How Air Quality and the AQI Are Measured
Anyone who grew up near Los Angeles prior to auto emissions regulations knows what bad air looks like. The same is true for many who have grown up in countries that have perennial air quality issues. But for most of us in the US, our air quality is pretty good, and except for the occasional “Spare The Air” day when atmospheric conditions cause smog layers to stay stuck somewhere, we don’t spend too much time thinking about it. But the recent fires in California and a new feature in iOS have made air quality and the Air Quality Index (AQI) headline news recently. Most of us don’t really know much about how air quality is measured, or the AQI, for that matter. As our local AQI soared past 200, I was quickly motivated to find out, and to pass along what I discovered.
Major Elements of Air Quality
PM 2.5 is the concentration of microscopic particles less than 2.5 microns in diameter. Those particles are found in vehicle and motor exhausts, as well as smoke from burning wood, oil, or coal. Indoors, smoking, cooking, and burning candles also contribute. PM 2.5 particles can be blown hundreds of miles, such as what happened this week, where air quality throughout the San Francisco Bay Area has been affected by a fire much further north. PM 10 refers to the concentration of particles less than 10 microns in diameter. While these can also come from some of the same sources as smaller particles, they are commonly from other sources, including dust, pollen, and mold.
Ozone can also be a major source of poor air quality. Yes, it’s great up high in the atmosphere, as it helps protect us from some kinds of radiation, but it isn’t great to breathe, or for our crops. Vehicle exhaust and industrial plants are major sources of ozone. Warm weather and lots of sunlight make it worse in the summertime, when it can be the primary factor in smog and poor air quality. Other pollutants that are tracked and can affect the official Air Quality Index (AQI) include carbon monoxide, sulfur dioxide, and nitrogen dioxide.
The AQI: A ‘Standard’ for Air Quality
In all cases, averages over some timeframe are used, but that can vary from eight hours to a day, depending on the pollutant and the country. As a result, it isn’t always helpful to rely on the “official” AQI when deciding whether it is healthy to go outside, as it may not reflect current conditions. Governments (the EPA here in the US) also only have a limited number of monitoring stations, so they may not reflect your local conditions. Apple has recently added an air quality section to its iOS weather app that passes along the most recent AQI average from the closest official EPA station. There is also a cool site that maps data from around the world.
Counter-intuitively, higher AQI values are worse. In general, anything under 50 is considered just fine by everyone, but as the numbers go up, different organizations provide differing recommendations on how cautious you should be. So, how you interpret the various AQI ranges is ultimately up to you. The more “at risk” you are, the more you need to pay attention. Over 100 and you’ll probably be able to see that there is pollution in the air, and maybe start to feel it a bit. Over 150 everyone should at least be thinking seriously about how much time they spend exerting themselves outside. Some schools start curtailing activities or even closing at that point. Over 200 and most schools consider canceling classes and sporting events. In our area, all the sporting events scheduled at Stanford University either had to be moved to a location with better air quality or were canceled during the worst of the smoke from the Camp Fire — even though that fire is well over 100 miles from the campus.
As far as the details, the formulas used to calculate the AQI in the US aren’t complex, but they’re based on a graduated table (more or less like US federal income tax rates) that maps concentrations of each measured pollutant into a numeric range. Then the highest numeric value (roughly speaking, the most dangerous pollutant at that time) is used as the current AQI measurement, that is in turn averaged over the day to get the final result:
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What to Do When Your Air Quality Sucks
Okay, great, so what do you do if you find out that the air outside isn’t good for you? Obviously, not going outside is one option. But if you decide to, then an inexpensive N95 (meaning it will filter out 95 percent of common particles) mask is helpful, at least for short periods of time. (Health professionals are cautious about recommending their use as they don’t want people to think they are perfect — they only reduce particles, not eliminate them). You will breathe more CO2 with one of these masks on, so they cause additional issues for anyone who already has respiratory problems. Pro tip: Buy them before something happens, as they tend to be hard to find once everyone decides to get one.
A step up from there are more expensive respirator masks that filter out 98 to 99 percent of all particles, and often have replaceable filters so you can use them for an extended period. Note that surgical masks do not offer protection from particulates.
Some air filters, like the Coway, have an integrated air quality sensor that gives you a rough idea of the current air quality. They also have an Auto mode that adjusts their fan speed to match the air quality. But if you want a better idea of what’s happening, you can get a hand-held air quality meter for between $80 and $200, depending on what features you want. We found one very helpful for measuring the effectiveness of our air filters, as well as deciding where we needed to place them in our home.
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