The Limits to Counting Particles

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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.

Thomas is a new Assistant Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.

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