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

Pollution particles come in a variety of sizes (Image Courtesy of the US EPA)Air quality can be affected by many different pollutants, but for typical air quality measurement, several categories are typically used. In most situations, particles are the major consideration. The most common measurements are PM 2.5 and PM 10, measured in micrograms per cubic meter.

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

Range of AQI values for the USARather than give people a spreadsheet of all the chemicals in the air on a given day, government agencies around the world got together to create a single number that would communicate roughly how healthy or unhealthy their air was. Most countries measure the same six pollutants we’ve already mentioned and use whichever has the most dangerous concentration to calculate the AQI, but details and interpretation vary from country to country. India includes measurements of lead and ammonia, for example.

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:

Mapping of pollutant concentrations to AQI values used by the EPA in the US -- Image courtesy Wikipedia

Mapping of pollutant concentrations to AQI values used by the EPA in the US — Image courtesy Wikipedia

Going Local: Getting Closer to Home With Purple Air

Purple Air map showing how pollution from wildfires can travel hundreds of miles -- in this case to the San Francisco Bay AreaWhile you can always get the official EPA reading of air quality for your area within the USA from airnow.gov, it is often helpful to have something that is closer to real-time, and a little more local. Depending on where you are in the country, that may be possible by using one of the crowd-sourced air quality platforms. My favorite is PurpleAir. They sell Wi-Fi-enabled monitors for just over $ 200 that will report several different pollution measurements to the cloud, and can be looked at online by anyone. IQAir is another company that provides crowd-sourced air pollution data from its monitors.

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.

Our hand-held air quality monitor showing the decline in PM25 after turning on an air filter in my officeHowever, if the air outside stays unhealthy long enough, even if you close your doors and windows things are going to start getting dodgy inside. In our case, after a week of unhealthy outdoor air, the air quality in un-filtered areas of our house had gotten to over 100. Room air filters can do wonders. When the smoke hit, we had a few old Honeywell filters that still do yeoman service, but also picked up the Coway 1512HHSEEAMAZON_ET_135 See Amazon ET commerce (Wirecutter’s top-rated, and recommended by friends), which does an impressive job. Air in my office was measuring over 90 at one point and the Coway brought the quality back to around 30 within an hour. There are also larger units if you’re willing to make the investment.

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.

Now Read: China’s problem with smog, captured by NASA’s Terra satellite, Enormous air purifier creates bubbles of clean air in polluted cities, and Scientists build smartphone sensors that can detect NO2 air pollution.

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