My Netatmo indoor sensor already reports CO2 levels (aka "hey Siri, what's the carbon dioxide level in here?") However the outdoor sensor's air quality readings still don't come into HK. So it's good to see some progress there.
What I'd really like to see is for Apple to finally acknowledge (and support) outdoor vs indoor sensors. In other words, if I've got an outdoor temperature sensor, I don't want it reporting that (those) temperature when showing me the range of indoor temperatures in the home.
