From: Walter Turner <wvturner at gmail.com> on 2010.02.15 at 13:07:57(20594)
I can’t add to the ideas Steve Marak presented, but I can try some numbers. I hope the readers of Aroid-L will work through my numbers here. It would be bad enough for me to look like an idiot because I overlooked something that changes the results by a factor of a hundred or so, but it would be worse for the wrong results to stand without correction.
I asked a friend who did his doctoral work on plant respiration and got the approximation that the emission of CO2 in the dark is unlikely to exceed 4 micro moles per square meter of leaf surface per second. This is the same as the O2 taken up. Say our plants have a leaf area of one meter. Let’s say for the sake of argument that both upper and lower leaf surfaces emit at that rate, so our plants use up 8 micro moles PER SECOND (please excuse the upper case, but when we think of a whole night the seconds really add up).
In an 8-hour night, we have the 8 micro moles/s x 3600 s/h x 8 h = 230400 micro moles of oxygen the plants take up. That is only millionths of moles, so it amounts to 0.23 mole. That got the number back down in a hurry.
How much oxygen was in the room to start with?
A room 4 m x 4 m x 2.4 m has 38.4 cubic meters of air. That is 38400 liters.
At room temperature, a mole of gas is about 25 liters, so the room has 1536 moles of air.
If we say the air in the room is 20 % oxygen, we have 307 moles of oxygen to start with.
How much of the total oxygen in the room did the plant use? It is 0.23/307 = 0.075 %.
What about our oxygen-starved sleeper?
A person breathes out about 900 g CO2 per day or about 300 g in an 8-hour night.
CO2 has a mole weight of 44.
300/44 = 6.8 moles CO2 emitted = moles O2 taken up by a person in the night.
The 6.8 moles of oxygen used by the sleeping person amounts to only about 6.8/307 = 2.2 % of that available.
How do the plant and the sleeper compare? The plant uses 0.23/6.8 = 3.4 % as much.
With all the guessing (“approximation” in science-speak), none of the numbers has any meaning unless it’s rounded off to only one digit. You can insist that a sleeping person uses less oxygen than an active one, the room is smaller and the plant larger, but that doesn’t change anything. A good-sized plant surface won’t reduce the oxygen content of the room by much more than a tenth of a percent, and that is only about three percent of what the inhabitant uses up.
If the sleeper is oxygen-starved, he or she probably isn’t breathing right. It can’t be blamed on the plant. I’ve known people who slept with their heads under the covers. That does scare me.
Walter Turner
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