You can buy a cheap sensor, like the MH-Z19B, connect it to a Raspberry Pi Zero W (or just about anything else), and set a reminder to open the windows once the

You can buy a cheap sensor, like the MH-Z19B, connect it to a Raspberry Pi Zero W (or just about anything else), and set a reminder to open the windows once the value rises above some threshold. Or just buy a finished product for this somewhere I guess.