Ask HN: Reliable offline air quality monitoring at home?

With all the news about gas stoves being more harmful than previously known, I'm looking into monitoring the air quality in my home. I don't have a gas stove, but it would be nice to have a better sense of what's happening to the air in my apartment.

I'd rather build something than buy something.

Can anyone recommend something offline / non-IoT / perhaps DIY they've had good success with?