Opening the windows is a solid choice. I keep a CO2 monitor in my office. So long as I keep the door open, levels usually stay below 800ppm, but if I close the door they can climb rapidly within an hour or two. I have a CO2 meter tied to an alarm to remind me to open the window if they get too high. I probably should have it set to 800, but I found that to trigger too often so I upped it to 1200.

I use a RAD-0301, which works fine as a monitor and an alarm by itself, but which I connect to a Raspberry Pi for dashboard purposes using the code shared here: https://news.ycombinator.com/item?id=22764488