Global said:
This is correct, but there's also a constant metabolic process that makes (most) plants breathe in oxygen and release CO2. This is cellular respiration, and it's shared with animals. It does occur during the whole day/night cycle, but since the volume of gas processed by respiration is sensibly smaller than the volume of gas handled by photosynthesis, the CO2 emission during light hours is relatively irrelevant for us. During night, though, photosynthesis shuts down and respiration continues, so plants do consume more oxygen than they release and release more CO2 than they consume. Hence the classic recommendation of not keeping plants in the bedroom. But you would need a lot of plants in a closed space to raise CO2 levels to a dangerous degree.
