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

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.