I guess same as you get enough oxygen at home even if you don't open the doors and windows at all.
this post was submitted on 27 Nov 2025
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It's called diffusion. Unless the greenhouse is hermetically sealed, and the rate of consumption does not exceed the rate of diffusion, the ratio of CO2 will be similar to that outside.
A simple vent will be enough if not just small gaps in the structure.
Also, most green houses have fans that can help circulate air in from the exterior.
I'm inclined to think the fans are more for fanning the plants so they can perspire and breathe more readily. Many plants need a breeze to grow well.
Usually both, in my experience.
You’ll have fans for plants that firm up stems and stuff, in addition to helping with transpiration.
Then you’ll have fans or passive vents near the highest point that move air outside, usually for the sake of venting excess heat and humidity. If you live somewhere cold, these probably stay closed through colder times and are active/open only during the height of summer (these are often the passive vents). If you live somewhere warm, it’ll probably always be active fan-driven, and usually running.
A lot of people have their ventilation system hooked to climate control sensors, so it only engages when it crosses a certain threshold.
The oxygen level inside is lower than outside. It might affect your health.
I can't tell if you're joking or not. Unless a home is small, effectively airtight, filled with people, and the doors and windows are never opened, oxygen concentrations aren't going to fall enough to be impactful.
CO2 levels can get too high on new construction, but that's a 600-1000 ppm increase.
Elevation will have a much larger effect on your body's O2 availability compared to what humans can deplete.
Plants don't actually use up all that much CO2. An entire greenhouse worth of plants wouldn't even consume as much CO2 as your body produces. Most oxygen produced on earth and CO2 consumed is done by algae. Land plants like trees can sequester more CO2 because they grow larger and use it to build their bodies but they do so slowly over the course of years.
So I guess plants don't suffocate in greenhouses for the same reason that you wouldn't if you sat around in a greenhouse. There is enough air infiltration through cracks and gaps in the walls that sufficient air gets exchanged to keep it breathable.
To put some math around CO2 usage:
The entire structure of plants is built primarily from CO2. A tomato plant and fruit grows from seeding to maturity in about 60 days, and will yield about a kilogram of dry plant mass.
That mass will be about 20% carbon, meaning each plant would need to uptake a net 3.3 grams of carbon - 12.3 grams of CO2 per day. A person exhales around 1Kg of CO2 per day, or about as much as would be needed to supply 81 tomato plants.
Most oxygen produced on earth and CO2 consumed is done by algae.
Thats more because of total biomass, and the difference is we're basically a ball of water.
Plants only absorb CO² and release oxygen when photosynthesizing, which requires sunlight. At night plants undergo cellular respiration where they absorb oxygen and release CO². Some sealed terrariums haven't had any water, air, or fertilizer for decades.
https://laidbackgardener.blog/2019/10/14/the-worlds-second-oldest-sealed-terrarium/
The same reason your house isn't a pure "air" environment.
Carbon Dioxide is heavier than all the components that make up the air you breathe. In a greenhouse they have fans that move air around, because stagnant air would generally be bad for both plants and humans. You'll notice a gap in all greenhouse environments to the ground.
CO2 gets pulled in, but eventually pushed out because of its weight, but enters in from the ambient atmosphere being injected by the fans.
Mostly it's fine as others have stated, but sometimes they pump CO2 in there. One example is Friðheimar tomato farm in Iceland, which enhances photosynthesis by using carbon dioxide produced from natural geothermal steam.
I'm so mad that there isn't a link to more info about that, because I'm wracking my brains and I can't think of any mechanism that makes sense.
Not sure how they capture the carbon dioxide, but it's delivered to the greenhouse itself from a giant tank outside the building. I realize that's probably not the part of the process you were interested in, but it's all I've got.
To add; I’d imagine decomposition of compost further contributes to CO2. The fact that CO2 is pumped in to some commercial greenhouses suggests CO2 is a limiting factor in plant growth, but I wonder whether hydroponic systems - lacking soil and hence decomposition - require more.