Science of Everything: Air Quality Index
By Stephanie Sorich, C2ST Correspondent
For the citizens of Chicago, the summer of 2026 has not been all sunshine and lake days. Between violent storms turning into flash floods and hazy skies brought on by wildfires, the weather has been a mixed bag. Those smoky days have caused Chicagoans to focus on a once less-considered aspect of the weather: air quality.
“The Air Quality Index (AQI) is meant to be a simple way to distill all of these different pollutants in the atmosphere that are hazardous to our health and provide a one-stop shop for the general public,” Dr. Anastasia Montgomery, atmospheric chemist and Chief Science Officer at Sponge, explains. “You can just look at the AQI and see what is currently dominating the pollution landscape.”
Originally named the Pollutant Standards Index (PSI), it was created by the Environmental Protection Agency (EPA) in 1976 after the passing of the Clean Air Act. Back then, the PSI was meant to standardize how cities and states reported air pollution. Now, as the AQI, Americans can receive information about potential hazards to their health.
When you check the AQI in your weather app, the first thing you see is a number. The Index is broken down into six levels ranging from good (0-50) to hazardous (301+). Wherever the number you see falls on this scale represents the level of pollutants in the air and the corresponding risk to your health. However, not all pollutants are as easy to see (and smell) as smoke, so the AQI also provides a measurement of the top five major pollutants: particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and ozone.
Wait, ozone? Didn’t we ditch our aerosol hairspray and old refrigerators to save the ozone?
“It’s actually the same chemical,” Anastasia confirms. “In the stratosphere (the second layer of the atmosphere), it blocks a bunch of high-energy UV radiation coming to us on Earth. So it’s a really nice protective layer, but it’s a nasty chemical. It’s less abundant in the troposphere (the Earth’s surface), but it is worse for our human health.”
Of the top five, the pollutant most noticeable this summer — smoke from forest fires — falls under particulate matter. It’s strange to walk outside and smell the smoke from a fire in Minnesota or Canada, and it’s a reminder of the far-reaching implications of climate change. Despite that, Anastasia says that air pollution isn’t following the worsening path of climate change: it’s actually gotten a lot better.
“Since 1990, pollution levels have decreased anywhere from ninety percent to thirty percent today, depending on which pollutant it is,” she says. “Because we said, ‘this is a problem, we care about it’, and we’ve had decades upon decades of research, legislation, continuous regulation, and updating with the latest science. I think the Clean Air Act is one of the best American success stories.”
On a local level, understanding which pollutant you are most frequently exposed to can be helpful for your health. Children, the elderly, and those with conditions like heart disease or asthma can be more sensitive to certain types of pollutants (which is why the AQI refers to them as “sensitive groups”). For instance, Chicago tends to see higher levels of ozone pollution, which can be detrimental to those with lung disease.
Even if you don’t fall into one of these groups, Anastasia says remaining inside when the air quality is unhealthy is key. If going outside is absolutely necessary, you can wear an N95 mask to filter out particulate matter, but no happy hour plans are worth risking your health. Spend a day inside, and hope that tomorrow brings clearer skies— literally.
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