Fresh air

I have a 4x8 tent in a 20x20 basement. I have the basement pretty sealed up so I’m not sure that there is a lot of “fresh air” that circulates down there. The furnace and AC ductwork is down there and I think there is a little air leakage that way but there are no windows and only one access door. I wonder if there is enough oxygen and carbon dioxide in the basement for proper plant development. I really have no idea how to quantify these values without appropriate meters. Is this something I should just now worry about?
 
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