors. Industries, factories, vehicles, mining, agriculture, forest fires, volcanic eruptions, and wind erosion all cause air pollution.
  3. According to the Global Burden of Disease report (2013), air pollution contributes to more than 5.5 million premature deaths every year. Another report by the International Energy Agency estimates the number to be 6.5 million deaths per year.
  4. Research has linked air pollution to multiple diseases: acute lower respiratory infections, chronic obstructive pulmonary disease, lung cancer, tuberculosis, low birth weight, asthma, and cataract.
  5. According to the WHO, 98% of cities in low- and middle-income countries with more than 100,000 habitants have unsafe levels of air pollution.
  6. Of the top twenty most polluted cities in the world, 13 are in India and 3 are in China. Delhi ranks as 11th most polluted, whereas Beijing ranks as 57th most polluted.
  7. Over half of India’s population—660 million people—live in areas with unsafe levels of air pollution.
  8. On average, Indians living in polluted areas will lose 3.2 years of their lives due to air pollution.
  9. In 2014, India and China tied at 155 among 178 nations in rankings measuring how countries are tackling air pollution in the world, despite both countries having some of the worst air quality in the world.
  10. Pregnant women who live in  high traffic areas have a 22% higher risk of having children with impaired lung function than those living in less polluted areas.

Winter (Air) is Coming

Another year, another Beijing winter. As winter arrives to Beijing, I got to wondering: how much worse is winter air?

To get to the bottom of it, I analyzed the last seven years of US Embassy PM 2.5 data. I found that the capital’s air has averaged 111 micrograms in the winter versus 92 micrograms for the rest of the year.


Just how bad is that? The WHO 24-hour PM 2.5 limit is 25 micrograms. That means Beijing’s summers average three times the limit, and winters average over four times limit.


Breathe safe this winter!


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).


Is Air Pollution a Problem in Mongolia?

图片 1

According to The Guardian, Ulaan Bator is the world’s second most polluted mega city. From 2008-2011, the average PM 2.5 level was nearly 150 micrograms. In 2014, Beijing averaged 98 micrograms; the WHO 24-hour limit is 25 micrograms.



Is it a bustling manufacturing sector? Manufacturing is still a tiny percentage of the Mongolian economy. Cars filling the streets? Not that either.


It turns out, most of Ulaan Baatar’s pollution is from people burning coal to keep warm. A little heat is necessary where winter temperatures dip to -40 C.

Yet all that air pollution causes 1 in every 10 deaths in the capital, according to an academic study. That makes clean air an urgent need in this city of 1.2 million people.

Smart Air will host its first-ever Mongolia workshops in Ulaan Baatar! Join us in the Mongolian capital September 11th and 12th.

Join us to learn how DIY purifiers can help remove dangerous particulate pollution from inside the home. Smart Air founder Thomas Talhelm will explain how even the priciest purifiers use a fan to push air through a HEPA filter. Workshop participants will make their own in just 10 minutes, and they’ll take it home to start breathing easier right away.


What time of day is PM 2.5 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 PM 2.5, and night time came in first:


I also know people who arrange their schedules to work out in the morning to avoid the worst pollution. But how accurate are our intuitions?

To get to the bottom of it, I analyzed thousands of hours of PM 2.5 data from the US Embassy in Beijing. When I finally got the answer, I was surprised:


Instead of during the quiet of night, it’s the afternoon–right around rush hour–that PM 2.5 is the lowest. So if you’re planning a picnic or insist on exercising outside, you’re usually best off between noon and 6pm.

What about other cities?

Is that how PM 2.5 generally works, or is it unique to Beijing’s activity or climate? Fortunately, US consulates in several other cities publish their historical data.


In terms of climate, Shenyang is pretty similar to Beijing, and its daily PM 2.5 patterns are very similar (although night time isn’t quite as bad as Beijing):



The pattern in Chengdu is similar. PM 2.5 is lowest in the afternoon and highest in the morning.



The picture starts changing when you get to the south. In Shanghai, PM 2.5 is lowest in the afternoon (like the north), but there’s also a dip in the very early morning:



Guangzhou’s pattern was virtually identical to Shanghai’s: a dip in the very early morning and a dip in the afternoon.


Bottom Line: When is it safest to be outside?

In all five cities, the afternoon had the lowest PM 2.5 levels. And in contrast to many people’s intuition, the night time had the worst air in several cities. Thus, you’re usually best off organizing your picnic or tai chi in the afternoon.

But keep in mind that the air is NEVER safe on average in any of the five cities at any time of day. So take “safest” with a grain of salt!


Open Data

All of the data is available from the US embassy and consulate websites. Thanks to Josh Malina for collecting and analyzing the consulate data.


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 that air pollution was a problem there too. But what about Guangzhou? Guangzhou doesn’t have winter heating. Does that mean its air is safe?

To answer this question, I analyzed all of the hourly data from 2014 from the American consulate in Guangzhou. Year to date, that data covers 4,572 datapoints. Then I calculated what percentage of those had PM 2.5 readings about the WHO 24-hour upper limit of 25 micrograms (which is about 77 on the US AQI scale).

Before I give the answer, take a guess at what percentage exceeded the WHO limit:

  1. 10%
  2. 25%
  3. 50%
  4. 75%
  5. 90%

Answer below


To date, 76.9% of the readings exceeded the WHO upper limit. The average reading was 53.4 micrograms–over two times the WHO limit.

Breathe safe, Guangzhou!


A New Way to Test Whether the DIY Works

So far, I’ve been testing air purifiers by taking a baseline measurement of particulate pollution in a room, and then turning on the purifier and testing whether the counts drop. I’ve used that method to test the DIY and more expensive machines.

However, I recently bought a second particle counter, so my collaborator Gus suggested another method: run one particle counter in the bedroom that has the purifier, and run another particle counter in a different room that does NOT have a purifier. The benefit of this method is that the control room represents the counterfactual–what would have happened if we hadn’t turned on the air purifier.

Thus, if a northwest wind hits Beijing and makes the outdoor air a lot cleaner, we can separate the effect of the outdoor air fluctuations from the effect of the purifier. In that situation, my old method would artificially raise our estimates of effectiveness. Changes in outdoor air can also artificially lower our estimates of effectiveness if the outdoor air gets dirtier after we turn on the purifier.

In previous tests, I corrected for this by averaging over multiple tests. I also analyzed the data after removing days in which outdoor air pollution fluctuated a lot (for example, I do that sort of analysis in the extra nerd notes here).

But it’s always nice to use different types of tests to make sure an effect is real, so Gus did this experiment. He set up one particle counter in his room and one in his kitchen: 1

He let the particle counters run for several hours, and then a timer turned on the Original DIY in his room. (The kitchen had no air purifier.) Here’s what happened:


The difference between the bedroom and the kitchen air quality can approximate the effect of the air purifier. It looks like Gus would have been breathing 16,000 of these 0.5 micron particles in the air in his bedroom if he hadn’t turned on his DIY purifier.

And it’s pretty clear that the kitchen air quality (where we don’t have a purifier running) is following outdoor air quality:


(Be aware that I’m overlaying these two lines on the same graph, but the Y-axes are different. This is NOT saying that indoor air is as bad as outdoor air. Indoor air is usually cleaner than outdoor air.)

Conclusion: Similar to earlier tests, the double particle counter test shows that the DIY purifier is removing particulate pollution from the air.

As always, I’m including more details for fellow data nerds below.


What about the 2.5 micron particles? In the main text, I present the 0.5 micron results, but what about the larger particles? The results there are similar:


Remember that because these particles are larger, they are more affected by people moving around. That’s probably why there are large spikes from 7 to 9pm, and why counts level off after midnight. However, even after the numbers level off, the purifier is reducing the counts in bedroom even further.

Timing: Careful readers will notice a slight lag between the bedroom and kitchen numbers. That’s because after entering its hourly mode, the Dylos particle counter takes its first measurement after a random number of minutes. (After that, it’s every 60 minutes.)

Therefore, we cannot guarantee the kitchen and bedroom numbers are taken at the exact same time. However, after the fact, we can tell when the measurements were taken. Thus, we know that the bedroom numbers were taken at 27 minutes past every hour, and the kitchen numbers were taken at 36 minutes past the hour. Those numbers are reflected on the X-axis.

Percentage effectiveness: I also computed the percentage reductions in particulates using my regular method of averaging the last four hours before the DIY was turned off. Then I compared that as a percentage of particulate in the kitchen.

According to that calculation, the DIY removed 86% of 2.5 micron particles and 87% of 0.5 micron particles. Compare that to 92% of 2.5 micron particles and 84% of 0.5 micron particles in my earlier tests. However, I am cautious about drawing strong conclusions from one day’s data. It would be best to average the results of several testing days.


More Data!

A while back, I posted data comparing how effective the DIY purifier was compared to a 6,000 RMB Blue Air and 11,000 RMB IQ Air. In that test, I calculated effectiveness as percent difference from outside air. At the time, I said I didn’t like that way of calculating effectiveness, but I did it that way so I could compare my results with the results of Dr. Saint Cyr.

Now I’m posting data from new tests that improve on the old tests in three ways:

  1. The new tests calculate effectiveness as the percentage reduction from indoor air. This gives smaller percentages because outside air is dirtier than inside air. In my opinion, this is a more rigorous way to test air purifier effectiveness.
  2. The new tests were conducted by my Smart Air collaborator Anna Guo in her apartment. This helps test whether the results are generalizable. In other words, this way we can know that my original results were not a fluke just based on the layout of my room or how well my windows are sealed. Anna’s room (15 m2) is also slightly larger than mine (13.5 m2).
  3. Finally, the new tests are with the Meidi fan I’m now shipping to people from my Smart Air site. Thus, these results are more relevant for people using our DIY filter kits. (However, the Meidi fans are very similar to the blue Kadeer fans I was using before. I haven’t seen a noticeable difference in the results with the two fans.)

Methods: Anna used my particle counter to take a baseline 2.5 micron reading before turning on the filter. Then she turned on the filter, put the particle counter on the other side of the room from the DIY purifier, set it to take hourly measurements, and went to bed.

To calculate effectiveness, we took the average 2.5 micron reading of the four hours prior to waking up and calculated the percent reduction from the baseline reading before turning the filter on.

Filter: We used the same DIY kit with the Meidi fan that we’re shipping from Smart Air:


Results: Here’s what the data looks like. This data is from September 9th:


In these real-world tests, we need to be careful to check that outdoor particulate didn’t go down during the test. If it did, we would think the decrease is due to the purifier when really it’s due to outdoor air changes. To test that, we can overlay the outdoor PM 2.5 readings from the US Embassy:


Outdoor air actually got slightly worse during the test, so the reduction inside is not because outdoor air got cleaner.

And here’s the average reduction in 2.5 micron particles over the three test nights:


Conclusion: The DIY filter removed an average of 92%. That’s similar to the earlier result of 94%. I suspect the new result is 2% lower than before because this new calculation uses reduction from inside air rather than outside air.

Again, the effectiveness ratings are similar to Dr. Saint Cyr’s tests with the Blue Air and IQ Air, although remember that his room is larger (20 m2), and the post does not specify the length of testing time or the particle counter used. Ideally we’d have tests with the same particle counter in the same room.

As always, I’m posting more details on the methods for fellow nerds here:



The Limits to Counting Particles

If Particle Counting has just one take-home message, it is that you can protect your yourself from particulate pollution in China, and it costs a lot less than filter companies want you to think. However, I don’t want to mislead people into thinking that turning on a filter will definitely solve any possible problem. I talked with Louie Cheng, who founded Pure Living China, a company that tests for pollutants in homes and offices, and we came to a few conclusions about the limits of filters:

  1. Particulates are not everything. 

Particulate pollution is a big deal in China, and it’s bad for your health. There are lots of studies out there showing this, but the one that pops into my head the most is the study showing babies exposed to more air pollution are born with smaller heads. If that’s not scary, I don’t know what is.

However, particulates are not everything. Gas pollutants can be a problem too. Although I suspect that almost every home in China has particulate pollution while only some have gas pollution, there are dangerous gases out there like radon, carbon monoxide, and the broad category of “volatile organic compounds.” Unfortunately, even the fancy filters have trouble consistently capturing a wide range of gases.

If you’re considered about gases, consider getting a home test from Pure Living China. They’re not cheap, but I’d consider it if I had unexplained health problems or a child at home.

I try to write “particulate pollution” rather than “pollution” because not all pollutants are particulates. However, it’s easy to just covert “particulate pollution” to “pollution” in our minds.

  1. Some particles are too small for filters.

A quick look at the size of different particles and gases makes it clear that particulate filters can’t get everything:


HEPAs are rated to .3 microns (micrometers), and I’ve seen reports that they can get particles of .1 microns, but there are still leftovers. For example viruses and gases will pass through air filters (but not bacteria). (This, by the way, explains what I call “the smoker paradox.” More on that later.)

  1. Opening your window is good (sometimes). 

The air outside in China is scary, but sometimes it is actually good to open your windows. That is because some pollutants can come from inside. Some common sources are formaldehyde used in some furniture, cooking without a good vent, and construction work.

In those cases, it can actually be a good thing to open your window, particularly on blue sky days.

Conclusion: Air purifiers will get a lot of pollutants out of your air, but there are limits to what you can filter out. Consider opening your windows on clean days–and particularly if you have renovation going on indoors or are cooking without an adequate vent.

Thank you to Louie Cheng for sharing his expertise.