Thanks for the link! I forgot this was a thing. I remember seeing this map on HN when the California wildfires were at their peak.
There's 4 active people using this in the city I'm in - between a million people. I really wish the device was cheap and reliable enough you could just pick it up at your local electronics store for 50 bucks. Having one with an onboard e-ink display seems very Kickstart-able. Or hopefully air quality measurement on smart watches will become a thing in the not too distant future.