How much green plant leaf is needed to balance the carbon dioxide output of a person?

And I mean this in a science-fiction-spaceship-balancing way, not a global warming kind of a way. Reading a bunch of classic Heinlein over this Christmas has brought to my attention that some writers expect to use plants to help or completely replace the carbon dioxide breathed out by the crew. Heinlein tossed out a figured of ten square feet of plant leaf to balance, on average, a person. I realize that he probably pulled that answer out of thin air, but is there a better answer nowadays? And bearing that in mind, does having a plant in my room (and I’ll estimate 1 sq. foot of English Ivy) help keep the oxygen count in my room up?

Asking the question indicates that you don’t understand the subject well enough to understand the answer. That’s not meant to be snarky, it’s just an observation.

If you understood the subject you’d realise that the actual question is “How much green plant leaf is needed to feed a person”. That’s because people only “output” CO2 by digesting food. That’s it. Total and complete. The amount of CO2 we produce is exactly equal to the amount of food we eat, and if we eat less food we produce less CO2.

Or to put it another way, in a self-sustaining spaceship of the type you describe, all the CO2 a human produces has already been paid for. Plants fix CO2 from the air in the form of plant material, we eat that plant material and re-release precisely the same amount of CO2 back into the air.

The only time you would need to have any additional plant material to balance the carbon dioxide output of a person is if the person were eating food form the hold that had been canned on Earth. In that case the person would be releasing CO2 that had not been fixed from the ships own atmosphere. However if that were significant you would have far bigger problems with oxygen and water balance than with CO2 balance. As the food is digested it will burn oxygen. If you try to solve that problem by using plants you need to realise that as plants produce oxygen they destroy water and increase in mass. Fluctuations in mass distrubution, oxygen and water levels are going to be more important on a spaceship than fluctuations in CO2, so if the changes in CO2 levels were significant enough to need to scrub with plants you’d be making far more significant changes to oxygen and water budgets.

So the short answer is that your spaceship is either self-sufficient WRT to its animal cargo or it isn’t.

If it is self sufficient and it’s growing all its own food then there is no need to work out required leaf area. You just need to produce enough calories to feed the crew, and in the process of producing those calories you will automatically and necessarily sequester enough CO2 to perfectly balance the CO2 output of the crew. How much leaf area you’d need to feed your crew depends on a huge number of factors including the species used, light, temperature, fertilizer regime and so forth.
If your ship isn’t self sufficient then CO2 buildup really isn’t your problem, it’s oxygen loss. If you try to counter the oxygen loss with plant life than you end up with problems budgeting all sorts of other gases.

From purely an oxygen standpoint though, lets say a ship, instead of taking an oxygen supply, took plants to produce oxygen. how many plants does it take to produce enough oxygen for a person to survive?

The answer is the same: precisely the same amount of plants as are needed to feed that person. No more, no less.

The exact answer depends, as I said, on what crops you are growing, how much light you have, how carefully you control your fertiliser inputs and so forth. For a person willing to live on chemically reconstituted algae you could probably get by with a hundred square metres per person. If the person wants a varied and natural diet it’s going to be a lot higher because most crops produce very little usable food per unit leaf area and produce no food at all for months at a time. If the person wants to eat meat it’s going to be higher still because you will also ned to feed the livestock.

But the basic answer doesn’t change. If you are growing all your own food you don’t need any more plants to produce oxygen. The production of the food pays any oxygen debt in advance.

If i was locked in an airtight safe on an empty stomach for a while with a plant I would die…I guess i am not understanding your answer. How much plants would it take for me not to suffocate locked in an airtight envoirment…what does eating have to do with anything? assume i am feed via an IV

Blake is working from a long-term assumption wherein plants take in the carbon dioxide breathed out by humans (which was attained from similar plants earlier) and thus there is a natural cycle of carbon dioxide transferring to and from the humans and plants. The crux of his argument is that all sustenance for the human crew is drawn from plant life; however, I would imagine most food would be in the form of prepared meals produced on Earth and not purely from the plants brought on board. I think there is a valid question here that he has summarily dismissed, which can be looked at using the assumption that the carbon-fixing plants are not to be eaten. The answer to this question is beyond my knowledge, but I assume looking at the rate at which humans expel carbon dioxide and the rate at which various plants fix carbon dioxide would be a good place to start.

If I’m understanding correctly, though, the only way for plants to fix carbon is to grow something. Either that’s food, in which case you might as well eat it, or it’s simply plant mass, and you’re eventually going to run out of space. That’s ignoring the issue of nutrients and such, but the basic point is that for it to work on the scale of years, it has to be a sustainable cycle, which means you have to eat the plants; otherwise, there’s nothing for the plant to turn the carbon dioxide into, and still have the same amount of plant left over.

Now, there may be another question lurking in there - what if we’re not looking for a permanently sustainable system, just one that will last, say, a year? We don’t care what shape the plants are in after the year. In this case, would it be possible to start with some larger-than-usual percentage of oxygen, and use plants to slow down our conversion of this (with stored foodstuffs) into CO2? My suspicion is that it’s going to be absurdly more expensive to cart all the plants around than to bring some carbon scrubbers, but at least it might be possible to answer, at the very least. Unfortunately, I don’t know nearly enough to figure it out.

Yes, because in an environment as small as a safe it is impossible with modern technology to balance the oxygen requirements of the two organisms. You will either suffocate or you will die from an excess of oxygen.
I thought we were discussing a hypothetical situation with future technology where we could balance these things out. If that is not the case then the answer is even simpler: no amount of plants will balance out your carbon dioxide outputs because by definition we can’t balance it.
Now if we are talking about a hypothetical situation with future technology where we can balance these things out then you are incorrect. If you were locked in an airtight safe on an empty stomach for a while with a plant you would be just fine. The plant would produce precisely enough oxygen to counterbalance your own needs and its own.

You’re right. You are not understanding the answer because the answer is : the same amount of plants as were needed to feed you. If you currently have 50, 000 calories in potentially consumable calories stored in you system then you will need enough plant growth to counterbalance that 50, 000 calories. And as I said earlier, how much that is depends on the type of plant, how fast you are using oxygen, the light levels and a bazillion other environmental factors.

I think that perhaps what you are failing to understand is that a fat person locked in a safe will require far more oxygen than a skinny person because the fact person contains far mood unburned food. Similarly a person who is asleep will require far less oxygen per hour than a person who is trying to dig their way out with a fork.

There’s no simple answer for how much plant mass you are going to need because it depends entirely on how much oxygen you will use and how fast.

The you will rapidly suffocate, not because of a lack of oxygen but because of a lack of space.

Contrary to high school science, plants as a kingdom don’t produce oxygen. Individual plants produce oxygen so long as they are actively growing. Once it gets dark they start consuming oxygen, and once they die they start consuming oxygen. IOW if you are being fed by a drip the plant will need to produce biomass at rate equivalent to the rate at which your IV is feeding you biomass. And all that biomass has to remain in the safe with you, intact, solid and never decaying.
So if your IV feeds you 1kg of dry biomass every day your plant will have increase in biomass by 1kg dry weight every day to supply you with oxygen. And because fresh plant biomass is >95% water the plant will need to consume 10 litres of water per day and increase in volume by increase in volume by 10, 000 cubic centimeters. Unless you are in one huge MOFO safe you are going to run out room in very short order.
The only way to keep that biomass down is to turn it back into gas in an oxygen neutral manner. You can’ t burn it because that isn’t oxygen neutral. Letting it decay to gas also consumes oxygen. So your only oxygen neutral way of reducing biomass is to eat the stuff.

If you are going down that route then using plants is a really, really dumb way to scrub CO2. If you have the luxury of infinite, unproductive increases in plant biomass you clearly have no mass or space restrictions or restrictions on available energy. In that case you are far better off just using chemical scrubbers. They are less energy intensive, more easily stored, don’t produce ever-increasing amounts of flammable gas as a waste product and can be readily regulated to meet demand.
In contrast plants are massive, they are hellishly difficult to store, they need shed loads of energy to keep them running, they can’t be adjusted to meet needs and they necessarily cause a buildup in oxygen which then needs to be scrubbed out chemically anyway.

The only reason anyone would take active plants (as opposed to seeds or culture vessels) on a spaceflight is because they make the craft self-sustaining. If you are eating food grown on Earth then plants have no practical value whatsoever.

It wasn’t dismissed. I explained at length that it isn’t a practical solution because of the inherent problems with budgeting other gases if anyone were silly enough to actually try it.

If you just want an answer to a hypothetical situation where a plant has to meet someone’s oxygen outputs, I answered that as well by pointing out that it depends on to many variables. It may be anywhere from a few hundred square metres to several thousand square kilometres.

Yep, and once the OP tells us what the metabolic rate of the people is, what plants he wants to examine and how they are growing we can make a start. Without that information the answer will vary by order of magnitude.

No. Every piece of food we eat will still be converted into CO2. The plants can’t stop our bodies doing that. All they can do is convert that CO2 into oxygen after the fact. The actual rate of conversion of oxygen to CO2 is constant.

Now if the question is whether plants can slow down the rate at which atmospheric oxygen levels decline, then the answer is that they can, of course. But they will do so at the expense of vast amounts of water and space and energy and in a very unregulated manner. It really makes no sense for a spacecraft to try to do this since it will always be less efficient and less sensible than what we can achieve chemically using current technology.

Hell, it’s going to be less efficient just carrying compressed air cylinders and replacing portions of the atmosphere periodically. Oxygen in air weighs less than the equivalent plant biomass, it takes up less space, it is easily regulated and it’s reliable. Hard to see why anyone would use plants.
I just can’t imagine why anyone would want ever do this. It becomes a purely academic exercise akin to putting plane on a treadmill, and like that exercise the answer depends on entirely on what assumptions we choose to make. There is no single definitive answer.

Kudos to Blake on the detailed answer.

A key point I think is this:

It’s not enough to have, say, a bunch of cabbages in the safe. You need to completely grow a new cabbage (or equivalent) for each cabbage (or equivalent) of food your body is using.

Don’t ask me why I picked cabbages.

Even if you’re eating pre-packaged meals, and not eating the particular plants that you’re using to take CO2 out of the air, you can still say “How much plant matter went into the food you’re eating?” (Even if you’re eating hamburgers, I think you could ask “How much plant matter went into growing that much cow?”) However much that is, that’s how much vegetation you’d need to grow to balance out the CO2 emissions.

Here’s a (perhaps overly) condensed version of what I think is the crux of Blake’s point:

(1) When you do stuff, you burn your body’s fuel (so to speak) and emit CO2. Let’s call the amount of CO2 you emit X.

(2) Let P be the quantity of plant matter you’d have to eat to put that stuff back (including CO2). That’s equivalent to what you had to eat in the first place to have the energy to do that stuff.

(3) If P is precisely enough to give you back X amount of CO2, then P must contain exactly that much CO2. (What I really mean is it must contain that much carbon and that much oxygen.)

(4) Thus, in order to grow P in the first place, we had to take X amount of CO2 out of the environment.

Note that whether or not we’re actually eating P is irrelevant. P just needs to be the amount we would need to eat to replace what we lost.

Right. So, you don’t actually need to eat the plants, you just need to chuck an amount equivalent to the canned goods you are eating out the airlock. And bring along enough water, energy, and whatever else is necessary to grow that plant matter that you’re chucking.

Or, you know, get a CO2 scrubber and an oxygen tank.

I think we’re talking past each other. I agree that bringing plants along into space is an infeasible method for scrubbing the air of CO2; however my point was that I thought that this might be an interesting intellectual exercise to demonstrate just how inefficient the method would be. I didn’t mean to imply that I thought this method was a reasonable solution.

This is what I was talking about before; given some set of assumptions, how much plant material do we need? In your first post you talked at length in general terms why the method is infeasible; however I thought a more concrete example would really demonstrate it better.