Air

Does your home or enclosed community have proper air production and filtering?

Plants
How much oxygen do the plants produce? How much carbon dioxide do they eliminate?

Pollutants
Do you drive combustion engine vehicles, or produce other sources of pollution? How are they dealt with?

Animal lifeforms
Be it humans, livestock, pets, or scenery animals, you must calculate how much oxygen is being used and carbon dioxide produced.

Oxygen from water
It is rather simple to build something which uses electricity to split water molecules into hydrogen and oxygen.