I used to do it by using an Awair device, I could feed it into HomeKit which is why I choose it in the first place. Then some weird crypto thing (planetwatch.io) happend which made them really expensive and I sold it for a lot more than I bought it.

Since then, I don't monitor any more and I don't miss it as I'm always airing the room multiple times per day anyway and I didn't really get something out of it except some pretty graphs.