Re: Plants cause oxygen deprivation at night???????
- Subject: Re: Plants cause oxygen deprivation at night???????
- From: Walter Turner <firstname.lastname@example.org>
- Date: Mon, 15 Feb 2010 14:07:57 +0100
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.
On Mon, Feb 15, 2010 at 9:11 AM, Steve Marak <email@example.com>
I assume that claim is based on the idea that while plants' metabolisms
produce a net surplus of oxygen, they do also use some oxygen, and that
therefore at night (when not photosynthesizing) they are technically
decreasing rather than increasing the oxygen in the room?
I have no actual measurements, but my immediate conclusion would be that
this is absurd from a purely logical standpoint, because anyone who sleeps
in a room with another person (or pets, for that matter) at night has
something with a much higher metabolism than plants using up their oxygen,
and no one has any concern about oxygen deprivation in that situation.
(For that matter, if it were true, even people with no plants in their
bedroom would be suffocated each night by all the evil grass, trees, and
shrubs outside ... where do they think the air in their homes comes from?)
-- Steve Marak
On Sat, 13 Feb 2010, ExoticRainforest wrote:
> I've been asked twice recently if plants can cause oxygen deprivation at
> night if kept in a bed room. I've never heard of such a tale but found a
> bunch of posts on garden websites today that make that claim. Can anyone
> with a scientific background elaborate on this one??
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