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 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.”
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.
The idea that microbes can remove VOCs has been exploited by Neoplants who has developed GMO microbes that are more efficient than regular microbes in removing VOCs. They are featured in their new product Neo Px
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://www.wolvertonenvironmental.com/NASA-Report-89.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
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.
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.
If you don’t want to know the facts, then you are correct – you don’t need science!
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.
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.
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.
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.
The ocean produces oxygen due to the plants that live in it.
True – but that is not the subject of this post.
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?
Plants in the home do make some people happy – get them for that reason.
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.
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.
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.
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
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.
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:)
I don’t sorry. Plants will do no harm and they do produce oxygen.
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.
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.