Across the world, billions of plants produce oxygen for billions of people. But that's on a giant scale. What if we scale it down to a single person in an airtight room? Maybe the classic sci-fi set piece of a heroic astronaut trapped on a broken ship in outer space.
How many plants would our galactic adventurer need to survive?
The short answer is 700 houseplants. That's the bare minimum. But it's complicated, so here's the long answer as well...
Let's imagine our astronaut is a woman called Lucie.
Lucie happens to look a lot like Neil Armstrong
And let's imagine Lucie is trapped in a plant-free room to start with.
Oxygen makes up about 20% of the air around us, but only 15% of the air we breathe out. With every breath, we use up a quarter of the available oxygen. So Lucie will tragically asphyxiate after breathing all the air in her floating prison four times.
In reality, it would be a lot sooner than that because the oxygen would be spread evenly around the room. But it turns out that's nowhere near the biggest issue in this scenario. So don't worry about it for now.
How much air do you breathe?
The average human breathes about 7-8 litres of air per minute. Across an entire day, that works out as about 10k-11.5k litres of air. The average woman is smaller than the average man, so let's take the lower number of 10,000 litres for Lucie's daily air consumption.
Ten thousand litres might sound like a lot. But that's because we only think about litres in the context of petrol or soft drinks.
An Olympic swimming pool contains 2,500,000 litres of water.
The biggest rocket ever made was still pretty tight on space, but they'll probably get more comfortable in the future. Let's say the room Lucie's unhappily stuck in is 4m long, 4m wide and 2m high. That's a bit bigger than the average bedroom and a bit smaller than the average living room.
Lucie's already having a bad day. There's no need to make her claustrophobic as well.
A room of this size has a volume of 4 x 4 x 2 = 32m3. And a single cubic metre is equivalent to 1000 litres – thanks metric system!
So Lucie's got 32,000 litres of air to breathe, which she'll get through in just over three days.
At that point, the oxygen concentration in the room will have dropped to 15%.
Is this survivable?
Yep! Lucie lives.
Out of breath
Mountaineers generally start experiencing altitude sickness at about 3000m above sea level. At this altitude, air composition is the same but the pressure is a lot lower. This means you get less oxygen in each breath, making the oxygen concentration feel lower.
The air gets pretty thin up there.
At 3000m, the equivalent oxygen concentration is only slightly below 15%.
So we have a good idea of what would happen to Lucie after her three days in space. She'll probably have a bit of a headache, feel a bit nauseous, have some trouble sleeping, and maybe get some weird dreams. Three days isn't long to acclimatise to 3000m, but it should be enough to prevent anything worse happening.
Three days later, Lucie will have breathed all her air twice. At this point, she'll be down to roughly 10% oxygen. That's equivalent to 6,000m altitude, a little bit higher than Kilimanjaro.
5,895m high Mount Kilimanjaro
Six days isn't long enough to properly acclimatise to these oxygen levels. Lucie might be able to survive there for a while, but it's not sustainable. And she'll use up more oxygen with every breath.
The most likely outcome is a pretty unpleasant death from either cerebral edema (swelling in the brain) or pulmonary edema (where you essentially drown in your own fluids). Not ideal.
But still, six days should be long enough for our intrepid extraterrestrial explorer to find a way out, right? Or maybe a passing spaceship will receive her distress call and come to rescue her?
After all, Matt Damon was growing potatoes and communicating with Earth in no time.
The silent killer
It gets worse. Oxygen isn't Lucie's biggest problem.
Allow me to introduce our old friend carbon dioxide.
At normal atmospheric concentrations (about 400ppm, i.e. 0.04%), CO2 won't cause us any problems. Well, not directly anyway. There's the whole issue of climate change back on Earth, but that's probably quite low down on Lucie's list of priorities right now.
More pressing is the fact that carbon dioxide becomes toxic at higher concentrations. At 5%, it's fatal.
Remember the concentration of CO2 in the air we breathe out? Yep, also 5%.
Lucie can only breathe all the air in an empty room once before the CO2 takes her out. In reality, it would probably kill her quite a lot faster. An hour of exposure to 5% CO2 will kill you, so prolonged exposure to 4% CO2 will probably do it too.
Without plants, Lucie will die from carbon dioxide poisoning much faster than she'll die of oxygen starvation. She's got three days tops.
Time to bring in the houseplants
How many plants would it take to produce enough oxygen to breathe?
With only three days to live, Lucie had better hope she's already got a lot of plants in the room to keep her alive. That's nowhere near enough time to grow an effective horticultural 'lung' from scratch.
So how many plants does she need?
Well, space is, as Douglas Adams famously said, big.
The chances of Lucie being rescued by a passing spaceship are pretty slim, so the oxygen and CO2 levels are going to have to stay stable for a very long time.
Using the same figures as above, Lucie breathes about 420 litres of air per hour, and that air is roughly 20% oxygen. So Lucie gets through 84 litres of oxygen every hour. Scientists have worked out that the average leaf (if there is such a thing) produces about 5 millilitres of oxygen in the same amount of time.
5ml of oxygen right here
A quick bit of maths gives us 84 / 0.005 = 16,800 leaves required. Your average mature house plant might have about 25 leaves, giving us 672 plants. Probably best to round up to 700 to be on the safe side.
We breathe out about one molecule of extra CO2 for every molecule of oxygen we consume, and plants do the opposite. That means these 700 plants should prevent carbon dioxide poisoning too.
So there's our (simplified) answer! If you're ever stuck in an average-sized airtight room, take a look around. If you see fewer than 700 house plants, you'll probably be dead in a few days.
Tight on space
Of course, nothing's ever that simple. The most obvious problem is where on not-Earth are those 700 plants going to go? Is it even possible to have enough plants to meet your oxygen requirements?
Lucie's 4m x 4m room has 16m2 of floor space. A selection of mature house plants would need at least 25cm of space in each direction, which works out as 0.0625m2 for every plant. Unfortunately, that means Lucie can only squeeze in about 250 plants. And she won't even have room to sit down. Food and water storage will eat into the available space as well.
Still, you could conceivably fit in enough plants if you've got a room full of shelves overflowing with greenery.
Lucie had better hope her sci-fi prison happens to be a vertical farm.
Put lots of these in a room and fill them with mature plants, and Lucie stands a much better chance of surviving.
Plants don't produce oxygen at a steady rate.
Plants produce varying amounts of oxygen at different temperatures at various stages of the growing cycle. Lucie's survival now depends on being stuck in a grow room with a temperature that doesn't heavily fluctuate.
Keeping the lights on
We also need to remember that most temperate plants only produce oxygen during the day. They switch to absorbing oxygen and releasing carbon dioxide at night.
Eight to twelve hours of darkness wouldn't allow oxygen levels to drop enough to be a problem. But Lucie's going to need twice as many plants if they're only producing oxygen for half the day. And squeezing 1400 plants into an already overcrowded room isn't going to be easy.
Ideally, Lucie needs the lights on 24 hours per day. She won't get much sleep, but being tired is better than being dead.
Not all plants are created equal
Plants don't all work the same way.
Lots of plants native to arid areas only respire at night. It's a survival mechanism, as keeping their pores shut during the day stops them losing too much water from transpiration.
But with Lucie's 24-hour daylight, these plants won't produce any oxygen at all.
Plants from dry regions need to do everything they can to conserve water.
The room could be stocked with a 50/50 mix of temperate and arid plants, which would allow constant oxygen production throughout the day. But you'd still need 1400 plants rather than 700. Perpetual artificial daylight is more efficient. Fingers crossed the lights are still working!
So what does Lucie need to survive her ordeal? She needs to get stuck in a reasonably sized vertical farm with at least 700 mature house plants, all from relatively humid areas, with the lights on 24/7.
It's not looking good.
Lucie's perfect plant – tropical with huge leaves!
Thinking long term
If Lucie's room meets all these conditions, she could theoretically breathe for weeks, months or years. Over those timescales, we have to start worrying about other materials in the room.
Just like the plants and Lucie, bacteria in the soil will also respire. They'll suck in precious oxygen and emit additional carbon dioxide. The plants should recycle this back into oxygen, but it's not always that easy.
In the 1980s and '90s, a group of scientists and a wealthy businessman built the largest closed system in history. Biosphere 2 was home to a group of researchers/test subjects for two years to see if they could survive in the airtight structure.
The project managers eventually had to pump in extra oxygen.
Why? The carbon dioxide produced by soil respiration reacted with the concrete walls, producing calcium carbonate and water. That meant the CO2 never reached the plants and was never turned back into oxygen.
After 16 months, this unexpected reaction was enough to deplete the oxygen supply to dangerous levels.
Biosphere 2, the largest closed system ever built. Photo by John de Dios under CC BY 3.0
Any oxidising materials like iron would also consume precious oxygen over time. So Lucie needs to be stuck in a room without any materials that might react with oxygen or carbon dioxide.
Growing the plants hydroponically (without soil) would increase the margin for error.
Life or death
Considering she's stuck in an airtight room with minimal chance of rescue or escape, Lucie probably doesn't have the best luck in the galaxy.
But it might not be the end.
If she's stuck in a hydroponic grow room with at least 700 mature house plants arranged on extensive shelves, and if none of those plants come from arid regions, and if the lights are always on, and if she also has a way to get adequate food and water, then Lucie could potentially survive longer than a few days. And if the room is built from inert materials or the surfaces are appropriately treated, she might even live until old age.
But probably not. We've never done this successfully even when we've tried to.
At least she'll be surrounded by plants