Plants, Fresh Air & Oxygen

Dear Nico,
I just finished reading your article on Indoor Air Circulation – great read. You discussed venting when using CO2, and I understand perfectly that you should shut the CO2 off while the exhaust fans are running, makes perfect sense to not waste it.

I am running a CO2 generator in what some call a ‘sealed room,’ I have no intake or exhaust fans running. I have three large circulation fans and seem to have a very healthy room. People have told me it can’t work because plants need oxygen. I’m running with the understanding that plants absorb the CO2 and create oxygen. Question: What’s your take on a sealed room? Am I missing anything?
— Thanks again, Dana

Greetings, Dana. Thanks for writing in and for reading!

This is an excellent question as it is something that is often overlooked by growers – fresh air exchanges within a garden. Of course, for growers in states where growing cannabis is still highly restricted, it is understandable that some may have trepidations about exhausting garden air outdoors, but with careful filtering it can still be done discreetly.

The primary concern with air exchanges is that plants need fresh air, just like humans and animals do. While plants do breath in CO2, if there is not an influx of fresh air carrying new CO2, plants can begin to suffocate over time, just as people would in a sealed room due to lack of oxygen. Of course, this is an extreme situation, but there are other more serious concerns here as well.

Without proper air exchanges the garden’s air will become stagnant, making it easier for disease (like botrytis or powdery mildew) and pests (like mites or aphids) to take hold. Sealed rooms without proper ventilation are also very susceptible to temperature and humidity problems. Heat pockets can form within the growroom and temperatures will start to rise. The same is true for humidity, especially if you are running a hydroponic system with frequent waterings.

Furthermore, to your point about CO2 versus oxygen, many people forget that while the plants breath in CO2, their roots actually breathe in oxygen. Bringing fresh air into your growroom helps replenish both oxygen and CO2 for your plants. However, if you look at Figure 1.1 below, you will see that there is very little CO2 generally present in the composition of fresh air – which is why many growers supplement fresh air exchanges with CO2 generators (just makes sure they switch off during any exhaust cycles!).

For all these reasons, it is always recommended to flush your grow room’s air at least once daily. And remember, we exhaust garden air near the ceiling, where heat build-up is most prominent and we intake fresh air near the ground, where air is coolest. It is very easy to attached activated charcoal filters to your intake and exhaust fans to ensure only what you want is coming in – and what you don’t want going out – odors – does not.

Figure 1.1 shows the composition of fresh air for intake into a grow room.

Thanks for reading everyone and remember: Grow… And help the world grow, too!

Got questions? Email ’em over to Nico at and be sure to put “Nico’s Nuggets” in the subject line!
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