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
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.
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
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.
Thanks – changes made.
“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).
Thanks – change made.
So, does that mean this is inaccurate as well?
http://www.ted.com/talks/kamal_meattle_on_how_to_grow_your_own_fresh_air
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.
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.
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.
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.
It likely gives off carbon dioxide and water at night, while the plant respirates.
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.
What about this guy? He’s doing research with air quality and several hundred people.
https://www.ted.com/talks/kamal_meattle_on_how_to_grow_your_own_fresh_air?language=english
I am in talks with Kamal Meattle – there does not seem to be a real study that shows plants reduce pollutants. His building is much more than just a bunch of plants. It also has a sophisticated air cleaning system. he conveniently forgot to mention that in his TED talk.
He is also not doing research. He does things in his building to try and improve the air and other people may or may not do any testing. So far he is unable to provide even a single such test result. When he does I will create a separate post on this topic.
>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.
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!
Brilliant info. Thanks for sharing your knowledge. It made alot of sense as i always thought this was indeed a myth
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.
Great info. Very helpful.
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.
I really need to find something that can clean the air in a 500 sq ft space and just a store bought air purifier is not doing the job so do i buy activate carbon soil or what? Chemicals make me sick and unfortunately we can’t stop others from spraying them or wearing them. Any suggestions?