Recently, the city I’ve called home for the past few months (Chiang Mai, Thailand) has been making it to the top of the most “world’s most polluted” lists on a pretty much daily basis. Today, the PM10 meter maxed out at 999 μg/m3 in one area, and the PM2.5 meter wasn’t far behind at around 850. For reference, the EPA defines an “acceptable” level as being below 12 micrograms per cubic meter (μg/m3). 12-35 is moderate, 35-55 is unhealthy for sensitive groups, 55-150 is unhealthy for everyone, 150-250 is very unhealthy, and 250-500 is hazardous. The EPA’s scale tops out at 500, since that basically doesn’t exist in the U.S.
I’ve been getting throat and nose symptoms, a dull headache, and general tiredness out of this whole experience, which hasn’t been great. I lived in South Korea for a few years, where the air quality hasn’t been amazing recently (it can go above 100, but I didn’t see above 200 happening much), but I never felt symptoms there. I’m not exercising, barely leaving the house, and generally in kind of a bad mood as a result of all this, which has left me some free time to obsesively research air quality topics. One of the things that’s caught my attention is the “Air Quality Index.” It’s essentially a market basket of various pollutants developed to help people better understand the impact of air pollution on health, which is great, but also confusing, especially when you’d just prefer to know exactly how much PM2.5 is being pumped into the air, since that’s what we’re really worried about.
Air Quality Indexes vs PM2.5
When you look up the air quality in a certain city, you’ll probably see an AQI, or an Air Quality Index–not an actual measurement of any one pollutant. There are a few reasons this is confusing:
- Different countries have different air quality standards. In China, a 10 is fine. In Canada, 10 is basically “we can’t count the moose in our backyard.” These different metrics are all calculated using different mixes of pollutants, each of which may be weighted differently, measured over a different period of time, and interpreted differently.
- Many AQIs (the EPA’s in particular) are non-linear, as they’ve been normalized to fit the basket of pollutants into a scale ranging from 0-N. That means that multiplying a number on the scale by 2 does not necessarily mean that it will be twice as bad for you–it actually may be much more than twice as bad.
- Many AQI measures can be pushed up by less harmful pollutants, which makes them fairly misleading when you really just care about PM2.5.
In general, an air quality index is one good way to get a quick grasp on air pollution, and the equations do make sense. However, in a situation like the one I’m currently in, what I really care about is the mass concentration per cubic meter of PM2.5, and I think it should be more of a general standard than it is. It lets you directly know what the content of the air you’re breathing is, isn’t skewed by different equations or national standards, and it allows for pretty much any air situation to be immediately understood.
When it comes to air pollution data, which we use to make large and small decisions on a regular basis, simpler is better for the public-facing stuff, and in this case that doesn’t mean giving people a mystery number and a color. Ideally, different AQIs would be available as part of the data presented by air pollution information sites, but more emphasis on the concentration would really help improve general understandability. Going with concentration data is more scientific, it’s linear, and it’s universal.
In the meantime, if you see an EPA AQI number anywhere, you can use this calculator to find the actual concentrations of each pollutant.