With the public release of iOS 12.2, Apple made a subtle change to iOS Apple Maps, adding an Air Quality Index (AQI) in the lower right corner. If you’ve not yet seen it, pull out your iPhone and take a look.

Had some interesting back and forth on Twitter this morning. There’s a lot of confusion about what these AQI numbers mean.

For starters, the AQI numbers in different countries mean different things. To understand the scale in your country, start with the AQI Wikipedia page, which lays out all the possibilities.

In the US, the scale goes from 0 to 500, with 0-50 being good, 51 to 100 being moderate, 101+ scaling from unhealthy to hazardous. Here’s a map showing the range of AQI throughout the US. As you can see, today is good in most of the US, but really bad news for Phoenix, Arizona.

Not all countries show an AQI in Apple Maps. It does show up in the UK, but their scale runs from 1-10. Not clear if Apple will roll out AQI for more countries over time.

If you’ve got an Apple Watch Series 4, you can see the AQI on the Infograph Modular watch face.