As fires burned tens of thousands of acres across Los Angeles County, officials were warning residents that the air was a “toxic soup” of pollution—fueled by the fact that not only vegetation but cars, buildings, homes, and all the plastics and electronics inside them were going up in flames. 

But to some residents’ surprise, the Air Quality Index (AQI) on their phones didn’t relay that same message. That’s because AQI doesn’t capture the full scope of air pollution—which, during the fires, was made up of toxins including lead, chlorine, and bromine. To give residents a fuller picture of what exactly was in the air around L.A., scientists with an air monitoring project made their advanced air pollution measurements available to the public.

The Environmental Protection Agency developed the AQI to measure five major pollutants: ground-level ozone, particle pollution (also called fine particulate matter or PM2.5, meaning particles that are 2.5 microns or less in size), carbon monoxide, sulfur dioxide, and nitrogen dioxide. PM2.5 and ozone tend to be the primary pollutants. The index gives all this air pollution a value based on the total mass concentration. 

That means the general AQI reading can lack specificity, says Roya Bahreini, a professor of........

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