Weather Why's: The air quality index explained

Hurricanes? We measure them with the Saffir-Simpson Scale.

Earthquakes? The Magnitude Scale.

Air quality? We have a scale for that, too: the Air Quality Index (AQI).

PHOTOS: Heavy wildfire smoke shrouds western Washington with hazy skies

With wildfires filling the air with smoke, millions of Americans have become familiar with those color-coded AQI maps. Green is good. Purple is bad.

But what do those numbers—56, 180, even 300-plus—actually mean?

Simply put, they're a standardized measure of how unhealthy the air is.

Here's the interesting part: the AQI isn't measuring just one pollutant, and it's not measuring "smoke." It calculates a separate score for several major pollutants, including wildfire particles (PM2.5), ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and larger particles such as dust (PM10).

Because each pollutant becomes unhealthy at a different concentration, scientists convert each measurement onto the same health-risk scale. Then they take the highest value. That number becomes the AQI.

That's why the AQI works for many different pollution events.

A smoky day in Seattle and a smoggy day in Phoenix can have the same AQI—even though they're caused by completely different pollutants.

So yes, the colors are pretty simple: green is good, purple is bad. But behind those colors is a clever system that translates very different kinds of air pollution into one easy-to-understand health score.