Tuesday, April 1, 2008

Indoor Plants as Air Purifiers

This is weird to write, but it's been bugging me for a while now, so I figure I have to. I see a lot of people asking – more on line than in-person at work, but there's some of both – about which plants are good at removing chemicals from the air, and where one might find these plants. Semi-recently, I had a customer who was very insistent that she had to have some plants from this specific list (out of Reader's Digest: she'd brought the magazine in with her) because those plants had been tested by NASA, and they were good toxin-removers, and anything else might be nice and all, but unless NASA had proven it would improve air quality, she wasn't interested. Another subset of customers want some plants because of their capacity to produce oxygen, and have been told by the deluded or unscrupulous that more oxygen is the Answer! at long last! if you're feeling tired or run down.

Pictures in this post are of various NOID mums from work,
and are only tangentially related to the topic at hand.

This is what we call junk science. Junk science is stuff that's true in some fashion or another, but horribly misunderstood, distorted, or incomplete. This is distinct from pseudoscience, which is stuff that isn't even a little bit true, but is dressed up in the language of science (vibrations, energy, metabolism, crystal lattices, toxins, quantum, etc.). Plants are tools of both, though not very often – generally the peddlers of junk and pseudoscience want you to buy something manufactured, not a plant, because manufactured things won't propagate themselves. If there's a plant involved, it's usually been ground to a fine powder, or reduced to a picture somewhere in the logo.

Part I: Oxygenation

Let's start with the oxygen claim, 'cause it's easier to take down. First of all, in order for this to work, we have to assume that your home is airtight. If you're going to be opening your doors and windows, then all bets are off, because any (allegedly) beneficial accumulation of oxygen is negated once all the air inside your home has been replaced with air from elsewhere, an event which is called an "air exchange."

This first qualifier excludes everybody who's reading this. The average home completes an air exchange about once an hour, and the EPA recommends sealing your home tighter, to extend this to about every 2 hours and 51 minutes. I'm not saying that the air in your home is identical in every respect to the air outside: things can build up. However, there's always a point where the oxygen you generate inside is balanced out by the amount leaving, whatever those relative amounts might be. At that point, there's nothing you can do to increase the average oxygen level in your home except to produce it faster (as by getting more plants) or get it to leave slower (as by sealing your home up even tighter). If all the air is turned completely over every hour, or even every 2 hours and 51 minutes, then that's not going to allow for a whole bunch of buildup of anything. Keep this in mind for later.

When photosynthesis happens, oxygen is produced. In order for the oxygen to leave the plant, though, carbon dioxide has to come in, one for one, and a molecule of carbon dioxide (CO2) weighs more than an oxygen molecule (O2). This means that the plant is going to put on mass over time, but we already knew that that happens: we call it growing. I'll spare you the chemistry, but when all is said and done, the plant adds one molecule of glucose for every six oxygen molecules it gives off.

So we can use this fact to do some math. I'll stick the math in a footnote, so as not to rile the mathphobic in the audience, but the upshot is, just to increase the oxygen content in a small bathroom, say 6 feet wide by 8 feet long by 8 feet tall, by a small (probably not even noticeable) amount, say 1%, over the course of a day (let's assume that the air exchange rate is once per day, which is more than eight times more airtight than the EPA recommends and is therefore damned unlikely) you'd have to have a plant in there that was putting on dry weight at the rate of 11.1 kg/yr (or 24.6 lb/yr).1 And this is to change the composition of air inside of a very tiny room by a very tiny amount. So, ma'am? That peace lily you're buying isn't even going to oxygenate your bathroom, much less your whole ground floor.

But – and this is really my main point here – that's okay. You wouldn't necessarily want it to, even if it could. Unless you are right this minute drowning at the bottom of a swimming pool, in outer space, suffering from carbon monoxide poisoning, adjusting to a move to a higher altitude, or in need of supplemental oxygen due to emphysema or some other lung impairment, you're already getting all the oxygen you need. Even if you feel tired and run down. You're probably not sleeping enough. You may not be eating well. You could be depressed. But lack of oxygen is not your problem. If everybody feels tired and run-down because of lack of oxygen, and oxygen only comes from plants in one's immediate vicinity, then: please explain this to the Inuit, who live a plantless existence on ice and permafrost for most of the year. (But explain it to them quickly! They have no oxygen!)

And furthermore (since this dead horse is just laying here, I may as well beat it), if you think about it, the odds are pretty good that just outside your door, or maybe the next county over, there is a very large biomass of plant life just photosynthesizing like mad, in the form of a lawn, or a farm, or a tree, or whatever, which probably puts on all kinds of biomass in a year. Unless you went to completely ridiculous extremes to fill your home floor to ceiling with houseplants (and I don't know what kind of insane person would do that2), any contribution from your houseplants is going to be completely swamped by the plants outside every time you open the front door.

And if that's not enough, most of the oxygen on earth doesn't come from land plants in the first place. Wikiposedly (I couldn't confirm this with anything more reliable, though it makes sense), 70% of the oxygen in the air comes from sea life, algae and cyanobacteria and such. Your spider plant, however big and lush, is just not that big of a deal.

Part II: Toxin Removal

The word "toxin" used to have a more specific meaning. I'm not sure who to blame for the change, but now instead of meaning "something that can and will cause you harm if it gets in your body," it seems to mean "tainted" or "contaminated:" it's more of a psychological word than a physical one. There are a lot of products out there which purport to cleanse toxins out of your body, but there's not a lot of detail on the questions you'd think would be important, like: Which toxins? How did they get there? What happens if I don't remove them? How do you know? A skilled charlatan will have a blizzard of junk science to throw at you; a less-skillful one will have to use pseudoscience and hope you don't know the difference; the newbies will change the subject.

That said, there really are, probably, some things in your air right now that aren't good for you. Exactly how bad we're talking depends on what it is, and where it's coming from, and how frequent your home or office's air exchanges happen, but sure. Not everything in your environment is as pure as the driven snow, the driven snow included.

That said, it's not likely that these chemicals are going to be what finally does you in. They may not even contribute to it, not even a little bit, or reduce your quality of life in any way. But they're also not going to make you healthier, so you may as well get rid of what you can, and if doing this involves a plant, well, plants are pretty, or at least can be, and having pretty stuff around improves your quality of life anyway. So go right ahead: I've got no problem with that. But keep in mind when you hear and read these things that the whole plants-will-save-us-all crap may have been just a weensy bit oversold.

Where this whole plants & toxins thing crosses a line, for me, is the point when people think that it is very, very important that they get a plant for this one, specific purpose. They don't care what the plant looks like, they don't care if they have the conditions to support it, they don't care about the price: if it's on the list, then they have to have it, and it's an emergency.

There are situations where it might actually be sort of an emergency to do something about chemical vapors in your home. For example, if you survived Hurricane Katrina and got moved to FEMA trailers provided by the government, who knew that the trailers' air contained levels of formaldehyde 75 times the maximum level considered safe for workplaces but stuck you there anyway. (Remember when I asked you to keep in mind how oxygen never gets much of a chance to build up in the air, because the air cycles through so fast? Imagine how much formaldehyde has to be being released, continuously, to build up to levels 75 times the amount considered safe. Imagine how much higher the concentration would have to be at the source of the formaldehyde, then, like kids playing on the floor next to off-gassing carpet, or sleeping with the head of your bed pushed up against an off-gassing wall.) Then, it might be kind of an emergency to get some plants. Though in that case, you probably don't have enough room or money for the number of plants it would take anyway, so the whole discussion is still kinda moot.3

Although different plants do seem to preferentially take up one chemical or another, I've had trouble finding anything reliable-looking on-line about any chemicals except three: trichloroethylene, formaldehyde, and benzene. So, in the unlikely event that you're sure you have an issue with one of these, do the research and find a plant that will get rid of it. Most of us, though, aren't going to know which chemical might be our main problem. Since 1) it's hard to find specific chemical/plant match-ups anyway, and 2) all plants have some capacity to absorb organic compounds from the air, I say buy what you like and don't worry about it. In general, plants with more surface area will absorb unwanted molecules from the air faster and more completely than plants with less. So you're better off choosing bigger plants rather than smaller plants, rainforest plants rather than succulents, fast-growing plants rather than slow-growing plants. But this isn't a life-or-death thing, and anyway, you're (in theory) going to be living with this plant for a while, so you may as well get something you enjoy looking at.

This is true even if it's not something that NASA specifically checked out.4 On the other hand, many of the NASA study plants are quite nice, including several that have already been the subject of plant profiles here at PATSP, so there's also no reason to exclude something from consideration just because it's on the list:

Aglaonema spp.
Chamaedorea seifrizii
Chlorophytum comosum
Chrysanthemum morifolium
Dracaena deremensis 'Janet Craig'
Dracaena fragrans 'Massangeana'
Dracaena marginata
Epipremnum aureum
Ficus benjamina
Gerbera jamesonii
Hedera helix
Philodendron hederaceum (a. k. a. P. scandens, P. oxycardium)
Philodendron selloum (a. k. a. P. bipinnatifidum)
Sansevieria trifasciata
Spathiphyllum spp.

The NASA recommendation was for one eight- to ten-inch plant per 100 square feet of floor space (some sources say six- to eight-inch; I'm not sure which was the actual original suggestion); based on my best guesses of how that scales up and down to four-inch plants and sixteen-inch plants and so forth, I think I have at least nineteen times more than the recommendation. We have really, really non-toxic air here. Or so goes the theory.

One note of caution: in some situations, having a plant inside can actually make air quality worse, at least in a sense: certain plants are known to cause allergic reactions in some people, and if dried sap, for example, is on the surface of a leaf and then gets stirred up by the wind, or a featherduster, or whatever, then it could, in theory, lead to some respiratory distress, though I was unable to find evidence that this had ever actually happened to anybody in any kind of life-threatening way. I assume that the same plants that are known to cause contact allergies would also be the worst offenders on this count, so Ficus spp., Euphorbia spp., Synadenium grantii, Hedera helix, Pedilanthus tithymaloides, Yucca guatemalensis, Agave spp., and so on, might not be the plants you want for this, if you're extraordinarily sensitive to plant materials in general. Or use pots filled with dirt, which also works, though not as well.5

I'd prefer that the people buying houseplants were buying them because they liked them, not because they're expecting them to do a job. And I'd really rather that the people buying them weren't expecting them to give them super-oxygenated energy! or remove toxins (for "toxins" read evil itself) from their environment, because this is a lot more to ask of a plant than most plants are capable of, and all involved are likely to wind up disappointed in the end.

[shrug] It's not up to me why people buy, obviously. I suppose I just don't relate to people who need an excuse.


1 Air is 78.08 % nitrogen, 20.95% oxygen, 0.93% argon, and the rest is miscellaneous junk (carbon dioxide, helium, neon, methane, ammonia, water vapor, ozone, nitrogen oxides of varying configuration, carbon monoxide, radon, krypton, sulfur di- and trioxide, hydrogen, etc.).

1 kilogram = 2.2046 lb.
The volume of one mol of any gas at standard temperature and pressure is 22.4 L.
The molecular weight of oxygen is 32.0.
The molecular weight of glucose is 180.16.
6x8x8 = 384 ft3. So then,

(384 ft3) x (28.3168 L / ft3) x (0.2095 L O2 / 1 L air) x (1 mol O2 / 22.4 L) x (32 g O2 / mol O2) x 1% = 32.5 g O2. This is the mass of one percent of the oxygen in the room in question.

(32.5 g O2) x (1 mol O2 / 32.0 g O2) x (1 mol glucose / 6 mol O2) x (180.16 g glucose / mol glucose) = 30.5 g glucose / day.

(30.5 g glucose / day) x (365.25 days / yr) x (1 kg / 1000 g) = 11.1 kg glucose / yr.
(11.1 kg glucose / yr) x (2.2046 lb / kg) = 24.6 lb glucose / yr.

This is a lowball estimate, because of course plants do things besides making glucose, like for example making proteins (for which they need nitrogen) and DNA (which involves phosphorus). Uptake of nitrogen and phosphorus, as well as the various other elements plants are made of, will increase the amount of mass involved still more.
2 my "office:"

3 So, you may be wondering how the formaldehyde got in there in the first place, and why it stays. Well. For reasons too complex to go into here, a lot of foams, glues and plastics are produced by reacting formaldehyde with some other substance (in fact, Bakelite, the very first plastic, was produced by reacting formaldehyde and phenol). There's always a little of the reactants left over, and they can only be washed off the surface. Other products that aren't actually made with formaldehyde can break down into formaldehyde (and other things) over time. A third group of things is deliberately permeated with formaldehyde to kill mold and bugs and such, a fraction of which stays in the product. So all three kinds of materials are going to be continually leaking a little bit of formaldehyde from the center of whatever it is, until it's all evaporated away, a process which is usually known as "off-gassing" in the environmental literature. Wikipedia has a list of stuff that (wikiposedly) releases formaldehyde, though this won't probably mean much to you unless you're fairly versed in chemistry.
There is the occasional product where release of formaldehyde is the whole point, as for example imidazolidinyl urea, an antimicrobial added to certain shampoos and other cosmetic-type products. Imidazolidinyl urea can protect a product against mold and other unwanted organisms because it is constantly releasing a small amount of formaldehyde, which is sufficient to kill anything off before it can get itself established; consequently, you can squeeze the shampoo out of the bottle in the shower without having to worry about it having embedded chunks of mold, which I think most of us would agree is a good thing. (I know Martha Stewart would agree that it's a good thing. Unless she makes her own shampoo. I bet she makes her own. So never mind.) Though I doubt the Katrina survivors' trailers have high formaldehyde levels in them because they're full of shampoo.
Indoor air normally contains about 10-20 parts per billion (ppb) of formaldehyde. The work-safe level is currently set at 8 ppb by the CDC. The average level in the Katrina trailers was 77 ppb, though there was quite a range: at least one test showed a level of 590 ppb, which is a whole hell of a lot.
4 The original NASA-approved list seems to have been expanded somewhat since the original studies were published, or at least you'll find longer lists than mine at some other websites. Whether this is because people spontaneously expanded the list to include their own favorite plants, or because of common-name confusion and people copying one another's lists, or because there were actually additional studies done more recently, I'm not sure. It doesn't change my basic point, however big or small the official NASA list is.
5 No, seriously. One of the NASA tests on Chlorophytum comosum and another plant I can't remember (Epipremnum aureum?) used a Plexiglas box containing a certain concentration of formaldehyde. In order to see how much of the formaldehyde being absorbed was the plant, instead of the soil, the pot, etc., they ran a control test with soil-filled pots of the same size. The soil removed a third of the formaldehyde on its own, taking the concentration in the box from 15 ppb to 10 ppb. It's easy to forget about the microbes in dirt, even sterilized dirt, but they're still there.


Heavy Petal said...

Hi! Just discovered your blog through Blotanical (yeah, I'm a bit slow on the uptake). I'm not sure how you managed to make plant science so funny, but I like it! I'll be back for more...

mr_subjunctive said...

Good to hear. Though I'm always more impressed with people who manage to take plant science and make it dull, 'cause to me it seems inherently interesting. They're like little robots! That are solar-powered! And build themselves! Out of air and water! Anybody who can make that boring clearly has powers beyond those of ordinary men and women.

Aiyana said...

"But keep in mind when you hear and read these things that the whole plants-will-save-us-all crap may have been just a weensy bit oversold."
This not only applies to what plants can do but also many of the other "green" suggestions coming out. This may offend some people, but this also applies to spending 40% more for organic food because of hype--and then complaining because they can't afford groceries.

Esther Montgomery said...

Well . . . there are always lettuces . . . and lettuces are so much more attractive than Aloe Vera!

I wish daffodils weren't so popular.

Three daffodils in a glass vase on the kitchen table used to send up my endorphine levels astronomically.

But someone had the idea of carpetting parks with them - looked good - so councils copied and decided to pack roundabouts and verges with the things so now I think "Oh no! Not another bank of daffodils!."

But single flowers - indoors - are very good for health, happiness and eyesight.


P.S. It's hard hearted and soft brained to use plants as mops for chemicals which humans shouldn't have put so exessively into the atmosphere in the first place. E.M.

Unknown said...

Wow! What a crazily informative post. I was googling around to find out how many plants I should have around my new apartment that sits right above a busy street. Glad I found this.

Kerry said...

Question: Why did you choose to use standard temperature and pressure (0 degrees C, 1 atm) for your calculation/estimation? No one keeps houseplants at 0 degrees C.
Also, I don't think you're giving enough credit to plants for their toxin removal capabilities. You apparently work in a garden center. Great, you get all the clean air you need. Some people are not so fortunate. Think about people working in a skyscraper office building in the middle of a concrete jungle. Sure, there is air exchange with the outside, but the outside air isn't that clean, either! It's also filled with incompletely combusted fuel, particulate matter (soot from diesel engines), and other nasty stuff. There's a reason we have building occupants who suffer from Sick Building Syndrome. These office-dwelling people might not be at immediate risk of death or anything (still nothing compared to FEMA trailers), but there’s no doubt that they are breathing in things that negatively impact their health (and productivity).
Two years ago, I competed in Intel International Science and Engineering Fair in Albuquerque, NM. The project that sat next to mine basically used the principals outlined from NASA research on toxin-removing plants to build a botanical air filter that maximizes toxin-removal capabilities (delivering air across moist meshy membrane where toxins could dissolve in the water and be easily taken up by roots, where bacteria could metabolize them). They performed their experiments in normal office buildings, taking air samples and analyzing them by GC-MS and found that their filter significantly reduced the concentration of various VOCs. Such a filter may be of limited use to you, but I’m sure they would make office-dwelling people healthier and happier. No need to downplay the science and label it as “junk science”.

mr_subjunctive said...

0C and 1 atm is "standard temperature and pressure," the conditions at which 1 mol of any gas = 22.4 L. The difference between that and 25C / 1 atm is negligible (look it up if you don't believe me); it's just that that's what everything gets calibrated against.

I concede that yes, for people in very air-tight buildings in large metropolitan areas, some form of air filtration is probably desirable, and it may as well be plants. I also concede that Sick Building Syndrome is a real thing, and that plants might be useful for combating it. However.

Putting a plant in a gigantic, cubicle-filled room is basically useless. It may be technically an improvement, insofar as it will absorb something that would otherwise have gone unabsorbed, but the air quality benefit is really minor, relative to the aesthetic benefit.

My point was not that this whole toxin-removal thing doesn't happen, just that it's been shamelessly overhyped. It is junk science to sell people plants on the grounds that they create oxygen -- it's not untrue, but the implication is clearly more oxygen = better, and that's wrong. It is junk science to promise people that because a plant can clear formaldehyde out of a small plastic box, buying a spider plant is going to give them more energy and improve their sleep and generally fix everything that ails them. Which are in fact frequently the claims, even if they're sometimes implied.

Furthermore, you're assuming that the plants aren't causing any problems on their own, like with ficus trees triggering allergies in some people, or pesticide residue on leaves or mold in the soil reducing the air quality. Not that those are necessarily a factor in all cases, or that I don't think that people should buy plants. I just don't think that Toxins! and Oxygen! are very good justifications for doing so.

Botanical biofilter-er said...

This is a very interesting article with well-developed arguments. Technically, there are a few errors. I think you may have confused part per billion with parts per million. OSHA (occupational health and safety act) publishes various concentration thresholds (based on exposure time. Their current TWA (time-weighted average for a 40 hr work week) for formaldehyde is 750 ppb and STEL (short term exposure limit) is 2000 ppb (taken from a recent MSDS (material safety data sheet).

To add to your 'oxygen argument', atmospheric oxygen (o2) concentrations are about 21% (or 210,000 ppm). In comparison, atmospheric CO2 is roughly 400 ppm (or 0.04%). Even if a plant were capable of removing all the CO2 from the air and producing O2, clearly this would have a miniscule effect on total O2 concentrations. In addition, in general, CO2 fixation (ie photosynthesis) only occurs when plants are illuminated, while respiration is an ongoing process. In the dark plants can be a net sink of O2 and a net producer of CO2. In certain conditions (especially low light) the 'plant microcosom' can be a net producer of CO2 and a net consumer of O2. People tend to forget about the rest of the plant system, but there is generally a tremendous amount of organic matter in potting mixes, which are cunstantly undergoing decomposition due to the microbes in the rhizosphere (ie. root zone). Also consider that many indoor plants functioning near the 'compensation point', which is the level of light intensity where plants are barely able to fix enough carbon to survive (ie. photosynthesis = respitation)

Many of the published studies were based on 'pull-down' experiments, where a plant was sealed in a chamber which was subsequently spiked with specific contaminants and then the headspace concentration was monitored over time. The problem with many of these studies is that the contaminant concentrations were far from realistic, often 100 to 1000x higher than is typically seen in indoor air.

Furthermore, an often overlooked conclusion of that early NASA and subsequent research is that the plants themselves have very little long-term impact on the concentration of gaseous pollutants. The bulk of the removal is due to the microbes (ie/ bacteria and fungi) that live in the rhizosphere. Many of these critters are highly adaptable and are capable of consuming a variety of the compounds that we call pollutants as food, in the same way that they break down other organic matter. In fact, several studies actually showed elevated rates contaminant removal when the aerial portions of the plants were removed. This was mainly because the leaves were actually blocking the air flow across the soil surface (ie. creating a boundary layer).

This, i believe, is the biggest hole in the argument that potted plants can have significant impact on indoor air quality: BOUNDARY LAYER. While the indoor air stream is not perfectly still, there is not a tremendous amount of air movement and mixing, particularly in the places that we keep potted plants (ie. corners). How can you expect a plant to have a significant impact on the overall indoor environment if there is really very little airflow around said plants (especially is the plants are, in turn, blocking the air flow over the soil surface).

Our research consortium have addressed these shortcomings of the potted-plant approach and have developed complex, plant-based ecosystems that bring the contaminated indoor air stream into intimate contact with the contaminant-degrading microbes present in the rootzone.

These active (due to airflow) living walls have been proven to efficiently trap and biologically degrade numerous gaseous contaminants, present at realistic indoor concentrations.

I invite you to check out www.naturaire.com

This company has commercialized technology developed over the last decade as a 'terrestrial spinoff ' of a biological life support research consortium (supported by NASA, CSA and ESA).

Chris said...

Well I guess I'll have to settle for having indoor plants because they're beautiful. ;-)

Anonymous said...

I was searching for how many plants I would need to increase the oxygen content in my house, and your post answered it very well and quite humorously.

I'm feeling kind of embarrassed now that after taking biology and chemistry I didn't think to calculate the mass the plant would need to gain to absorb all that CO2. Nice post.

Unknown said...

Here's some "real" science analysis that was done to vet the truthfulness (or lack of same) of the "plants for clean air" zealots' claims. Turns out you'd need a lot more plants in your home or office to do what opening a window or ventilating your HVAC system better will do a lot cheaper, and with a lot less trouble. And I'm in the tropical plant business...if you're honest with people, and show them that plants beautify interior spaces, they'll buy just as many plants as they would if you tried to snow them with junk science. Maybe even more.