If you live in the US, the EPA has a website to check outside air quality https://www.airnow.gov/ https://www.airnow.gov/ There was a recent thread[1] about a

If you live in the US, the EPA has a website to check outside air quality https://www.airnow.gov/ https://www.airnow.gov/ There was a recent thread[1] about air quality sensors a few months ago. From that thread there were a lot of recommendations. I ended up buying the Airthings ViewPlus[2] because it covered the widest range. If it's too pricey for you, AirGradient[3] sells a DIY "Pre-soldered" kit for cheaper that I strongly considered as well. The article in that thread was about how "Many indoor air quality sensor products are a scam," but like people mentioned in the comments, even if they are not 100% exactly accurate, you can still see that something has changed and you can crack a window, or close one if PM2.5 from outside is too high. 1. https://news.ycombinator.com/item?id=33025995 https://news.ycombinator.com/item?id=33025995 2. https://www.airthings.com/view-plus https://www.airthings.com/view-plus 3. https://www.airgradient.com/open-airgradient/kits/ https://www.airgradient.com/open-airgradient/kits/