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A Garden Myth Is Born – Plants Don’t Purify Air

Since it is Christmas, I thought it would be appropriate to write about the birth of a myth. Gardening information is full of myths–untruths that seem to take on a life of their own. As part of my effort to understand these myths, I also want to understand how myths get started. The history of most myths has been lost but every once in a while I am able to find evidence that clearly shows how a myth is born.

Understanding how a myth is born can be very educational. It provides insight into how the human mind works. It also trains you to spot future potential myths and to better evaluate the information you are reading. So let’s celebrate this holiday by peering into the start of a myth.

a myth is born - plants don't purify air

A myth is born – plants don’t purify air

 

A Garden Myth is Born

A few posts ago I wrote about the Air Purifying Plants Myth. Almost every web site that discusses this topic refers to an earth shattering research paper that supports the idea that adding a few plants to the home will purify the air. The research work was done by NASA–what better reference can you have–maybe that is why everyone uses it? If NASA says it–it must be true.

The NASA paper must be the start of this myth and so it is a good place for me to start. Interestingly, virtually none of the web sites give a reference for the paper. What this usually means is that none of the authors have actually read the paper they are quoting. After a few clicks, I found the original research paper, 1989 (ref 1).

This study concludes:

“House plants along with activated carbon plant filters have demonstrated the potential for improving

indoor air quality by removing trace organic pollutants”

and

“the plant root-soil zone appears to be the most effective area for removing volatile organic chemicals”

There is nothing in the conclusion of this report that says houseplants will improve the air quality of our homes. In fact it specifically says plants + carbon filters. The reason for this is that much of the study centers around a special container that grows the plant in activated carbon instead of soil. We do not do this in our homes!

The second quote is also critically important. As I’ll discuss shortly, when soil was used, it was much more effective at removing pollutants than the plants themselves.

Neither of these conclusions are mentioned in any of the web sites that promote the use of plants to purify our air, but they do quote this reference as their primary source of information. Let’s look at several reasons why authors make this mistake.

For the rest of this blog I will only use data for plants growing in soil and ignore the experiments for plants growing in charcoal. If you are interested in the charcoal studies see reference #1.

Selective Reading

One of the biggest problems people have in understanding facts is selective hearing or in this case selective reading. They read everything, but they only pull out the facts that support their ideologies. In this case the use of carbon filtration, and the importance of soil were completely left out. It is a pretty big mistake since one of the main goals of the study, according to the introduction of the study, was to test the plant carbon filter combination.

The popular press also left out a very important word, “potential”. This is done all the time with scientific studies. The scientist finds some facts, and then proposes a possible future use of their findings. The popular press leaves out key words like ‘potential’ and ‘possible’, and jumps to the futuristic positive conclusion. They turn possible future ideas into today’s fact.

Reported Facts

What are the reported facts? I’ve read through a number of web sites reporting on the NASA study and picked out a few facts that are commonly reported.

1) plants clean 90% of chemicals in 24 hours

2) Use 1 plant per 100 sq feet of home for most effective air purification

3) The best 10, 15, 17 or 20 plants are listed by name

It is interesting that most sites say nothing about how many plants you need, or what size they should be–a few do mention point #2 above. It kind of makes sense that if plants are going to be used as a cleaner of chemicals, the size and number of plants should be an important piece of information? I guess I am just being too logical.

Let’s have a look at each of these so-called facts.

Plants Remove 90% of Pollutants

That is quite a high number and clearly stated. With the right kind of plants your pollution should be 90% less than before you bought the plants. But what did the research find?

The NASA study only looked at 3 chemical pollutants. There are hundreds of chemical pollutants, so even if plants removed the 3 that were studied, it would be incorrect to report that plants removed 90% of pollutants – most pollutants were not studied.

For benzene the researchers reported a 50 to 90% removal rate in 24 hours. For trichloroethylene it was 9 to 23%. The study only reported the results for 7 of the 12 plants used. Preliminary testing for the other 5 plants had values so low that researchers felt it was not worth continuing the testing with them. The popular press decided to use the highest number in the report, namely 90%, or more correctly 89.9%.

The 90% was only found for one plant type out of 12, and for only 1 pollutant out of 3. And it was not 90% – it was a value somewhere between 50 and 90%.

Pollution Free in 24 Hours

Wow–in 24 hours your home is pollution free! Or at least for the 3 chemicals that were tested. Turns out even that is not an accurate statement. The NASA work was done in a lab using closed chambers. A plant was placed inside the chamber, and a chemical was injected. The amount remaining in the chamber was then measured over the next 24 hours.

Homes don’t work that way. In our home, the manufactured stuff we have (furniture, carpets, flooring, house cleaners etc) is constantly adding new chemicals to the air. As soon as some are removed, the stuff adds more. Think of it as a conveyor belt delivering chemicals. For you to be pollution free, you need to remove them as fast as they are being added.

None of the testing done by NASA looked at the home situation.

This is a very common source for the birthing of myths. It makes a lot of sense for researchers to use simple conditions that are well controlled in the lab. It is the best way to pin down certain facts. The problem is that most of the time the results of such tests can’t be applied to “the field”, a term used for real life situations, which in this case is our home.

Based on this report, and any report that I have seen on this subject, any statement about homes being pollution free in 24 hours is nonsense.

One Plant per 100 Square Feet

The statement about using 1 plant per 100 sq ft did not come out of the NASA report. I am not sure where the number originated, but the Associated Landscape Contractors of America (ALCA) does recommend this number.

How does this compare to the research?

The experiment used two different sized chambers, I assume for different size plants. One was 15 cu ft in volume and the other was 31 cu ft. A 100 sq ft room with 8 foot ceilings would be 800 cu ft in volume. If we are looking at removing pollutants from air it is important to look at volume, not the size of the floor space. This means that for you to mimic the research and get the same results, you need to have 50 small plants or 25 large plants for each 100 sq ft room.

It is clear that anyone who suggests 1 plant per room has not looked at the research.

Best Plants for Cleaning Air

One web site reports the following:

“Best air-filtering houseplants, according to NASA –If these plants are good enough

to filter the air of the space station, surely they’re good enough for your home.”

It simply lists every plant used in the study, even the ones that removed very little pollutants. This is not a list of the best plants, it is a list of every plant used in the study and it also has nothing to do with the space station which was launched 10 years after the work in the study was done!

For most other lists on the net, there is no reference to indicate where they are getting their data. One even reports that Orchids are good air cleaners, which I doubt, since Orchids are some of the slowest growing plants available to home owners–they are the sloths of the plant world.

One post lists Aloe vera as the top plant–but the NASA study showed it was one of the worst in removing formaldehyde–it was dropped from the main part of the study because it was so ineffective.

There have been other more recent studies, but I think that in many cases people are just making up lists to have something to post.

Are Plants Responsible For Removing The Pollutants?

One of the most interesting results from the study is the observation that plants are not responsible for removing most of the chemicals tested. Which means that any web site reporting that plants are cleaning the air are not reporting the facts.

The experiment was carried out as follows. A plant was placed in the test chamber, a chemical was added and the amount of chemical was measured for a 24 hr period. A few weeks later the same plant was retested but this time all of it’s leaves were removed before being put into the chamber. A similar pot with just fresh soil was also tested.

This is what the study found for the removal of benzene by Dracena marginata.

  • Plant with leaves removed 58%
  • Plant without leaves removed 50%
  • Fresh soil (ie no plant or microbes) removed 20%
  • Leak Test (nothing in the chamber) removed 7%

There is no indication in the study about the accuracy and repeatability of the numbers. Statistically 58% and 50% could be the same (ie not statistically different) in which case plants removed no benzene, but lets say the numbers are accurate.

Plants only removed 8% of the benzene, not 90% as reported by the popular press and Dracena is frequently on the best 10 plants lists.

The roots and microbes removed 30%, and more recent testing indicates it is the microbes, not the roots, that are responsible. The microbes are much more effective at removing chemicals from the  air than the plants. Even fresh soil with no plant or microbes, at 13% (20-7), is better at removing benzene, than the plant.

When a plant was exposed to benzene for a 6 week period, the % removal increased and so did the bacterial count in the soil, showing a correlation between the number of bacteria and benzene removed. The benzene was feeding the bacteria, and so they multiplied.

The NASA study was poorly done in a number of respects, and some of these issues are dealt with in more detain in references 2 and 3.

A Gardening Myth is Born

The NASA study shows that plants remove a small amount of certain chemicals from the air. A 1500 sq ft home would need around 400 large plants to remove most of the tested chemicals–something that is not practical. Reports that list the best plants for the job are probably not valid lists. The microbes in the soil of the pot are more efficient at removing chemicals than the plants themselves.

Reporters who write about the ability of plants to remove pollutants either have not read the reference they quote (most likely case) or they have cherry picked the data that suits their story. Most have probably just reported what previous reporters said. The original reporters made the following mistakes:

  • ignored the lab conditions used to carry out the experiments
  • used the very best number in the report, ie 90%, and extrapolated it to all plants and all chemicals
  • extrapolated results for 3 chemicals to “all pollutants”
  • completely ignored the scientists own conclusions, namely microbes and charcoal filters remove most of the chemicals

As reported previously in Air Purifying Plants – Do They Work?, further research by others, in field conditions (ie office buildings), have not shown any changes in chemical levels due to plants. The idea that plants can clean the air in your home is a myth and now you have some insight as to how such a gardening myth is born.

Kamal Meattle – Plants and Air Purification

Kamal Meattle presented a very convincing TED Talk video on line promoting the idea that plants purify air. You can see the video and read the full story at, Kamal Meattle – Plants and Air Purification.

Do Houseplants Increase Oxygen Levels in the Home?

Several people commented on my posts that houseplants were still valuable in the home because they increase oxygen levels and that makes us feel better.

I have now looked into this claim in Do Houseplants Increase Oxygen Levels?

references:

1) Plants Remove Air Pollutants: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930073077_1993073077.pdf

2) How Well Do House plants Clean Air?: http://www.buildingecology.com/articles/critical-review-how-well-do-house-plants-perform-as-indoor-air-cleaners/

3) Can House Plants Solve Indoor Air Quality Problems: http://www.practicalasthma.net/pages/topics/aaplants.htm

4) Photo Source: NASA –greenhouse for Mars

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Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

103 Responses to 'A Garden Myth Is Born – Plants Don’t Purify Air'

  1. Yassine, Ben says:

    According to the study of NASA, plants performed differently in removing pollutants; so if it’s not true, how would you explain some plants removing pollutants a lot more than other plants, while all have the same potting mix? do you suggest that they are just conveyors, who transfer pollutants to the soil for bacteria ? if yes, then the myth is true in a way.

    • You can ask two different questions:

      a) Do plants remove VOCs from air?

      b) Do plants reduce VOCs in a home?

      Two very different questions. I never said that plants do not remove VOCs from air. Several studies have shown that to be true and different plants have different abilities to do this. In the NASA study, microbes and the soil were more efficient than plants at doing this.

      However, this does not mean that question #2 is true. In homes the rate of VOC production is 24/7, not a single injection. The relative size of the chamber – the home, to plants is much different than these chamber experiments. There is no proof that plants reduce the VOC level in a home.

  2. THIS IS GOOD EDUCATIVE SITE. I WISH TO KNOW IF ITS A GOOD IDEA TO KEEP TWO DWARF PLANTS IN BEDROOM TO KEEP AIR QUALITY BETTER ESPECIALLY IN WINTER.
    THANKS. BOXBOROUGH MA USA

  3. Hew says:

    Thanks. Would you also edit https://en.wikipedia.org/wiki/NASA_Clean_Air_Study based on your knowledge? It would be helpful, I think.

  4. hasuhonere@inaby.com says:

    “This is what the study found for the removal of benzene by Dracena marginata.

    Plant with leaves removed 58%
    Plant without leaves removed 50%
    Fresh soil (ie no plant or microbes) removed 20%
    Leak Test (nothing in the chamber) removed 7%”

    issues
    1. there is no fresh soil with no microbes.
    2. the plans have the soil any way so why separate stats? (no one will grow dracaena in hydroponics) so the relevant stat is 58%.
    3. wtf is “plant with no leaves”? dead & dry plant? leaves just chopped off? this changes a lot.

    Leak test 7%
    Just the soil removes 13% (20-7) (how can you have soil with no microbes btw?)

    Plant 23% (50-20-7) (dead or almost dead plant?)

    Plant with “leaves removed” 31% (58-20-7)

    But you say:
    ” …in which case plants removed no benzene”
    The plant+soil clearly removed 50% while just the plant 30%.
    that is far from zero.

    “Plants only removed 8% of the benzene”
    (p.s you just clamed NO benzene now 8%?)

    58-20-7= 31% not 8% (math?)

    “The roots and microbes removed 30%”

    Nope. 50% is the plant+its soil + 20% is just from soil.
    You cannot divide “roots” living plant.

    “Even fresh soil with no plant or microbes, at 13% (20-7), is better at removing benzene, than the plant.”

    First how can you say “fresh soil with no microbes”? there is no such thing. fresh soil HAS microbes.

    Soil is at 13%, plant 23%. how 13% is better than 23%?

    • Plants with leaves removed 58%. Of that 50% was due to the soil under the plant. That leaves 8%.
      “There is no indication in the study about the accuracy and repeatability of the numbers. Statistically 58% and 50% could be the same (ie not statistically different) in which case plants removed no benzene,”. Without the statistics you don’t know if the 8% is 8% or 0%.

      If you want to include the leakage amount it would be deducted from the 58% and the 50%, still leaving a difference of 8%.

      The 20% is for fresh soil and does not enter into the calculations.

      • hasuhonere@inaby.com says:

        “Of that 50% was due to the soil under the plant.”
        where are you getting that number from?
        when in your words:
        “Fresh soil (ie no plant or microbes) removed 20%”

        also why not address my other questions:

        1. what do you mean by “fresh soil with no microbes”?

        2. what do you mean by “plan with no leaves”?

        • Did you read the original paper?

          Fresh soil with no microbes is new soil that has probably been sterilized so no microbes.
          Plant with no leaves is the pot where the plant was growing, after the leaves and stem have been removed.

  5. Antti P Nyymi says:

    Just to note.

    It is not the plants which clean the air. Air is cleaned by the root microbes. The plant is there only to provide the proper environment and support for these microbes. Selection of the plant provides the root microbes and therefore the plant type is critical to the cleaning factor.

    And what comes to number of the plants, a proper number would be roughly a 100 per person. So 4 people house would need 400 plants.

    This is not practical with traditional plant growing, unless you live in a greenhouse. Therefore the solution is to enhance the situation by removing soil and accelerating the aerating effect of the microbes by pushing air through the root system.

    Below is the way to do it commercially:

    https://www.naava.io/en/

    • The studies so far, included the activity of microbes in soil and found no effect on pollutants in the air.

      Your product looks good – but the promotion for it is based on lies. It won’t purify the air. If you disagree – where are the studies to show it works?

      Be honest with your customers and sell it on its real merits – ascetically pleasing and people enjoy being near plants.

      • Adrian says:

        My friend sent me the Naava product video, and it seemed to good to be true (pseudo-science). I googled a bit and found your site Robert. I want to thank you for taking your time to debunk these myths!

  6. Thokhir Shaik says:

    Thanks for your information regarding VOC’s. I was searching for some indoor plants to increase the oxygen levels in the house but i was navigated towards the air cleaning plants and i was shocked. I am a biology researcher but the claims about air cleaning by plants was mislead me for a while, after reading your post i recollected some facts like uptake of organic chemicals occurs majorly from roots and most of them were degraded by soil bacteria or symbiotic bacteria present in the roots of plant. Some amount of organic chemicals that plant required only metabolised in plants.

  7. Anthony Dsouza says:

    Hi Rob, I saw some of your comments on the snake plant but wasn’t able to understand the write up however, I would like to know will it help in keeping the snake plants in my bedroom or should I avoid it? Or, is there any such plant that gives oxygen at night so that it could be retained in my bedroom, living room as I feel breathless sometimes in the night.

  8. Kyle McLeod says:

    Hey, thanks for the insight. I was about to spend $200 on plants for formaldehyde removal. That still leaves me with the problem of how to remove the formaldehyde from a friend’s condo unit (it was tested to have .1 PPM where .08 PPM is dangerous. It literally starts to burn your eyes in maybe 15-20 minutes and somehow this friend and her two daughters live there.). So, with that said do you have any suggestions of how to reduce the formaldehyde? It’s winter in Utah currently so it’s tough for them to open the windows and ventilate the place.

    The best idea I have is to keep it cold and dry (supposedly helps. And yes they’ve tried to remove particle board etc. from the place as much as they can and they’re looking hard for somewhere to move. They’ll likely have problems wherever they go as they’re so sensitive to things so they’ll need help regardless).

    Any ideas would be greatly appreciated, they’re hugely needed.

    thanks.

  9. Pete says:

    Hi Robert,

    I am a bit lost on what you have written about voc off gassing reaching an equilabrium? Why does this happen, surely the carpets, furniture etc dont stop off gassing or do you mean that the plants can only absorb so much voc and that then they stop and therefore the air would return to its previous state plant or no plant until fresh air replaced the air and the cycle started again (so equlibrium of voc percentage in air is dependent on how often air from outside removes inside air)?
    The reason I ask is because I take it you are not denying that plants can absorb voc from the air but just that the amount removed would require to large a number of plants in a home environment to be effective. As there are some comercial companies that are claiming specific plants or moss like plants can replicate a household plant 64 to 1 which would then make them more effective for filtering air in a home or office environment.

    I havent seen mentioned anywhere in the filtration claims as to where the voc compounds go once within the plant/soil. At some point without regular replacement maybe the plants/soil themselves would off gas the voc they had initally taken in anyway?

    • Many, if not all chemical reactions reach a point of equilibrium. Consider this simple example. You have a glass containing dry air. You pour some water into the glass. The liquid water starts to evaporate. As this happens, water vapor starts to fill the glass and then it starts to move out into the room. If you measure the amount of water vapor you find the most right above the liquid water, less near the top of the glass and even less at the other side of the room. As the amount of water vapor above the liquid increases, it slows down the rate at which liquid turns to vapor. As the water vapor in the room reaches high levels, the rate of evaporation becomes even slower and at some point it stops. The evaporation of water has reached an equilibrium.

      But even before it reaches equilibrium, the rate of evaporation slows.

      In homes we have an additional factor. Air moving into the home is constantly refreshing the air. As some air leaves the home it also takes VOCs with it. this rate of removal is greater than the removal by potted plants. So the plants, assuming a small number, don’t affect the concentration of VOCs in the room.

      When plants and microbes absorb chemicals two things can happen. They can accumulate in the organism. This happens with heavy metals for example. Plants can be grown in contaminated soil and they accumulate the contamination. The other thing that happens, especially with organic molecules like VOCs, is that the organism digests them. They are able to break up the molecules into other molecules which eventually end up as CO2 and Water and maybe some nutrients. Bacteria are particularly good at this.

      Your comment “plants/soil themselves would off gas the voc” is interesting. This may happen. What does happen for sure is that plants actually produce VOCs. These are not the VOCs from the house furniture, these are new chemicals made by the plants. I have not looked into the amount they produce, but it is probably fairly small.

  10. Han says:

    You mentioned that “The idea that plants can clean the air in your home is a myth” yet you do not deny that plants do absorb pollutants from the air, isn’t this contradictory? I understand that you believe the percentage purified is too small, I personally think 8% is a good amount I’m better off without. I do understand that the point of the post is to debunk the myth that plants purify air to a maximum of 90% but in the same time I see from the comments that readers are now reconsidering getting plants in their homes, those plants actually have other benefits one of them as you mentioned it makes some people happy. I think to have a low maintenance plant that absorbs 8% pollutants and increases oxygen levels – let’s say just 5% so you don’t mention it could be too small 🙂 – and it also makes me happy is a wonderful deal! I appreciate if you add a part in your post to mention the benefits of having plants at home just so some people are not turned off by this. Thanks for this post

    • I have discussed this in other comments.

      The 8% is the number reported in the study. Since no statistics were done we don’t know if this is 8% or 0%. But that is not the point.

      In a home, the amount of VOCs produced is higher than what the potted plant removes, so the level in the home is the same with or without plants. The main reason VOC levels stay low in homes is that fresh air enters the home from outside.

  11. Mark Borders says:

    I meant to say- since plants take in carbon dioxide.

  12. Mark Borders says:

    I found this website while searching for an answer to the question- do plants do better with more people in the room since plants take in oxygen?

  13. John says:

    Seems odd to suggest it’s not worth removing VOC’s from your home since more will just be created. Wouldn’t VOC’s just continue to build up if not removing them at all? I’d rather make some effort to remove harmful chemicals than just give up because I can’t get them all. Also, you mention that its the soil that actually filters the air not the plant, and the plant only increases the effectiveness, so that still doesn’t seem like a bad thing?
    Studies aside, common sense dictates something must be removing VOC’s from the air right? Else at the rate we pump them out we’d all be dead by now surely? So if not the plants, what is cleaning the air?

    • Actually, having VOCs in the air will slow down the release of more VOCs from material, as things reach an equalibrium. I have no idea how effective this is.

      Homes are ventilated and most VOCs are lost to the outside air. That is the main reason they don’t build up to dangerous levels in the home.

      It is not the soil that removes them – it is the bacteria in the soil. What the studies show is that the whole pot – soil, bacteria and plants – have no effect on VOCs.

      • John says:

        I’m having some issues how these statements aren’t contradictory…

        “90% was only found for one plant type out of 12, and for only 1 pollutant out of 3. And it was not 90% – it was a value somewhere between 50 and 90%”

        “Plant with leaves removed 58%
        Plant without leaves removed 50%
        Fresh soil (ie no plant or microbes) removed 20%
        Leak Test (nothing in the chamber) removed 7%”

        “What the studies show is that the whole pot – soil, bacteria and plants – have no effect on VOCs.”

        So the first statement you’ve said that at least one plant removes 50-90% of at least one pollutant.
        In the second statement its clear that a plant with leaves removes more of a pollutant than a plant with no leaves by 8% and removes more of a pollutant than no plant at all by 51%.

        So how do these not contradict your statement that plants have no effect on VOC’s?

        If your argument is that the VOCs are replaced as quick as plants can remove them, that is different than saying plants don’t remove VOC’s from the air.

        • All of the numerical numbers are for plants tested in small chambers. None of that data applies to a home. In addition to the quotes you used, I also said that “There is no indication in the study about the accuracy and repeatability of the numbers. Statistically 58% and 50% could be the same (ie not statistically different) in which case plants removed no benzene”. So in fact the study does not show that plants reduced the VOC by 8%.

          I don’t see the last quote in this post. What i did say was “As reported previously in Air Purifying Plants – Do They Work?, further research by others, in field conditions (ie office buildings), have not shown any changes in chemical levels due to plants”

          The study that did look at the effect of plants on air in a building found no change in VOC’s

  14. Alex Murray says:

    I not only found your post well reasoned and well laid out, but I thought you handled questions and rebuttals coolly and gracefully. In short, we all need to be prepared to do a little bit more research ourselves!

  15. Alf says:

    Valuable clarification. Thank you for the analysis. But the language goes too far and edges into the realm of hyperbole, becoming somewhat tortured in the process. Two examples:

    1. ‘Plants don’t purify air.’ Incorrect. The studies analysed shows that they do. When push comes to shove, the post retreats into qualifications like ‘in the home’ or even ‘in the typical home’

    2. The NASA study finds an 8% difference between ‘plants’ and ‘plants without leaves’, which you interpret as ‘plants contribute only 8% to detoxification’. Strange definition of the word plant! Does a deciduous tree cease being a plant when it has lost its leaves?

    Finally, absence of evidence does not mean evidence for the negative statement. The proper conclusion, if you accept all the hedging language of the post, is that we don’t know if plants purify air (in the home). They may purify it, or they may not. You cannot conclude, on the basis of the arguments in this post, that plants do not, because you have not demonstrated this. Negative statements need their own evidence.

    • Plants don’t purify air in the home – the blog is written for home gardeners. Agreed some liberties are taken in the title, which by necessity needs to rank high on Google.

      In the study the leaves of the plant were cut off, the roots were left, so it is a plant without leaves.

      The conclusion is not based on just this one study. “As reported previously in Air Purifying Plants – Do They Work?, further research by others, in field conditions (ie office buildings), have not shown any changes in chemical levels due to plants. “

  16. Tatu says:

    This is an interesting topic and I spent a few hours reading things. Naava.io is an air purifier using plants (like naava, I’m from Finland but I’m not affiliated with them) – at their website under Science, they list this paper: http://www.sciencedirect.com/science/article/pii/S0360132310003008

    and if you don’t have Access to Sciencedirect you can have a look at this: http://surface.syr.edu/cgi/viewcontent.cgi?article=1062&context=mae_etd which is basically the same article (I had a look at the original paper too since I still have my Uni credentials to Elsevier).

    One of the most interesting notes in that paper: “…DBAF had high initial removal efficiency for formaldehyde and toluene even without plants in the bed.” — “While the moisture in a wet bed had the scrubber effect for water-soluble compounds such as formaldehyde, presence of the plant increased the removal efficiency by about a factor of two based on the results from the reduced-scale root bed experiments.”

    (DBAF stands for Dynamic Botanical Air Filtration system.)

    Robert, did you know these articles before? What do you think of them?

  17. manoj kaushik says:

    Dear Sir, I am new to airpurifiers, your study is well researched and the best thing is you have given reply to all the questions very authentically,we need logical people like you to make earthe a better place
    regards
    manoj kaushik

  18. Great job. This just teaches me to google: (anything) scepticism.

  19. Chax says:

    This is silly… of course plants improve air quality. Our own biology relies on that fact.

    • I guess you did not read the post! In fact our bodies are quite good at dealing with VOCs and other chemicals on their own without plants present.

      • Chax says:

        I read it… but you’re just click baiting with the title. Our bodies ability has nothing to do with the fact that plants do in fact clean the air lol, I mean have you never slept in a woods? I don’t even need science to confirm the obvious here.

  20. Sean Petrash says:

    I appreciate the diligence level and you will have spent more time researching this than I would ever care to.
    My question is two fold…
    You acknowledge that the VOCs were reduced by the soil but not the plant?
    I don’t have much experience with microbes but in the little bit I’ve learned, microbes can metabolize just about anything. It stands to reason that these microbes that are living co-symbiotically with the plant can also metabolize the VOCs? Different plants put off different levels of chemicals and thus have different microbiota and biofilms.
    So are you saying specifically that plants don’t filter air but microbes do?

    • Not really. A potted plant does remove VOCs. Most of that metabolism is due to the microbes, but the plants also contribute.

      The issue is that the amount removed per potted plant is small compared to what is present in the room. As potted plants remove some VOC, the material in the room is just replacing it, so that potted plants don’t affect the concentration of VOC’s in the room.

      • Sean Petrash says:

        But as I understand it we don’t have data on the volume metabolized or the rate. We need some more science!

        • The NASA study did measure the rate of metabolism. Not sure what you mean by “volume metabolized” – the volume is unimportant.

          So far I have only found one study that looked at a field (ie office) situation. More data would be better. So far we can say that there is no evidence that plants reduce VOCs in the home.

  21. Steven Costabile says:

    So, let’s shelve the term “air purifier” and talk oxygen production. which indoor plants, that require little to medium amounts of light, produce the most amount of oxygen? Do any plants produce a significant amount more than any other indoor plants? And all that being said is it reasonable to think (assuming I have an almost airtight environment) that I can increase the oxygen concentration by 10% or more? and if that’s possible, you know I gotta ask, how many plants of that type would it take to accomplish this goal with a 9500 cu. ft area?

    • No idea. The article is not about oxygen levels.

    • apv says:

      No. Most obvious reason, putting an oxygen tank equal to 10% the air volume of the house wouldn’t raise the level 10% because your house is not airtight—good thing too, you’d die if it were—and gasses like to equalize.

      The majority of the oxygen you breathe doesn’t come from trees or terrestrial plants at all. We could cut down every tree in the world and probably not notice—it would be measurable but just breathing, you probably couldn’t tell—a change in oxygen levels. The oceans provide most of the oxygen we breathe.

      The amount of oxygen that comes from houseplants is miniscule. Wall to wall plants on the walls, and most of the floor might do what you want but it would take more space than you posit, you’d need thousands of plants.

      What you want is probably a bad idea even if you can work it out. Oxygen is toxic on a few fronts. Like chemists say: the dose makes the poison. We’re evolved and adapted to the levels that are currently available and, long term, we deal with less much better than we deal with more.

  22. Sana says:

    I was excited about planting trees in my house and having a one on my computer desk, but right now I think its not much needed as they don’t purify the air around us. What do you say? Do I need to have a one in my home and one on my desk?

  23. Budd Margolis says:

    You should remove this article as it is filled with misinformation. An indoor plant’s ability to remove these harmful compounds from the air is an example of phytoremediation, which is the use of any plant — indoors or out — to mitigate pollution in air, soil or water. Indoor plants remove pollutants from the air by absorbing these gases through their leaves and roots.

    • Maybe you did not read the article? I never said plants do not absorb gases from the air. What I did say is that they do not purify the air.

      If you have a reference that shows a reduction of VOCs in homes or offices, I’d like to see it.

  24. Mark Hill says:

    Actually some of your reasoning is flawed. Take the benzene removal as an example. Whether the plant has leaves or not is of no consequence. The difference between virgin soil and the plant is 30% and the plant WITH leaves intact 38%. That difference is huge.

    Of greater interest would be an analysis to determine HOW the plants absorb the benzene and what they do with it once they’ve absorbed it.

    Based on that, you’d have a better idea of the long term effects on household pollutants, which are replenished to a degree after every application of the contributory products.

    • Which reasoning is flawed? using your numbers only 8% of the benzene was removed by the plant. That is not ‘huge’. If I remember correctly, most of the plants did not even show that amount.

      The statement “household pollutants, which are replenished to a degree after every application of the contributory products” is not correct – assuming i am reading it correctly. The household products that produce these chemicals leach them for a very long time after installation. It does not depend on new applications.

  25. Al VM says:

    Dear Robert, I hope you read this paper and perhaps consider to make some review of your previous post.
    Research was done before 2007 by the UTS Faculty of science, in Australia, showing that, also in real world-office conditions, potted-plant microcosm (PPM) worked very efficiently in doing VOC removal.
    So, where’s the myth then?

    http://www.inive.org/members_area/medias/pdf/inive/iaqvec2007/tarran.pdf

    • I did have a look at the study. Was this paper published? The reference does not indicate it was published.

      The section of most interest is “POTTED-PLANT VOC REDUCTION IN THE ‘REAL-WORLD’ – OFFICE STUDY”, but all that is provided are the conclusions. Where is the data and the experimental setup information? Without that we can’t really evaluate the conclusions. I would be very interested in seeing the full published paper for this section.

      • AL VM says:

        This paper was published at the “Proceedings of Sixth International Conference on Indoor Air Quality, Ventilation & Energy Conservation in Buildings – Sustainable Built Environment, Oct 28-31, 2007,Sendai, Japan, Volume III, 249-256”

        Here you have the link to the Researchgate:
        https://www.researchgate.net/publication/228639007_Use_of_living_pot-plants_to_cleanse_indoor_air-research_review

        You have reason about the most interesting part concerning to real world – field study.
        As long as that paper I posted before was a research review, it mentions the research done by Ronald A. Wood et al. at the University of Technology Sydney (UTS) which was published as “THE POTTED-PLANT MICROCOSM SUBSTANTIALLY REDUCES INDOOR AIR VOC POLLUTION: I. OFFICE FIELD-STUDY” (Water, Air, and Soil Pollution. September 2006, Volume 175, Issue 1, pp 163–180)

        Here you have the link to its Researchgate information:
        http://link.springer.com/article/10.1007/s11270-006-9124-z

        and here you can download it for free:
        http://www.urbangarden.co.nz/media/1286/potted-plant-microcosm-office-field-study.pdf

        I hope this time you have enough data to contrast.
        Kind regards,
        AL

        • I have reviewed the last reference you provided. My conclusions are:

          This study was done using three buildings at the University of Technology, Sydney, Australia. Zero, 3 or 6 plants were put into various small offices and total VOCs were monitored over time. There was no reduction of VOCs in any of the buildings as a whole. Building 1 was an administrative office building and there was no reduction of VOCs in the offices with plants.

          Building 2 and 3 were mixed use offices containing both offices and laboratories. Some reduction of VOCs occurred inside the offices with plants. There was no difference between using 3 or 6 plants and the reduction was only seen when the offices had high levels of VOCs present. These high levels were above that found in a normal home.

          In conclusion, the study indicates that adding such plants would have limited effect, if any, in a home with average VOC levels.

          It is not clear why buildings were chosen that contain laboratories? One can expect them to add extra VOC to the air of the building. The results show that the office-only building did not see a reduction of VOCs. The effect of the laboratories is not discussed.

          For the majority of the study, VOC was measured using a photoionization detector. This is a quick way to measure total VOC but it does not provide a good understanding of which chemicals were present and which were reduced. Photoionization can also measure non VOC gases. It is surprising that proper air samples were not taken and analyzed by GCMS.

          VOC in the total building is not discussed or measured. The report does say that the values found in offices with zero plants were the same as monitoring in the 4 weeks prior to plants being added. One can infer from this that the air in the general office building was unaffected by the plants. So this study does not show that plants purify the air in the building. It essentially tests larger containers than were used in the lab studies, namely the offices.
          No information is provided for the offices. Were they occupied? How much? Were doors kept closed? Etc. We only know an approximate size of 11 sq m (120 sq ft). No individual office data is presented.

          Building 1, the office-only building, did not see a statistically significant reduction of VOC. Building 3, containing labs and no air conditioning, did see a reduction when VOC levels were above 100 ppb.

          No difference was noticed between using 3 or 6 plants, which is unexpected. The explanation is that 3 plants were enough to reduce VOC to a level, where plants were no longer functionally removing VOCs. The argument is made that below the 100 ppb level enzyme induction is not initiated in the plant or soil bacteria. For this to be true, the induction level would need to be the same for all the VOCs and for all the organisms involved. It seems more likely that this lower level would vary based on the VOCs present, and on the organisms present. Since there is no data on either I believe that the explanation in the report is mostly speculation at this point.

          Assuming the 100 ppb level is real for most combinations of VOC and organisms, what does this mean for the average home? The average home VOC level is 150 ug/sq m, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4057989/. Assuming an everage molecular weight for VOC (120), 150 ug/sq m would be 30 ppb. This value is well below the 100 ppb threshold which means that the plants would have no effect in removing VOC from the average home.

          Most buildings will have TVOC levels ranging from 100-500 µg/m3 from http://www.critical-environment.com/blog/know-the-air-you%E2%80%99re-breathing-volatile-organic-compound-2-of-4/. Plants would not have an effect on reducing VOCs for most buildings.

          The major problem with this study is that it looks at very small rooms inside an office building. Any effect that was found does not translate into the whole building. The study does not show that the VOC levels in a building can be reduced by potted plants.

          An additional review by Girman Phillips be found here: http://www.buildingecology.com/articles/critical-review-how-well-do-house-plants-perform-as-indoor-air-cleaners/at_download/file

      • David says:

        I think you are doing a great job answering to people who have not read your article, or present information that might not be valid or published in any Scientific journals etc..

        After reading the NASA article, I was going to buy plants to help remove some of the contaminants in my one bedroom apartment, but thanks to your article I’ll just buy them for their beauty.

        Actually after reading your article I had more questions like:
        How many plants would I need per person to remove toxins in the air?
        I found that it would be to many to make it practical. Some say 10 trees per person.

        I think I’ll just open most of my windows a bit – that’s like having ten trees filter my place.

  26. Sheila Hunt says:

    I was hoping this info was correct!! I only have one lung and am looking for ways to increase and purify the oxygen in my home. Do you have any suggestions besides just an air purifier? Thank you:)

  27. artpodblog says:

    Its not a myth that plants create oxygen and consume carbon dioxide, thats primary school level science.

    • You are correct. But this post is not about oxygen levels. When people claim that plants clean the air they are referring to the removal of organic compounds, not increasing oxygen levels.

      Maintaining oxygen levels in most homes is not a big problem because the homes are not very air tight.

  28. J says:

    So all the studies quoted here are misrepresented, or bogus? https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3230460/#__ffn_sectitle

    • Why do you say they are all bogus?? I did not say that!

      There are 17 references. Based on the titles, 11 have noting to do with the topic of plants cleaning air. #4 is the NASA study I reviewed – it does not support the idea of plants purifying air. #17 is a book based on the NASA study.

      That leaves 4 articles that were not reviewed in this post and that deal with the topic. I don’t believe I have made any comments about these 4 papers?

      Ref 11 – tested potted plans in chambers – essentially the same experiment as the NASA study. For the same reasons as the NASA study it will not show that plants will clean air in homes.

      Ref 8, 12, and 16 sound like similar tests. In fact the title of ref 16 suggests plants produce pollutants.

  29. Kathryn Alma-Nihte says:

    Oh No!! I was so wanting this myth to be true! I also heard that Magnolia trees suck up exhaust from the cars and also pump out a fair amount of oxygen. Is this also a plant-cleaner myth? Is there any plant that in fact does eat up pollution?

    • All plants pump out oxygen. Many plants will also absorb pollutants from the air and from soil. The problem is that the amount they remove is so small compared to what we create that it does not have much of an impact. It is also important to ask what the plant does with the pollutant once it is absorbed. Some chemicals will be broken down into safe chemicals. But others are not.

      Consider the magnolia. Assume it absorbs car exhaust. That means it absorbs lead. But when the leaves drop in fall, lead will still be in them. Lead is not converted to anything else. If you now use these leaves in your vegetable garden you will be adding lead to your vegetables – not a good thing.

      • Kathryn Alma-Nihte says:

        Good thing gas is unleaded. But yes, it is a problem for sure, when a chemical is absorbed it never just disappears. I imagine there are more toxic chemicals we know nothing about in gasoline. Also, it bodes unwell for so many pollinators sucking up nectar that is laced with chemicals. Thanks for your reply!

      • I thought it was illegal to put lead in gasoline since the 80s. How can car exhaust still contain lead?

        • Just because we don’t add lead does not mean it does not contain lead. Do you have a reference to show it is lead free?

          • I was asking you what you are basing your claim that unleaded gasoline 1) contains lead and 2) contains lead in high enough levels people should be concerned about eating food grown from soil fertilized by vegetation which grows by the road (which I guess also means we should be concerned about lead poisoning from any food grown near the road…). If you can’t answer how you know unleaded gasoline contains lead, then just say so.

          • What I said in reply was “Just because we don’t add lead does not mean it does not contain lead. Do you have a reference to show it is lead free?”

            So I assume your either did not find a reference to support your position, or you did not bother to find one.

            Here is mine: “First, the lead content of gasoline was fixed at a maximum of 0.4 grams per liter in 1981. In 1985, the EU mandated that unleaded gasoline levels of 0.013 g Pb/l be available in all member countries.” http://www.worstpolluted.org/projects_reports/display/66

  30. debwain says:

    I’m all for debunking myths but I’m also for accuracy & editing. You’ve got a few typos in this article which make it inaccurate–at one point you have “plants growing soil” instead, I assume, “plants grown in soil”. But more importantly, the date of your NASA reference is wrong. The paper is dated 1989 not 1998. Big difference & important referencing information.

  31. Janet says:

    “Understanding how a myth is born can be very educational. It provides insight into how the human mind works. It also trains you to spot future potential myths and to better evaluate the information you are reading. So let’s celibate this holiday by peering into the start of a myth.”

    Ha ha, apparently your mind thinks there’s too much sex on holidays! I trust you meant “celebrate”, not celibate).

    • Yes, it is inaccurate. But if you find a study that supports his argument – please post.

      In the report, according to the TED video, they have 1,200 plants, waist or shoulder high in a 50,000 sq ft building – but no indication how or where the plants are located.

      The claim is that the plants increase oxygen. No one doubts that. But no data was presented to show how much the oxygen levels went up.

      In the video there is no claim that the plants in the test reduced pollutants. There was no mention of even measuring them. although in the introduction the speaker implies that this takes place.

  32. D T Nelson says:

    You are a little off in your assessment of the report as well – especially of needing 25-50 plants per 100 sq. ft.. If you look closer at the NASA report, you will see that, essentially, the plant (with the activated charcoal in the potting soil) is acting as a filter in a machine that pulls in the surrounding air, through the soil, and pumps it back out. It is like all the air cleaning machines out there, only it is using a plant with soil containing activated charcoal in the soil as the machines filter. No need for a lot of plants. But it would require several in each room to cover all the various contaminates. It would also suck up a lot of electricity.

    • The NASA study looked at regular pots as well as special pots containing charcoal. My post says nothing about the charcoal pots since most people don’t use them or have them available.

  33. IG Stark says:

    What about the snake plant? I believe it purifies the air, no? Even if a little bit, that’s better than nothing. I would like to put about 5 large snake plants in my bedroom to purify the air. I read that they give off oxygen at night. Is this true? Please let me know if they are at all dangerous to keep in the bedroom. Thanks!

    • Plants give of oxygen during the DAY, not at night. At night they produce CO2.

      Plants remove very few pollutants from the air. The microbes in the pot remove more, but the problem in homes is that the amount of pollutants the home gives off is larger than what microbes absorb. If you really want to reduce pollutants in the home – change the way you live, and what you use to in the home. Get rid of carpeting, artificial wood products, and all solvents.

      • D T Nelson says:

        Snake plants are unusual in that they give off oxygen (and take in CO2) at night rather than the day – the opposite of other plants.

        Yes, I was looking at getting plants to help clean the air in my house until I read the NASA report and saw the claims were a load of hogwash. I’ll still get plants, though. But only because they increase oxygen, absorb CO2, increase humidity (very dry air where I live), and look nice.

        • Snake plant uses CAM processes for photosynthesis. They close their stomata during the day so they can’t take in CO2 during the day. Instead they do it at night and use it the next day for photosynthesis – which only happens in light. The oxygen produced is stored and released at night but the amount released is quite small.

          Plants also use a process of respiration where they turn sugars into energy, and this requires oxygen. This happens day and night. CAM plants efficiently use the oxygen produced in photosynthesis for respiration – releasing only the excess.

    • tom says:

      It likely gives off carbon dioxide and water at night, while the plant respirates.

  34. Mainer says:

    Interesting analysis- but I’d suggest some further reading. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/of-39.pdf
    Note the lengthy list of organizations and papers referenced and extensive studies done on many plants WITHOUT activated charcoal.

    As to the grape whipping- super cropping or stressing plant stems to increase size, strength and nutrient moving potential is well known. If you don’t think it works- try it.

    I’d replace some skepticism with curiosity … there is wisdom in experience..

    • The reference you have given is just another source of information that is presenting miss-information. It has no data in it to back up the points it is making. So I had a look at one of the key references: http://www.wolvertonenvironmental.com/air.htm, which is an engineering company reporting information, again without any data. It refers to the older NASA study I discussed as well as a newer one about the Biohome – it provides no references. But based on it’s report, the Biohome studied plants crammed into an artificial small chamber – not anything close to the situation in a home.

      If you a reference to a study that measure pollution reduction in a home type setting – I would like to see it.

      It is true that ” there is wisdom in experience”. But experience also breeds misinformation.

    • Tom King says:

      >I’d replace some skepticism with curiosity

      I think you missed the point of this well-written article. Skepticism defends you from the false ideas that your curiosity discovers.

  35. Roger Brook says:

    What a brilliant analysis of how honest scientific research is garbled and exploited by the ignorant.
    In this case the research itself was not too hot!

  36. tolga erok says:

    Brilliant info. Thanks for sharing your knowledge. It made alot of sense as i always thought this was indeed a myth

  37. Nic Combrink says:

    I agree, there are many myths in agriculture. What about this one; that a non-fruit-bearing young Avocado tree can be forced to bear fruit by hammering a nail into its stem? I am no horticulturalist but realise that seed produce trees with a long vegetative period. My theory is that the stem of a seed-tree will be thick and strong enough to be nailed and that this will also be the stage where the tree will naturally change from vegetative to reproductive. Your opinion?

    • I have never heard that one before. Your explanation sounds logical and is a good example of how some myths are born. You have a problem, and you wait for it to fix itself. It doesn’t. So you try something, on your only tree. It works–you suddenly have fruit. You tell the world that a nail solved the problem.

      In this case the nail had nothing to do with solving the problem–the events of having fruit and hammering in a nail are unrelated. That is one reason scientists always use controls–lots of trees in the same study.

      It is also quite possible that it is an old tail that no one ever tests.

      I have heard that if you whip grape vines with a chain they produce more heavily. This came from a professor in horticulture who I do respect. He says the practice is quite common and has been ‘scientifically’ shown to work. It is on my list of possible myths for me to investigate. So many myths–so little time.

    • Rob says:

      Great info. Very helpful.

    • Will Wheelahan says:

      Avacado trees are interesting, I read that if you cut some branches off they “sometimes” stop producing for two years. Mine did. Trauma, saves energy?

      Maybe the nail is something like tree responding to fear of death, or iron in nail or something?

      It was obvious the research is being used and abused or lacking. So came here to work out the truth and find some good research articles. Come on fellas.

      I have always thought the plants would out compete the black bacteria (mildew) that grows in dust from car tyres etc. So do rubber plants improve or worsen the rubber problem ( my guess is they eat a fair bit of the rubber dust for you before it settles). Someone must have done some good research on this? My theory is if you do not vacum that dust, it is no good, because the bacteria gets a fair way to producing LSD.

      Just do not rub the leaves, plenty of morons cleaning leaves without gloves on youtube. ;op . Hose in yard or give them the odd shower.

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