How many plants would be needed to produce oxygen enough for 20 humans?

Does it depend on the type of plants and how much sunlight they get too?

• Sure it depends. Is it a 1 cubic meter tanks full of cyano algae or a apple tree? With a 24h equatorial sun cycle or UV lamps? – jean Apr 13 '18 at 20:05

A human being uses about 550 liters of pure oxygen (19 cubic feet) per day 9.5 tonnes of air in a year, but oxygen only makes up about 23 percent of that air, by mass, and we only extract a little over a third of the oxygen from each breath. A 100-ft tree, 18" diameter at its base, produces 6,000 pounds of oxygen." "On average, one tree produces nearly 260 pounds of oxygen each year.

https://earthscience.stackexchange.com/questions/13963/which-plant-produces-the-most-oxygen

https://biology.stackexchange.com/questions/72903/what-is-the-most-nutritious-plant-in-the-world

Which is, very roughly, seven or eight trees' worth per person X 20 = 160 trees.

http://www.sciencefocus.com/qa/how-many-trees-are-needed-provide-enough-oxygen-one-person

Soil nutrients also play a big part in plant.

The plant would have to thrive in an environment like the ISS dry and cool. I presume that the environment currently is best for the electrics first and people of the ISS. Trees would not work by current technology.

I keep thinking algae.

An exchange of CO2, human waste and algae can exist in a microgravity. The CO2 would be injected into the bottom of transparent pressurized tubes stored on the outside of the ship composed human organic hydroponics and algae. The gas would slowly rise through the algae and converted to O2 then collects at the top where abstracted.

This article addresses much the same question. The quick summary is about 300 to 500 typical domestic houseplants per person, with significant caveats, more like 700 to be safe. So 6-14 thousand such plants for 20 people.

Another approach based on some data from other answers on this site is to observe that the net amount of oxygen produced by a plant is basically determined by the amount its dry mass increases through growth (including in the dry mass any leaves shed, etc.). Each mol of glucose produced by photosynthesis, results in six mol of $O_2$ being released

We see below that our 20 humans need 17kg of oxygen per day between them, which is more or less 1000 mol. This means the plants need to add 160 mol of glucose to their dry mass, which is about 30kg. So you need enough plants to produce 30kg dry mass of new plant per day. This paper reports a doubling rate under "optimal" conditions of "between 0.47 and 1.22 divisions per day" for a species of microalgae, so 60kg (dry mass) of that might be enough if optimal conditions could be maintained. This thesis gives a dry cell mass for a couple of species of microalga around $10^{-11} g$ per cell, so this suggests an answer of about $6 \times 10^{15}$ (admittedly rather small) plants to supply oxygen for your 20 people. Finally this paper gives a number of figures (table 2) for the algal dry mass per $m^3$ of algae-laden water, at around 0.1-1 $kg/m^3$. On this basis, you would need 60-600 $m^3$ of algal water (plus the pipework to circulate it and the lights to illuminate it).

• Is "typical domestic houseplant" MKS, cgs, or imperial units? I suppose this one doesn't count? – uhoh Apr 16 '18 at 8:22
• I don't know. I think 6-14 000 of those would produce plenty of oxygen for 20 humans for as long as they'd need it -- which wouldn't be very long. – Steve Linton Apr 16 '18 at 8:41
• The latest issue of Ad Astra calls for 22 cubic meters of "greenhouse" volume per person, and that also supplies about half of the needed calories. See University of Arizona Controlled Environment Agriculture Center and the Lunar Greenhouse project. – MBM Apr 16 '18 at 23:31
• For those less knowledgeable like me, that's Ad Astra Magainze, not the rocket company or the upcoming film, or the journal or the academy all of the same name. – uhoh Apr 22 '18 at 6:47

Plants produce oxygen by photosynthesis, where they convert light and CO2 into sugars which they can use to grow. Luckly for us, the oxygen is a waste stream which we can use.

$${{6\ CO_{2}+6H_{2}O+energy\longrightarrow C_{6}H_{{12}}O_{6}+6\ O_{2}}}$$

This process requires light, so the amount of light the plants are able to receive is an important factor. Also the concentrations of either $CO_2$ and $O_2$ are important, where each plant type has its own preferences.

However, if one would send plants into space to provide oxygen, it can be safely assumed that they would create an environment in which light would be available 24/7 and the gas composition would be optimal.

Plants vary largely in sizes and types, so the typical plant production metric is yet to be found. I was able to find a value for small scrubs, which apparently produce around 0.69g $O_2$ per $m^2$ of leaf surface per hour. [1] With 24 hours of light, this would produce 16.56g of $O_2$ a day per $m^2$.

According to NASA, a human consumes 840 grams of $O_2$ a day. [2] So 1 human would require roughly 50.7 square meters of leaf surface.

So for 20 humans, a leaf surface area of 1014 $m^2$ would be required. Now just look how large you want you plants to be and you know how many you require!

[1] http://www.saps.org.uk/saps-associates/browse-q-and-a/463-how-much-oxygen-does-a-houseplant-give-off-in-a-day [2] NASA: ' Closing the loop: Recycling water and air in space'

• The gas composition in the green house should be optimal for the plants, but also breathable for the astronauts. The plants may tolerate a higher carbon dioxide content than the humans. – Uwe Apr 16 '18 at 9:32
• " a leaf surface area of 1014 m2" ==> Would all that surface need to be directly under sunlight? Dont you need to consider shadows from one leaf to other – Xinus Apr 16 '18 at 10:05
• @Uwe, I agree. However a quick search shows me that plants (winter weat in this case) have an optimum at 890ppm, which is acceptable for humans. If some plants prefer higher concentrations I would start thinking about a uninhabited greenhouse – Martini Apr 16 '18 at 10:18
• @Xinus, Sadly I cant access the book that my value originates from, so a cant be definitive here. I would guess that the leaves require a level of lighting. With closely stacked vertical plantations with LED's in between it might be possible to have a decent amount of surface area per volume area – Martini Apr 16 '18 at 10:22
• "create an environment in which light would be available 24/7". That sounds like a question for Biology.SE - it this indeed the most efficient way? Or could you optimize the duty cycle? (Besides, it's mars, it'd be 24:37/7;) ) – MSalters Apr 17 '18 at 16:52