In Japan, the building code requires every room to have an air vent [1]. Initially I was skeptical of them and was looking to close them to save on heating costs in the winter. But after doing a little research, I decided they are probably best left open to reduce condensation and mold growth.

Now I'm curious how effective they are at keeping down CO2 levels at night. I would like to try this experiment in my house.

[1] https://resources.realestate.co.jp/living/japanese-apartment...