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
Hi Robert,
I enjoyed your article and agree with your findings, I do however believe that the process of phytoremediation for cleaning air can be very important in our homes and commercial buildings if it is done properly. As you have pointed out its all in the soil. Commercial applications of phytoremediation is being done every day for all kinds of chemical spills in the ground so the science works, its the application in most systems that does not for air remediation. Both Wolverton, Darlingtons systems and (Naava) which is really Wolvertons system rely on artificial substrates for the plants and do not produce dynamic bacterial colonies that soil can offer, this is where the money is with the process. I run a company called AgroSci http://www.agrosci.com and developed the IP for the company where we use a process called Aerogation that directs a small amount of moisture with air directly into the root zone of the plant, this exposes all the air directly to the root system and does a remarkable job of removing toxins, particulates and odors. It is slower because you cant move huge amounts of air through soil but over time it adds up and the air is cleaned. We have research done by Professor John Dover at Staffordshire university as a pass through test, air brought in the system and moved through and out we got very good results, I would be happy to share it with you. You are absolutely correct chamber tests do nothing to show how a plant can clean air in your home, it would be equivalent to putting an oak tree in your living room. I know our system is working because we have done the research and because we do not need to add fertilizer to the system, there is enough in the air, particulates, chemicals going into our system that is being consumed by the bacterial colonies, that grow, flourish, and die off leaving organic material for the plants to consume. The plant adds beauty, and most importantly provides bacterial inoculation for system to break down the pollution. The soil food web is working over time here, sugars created from photosynthesis feed the bacteria which grow in around the roots zone, migrate out into the Aerogation channel as chemical compounds accumulate in moist oxygen rich environment, colonies grow and adjust to the particular compound in the air specific to the environment. It is a beautiful and functional way to use the plants we all LOVE!! in the end, that’s what its all about!
I have written an updated post to this topic but it is not posted yet. The conclusion does not change. I have been in touch with Dr. Stanley J. Kays, who has done some work in this area, and he too feels that when air is moved through the soil, it might result in a way for homeowners to use plants to clean their air.
The downside of any such system is the need for special pots and electricity to run them.
But there is also a differnce by using soil and hydroponics, right?
I don’t think the effect of the plant would be any different – why would there be a difference?
If you compare a potted plant and one in hydroponics there would be a difference since you lose the effect of the microbes.
Hi Robert,
Some power is needed, but it’s low, our fans even for a wall that’s 10 by ten use a fan that’s about 25 watts, lighting for a wall that size with an LED grow light uses about 120 wats, a wall this size can run 50+cfm directly through the root zones of about 400 1300cc plant containers, removing all particulates, odors and 60+% of VOC’S per pass according to our research, 24/7
Mark
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.
Lol, I’m not sure why you consider the experiment chambers different vastly different from a home setting, unless your home is in space. The point of the article is that there are some purifying benefits from the introduction of a plant system. Nobody or very few people are going to be interested in placing a pot of microbial rich soil in their homes. A plant on the other hand, is very attractive. So by introducing the plant into the home, they are also introducing the soil cocktail which you tout as so effective. Either way, it’s hard to deny that as a result of the introduction of a plant(including soil) into the home environment, positive benefits would be achieved. This doesn’t seem like a myth, even after reading your article.
Your conclusion is wrong. There is no evidence that there is any benefit to air quality by putting plants in your home.
Hi Robert,
After looking at several comments and your attempt to address them, I think part of the problem is that most people commenting here do not understand the concept of equilibrium, especially when it comes to concentrations of gases in their home. I believe that most of the exchanges distill down to this: someone says, “Aha! The plants DO remove VOCs, even if it’s just a little, so you’re wrong!” and your reply is that the effect is miniscule and in an open environment. Perhaps there should be an article about how the reduction in VOCs is made up by the off-gassing of everything inside the home and the exchange of air with the outdoors.
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
It is good to keep them there, but they won’t improve air quality.
What about others who used the scientific method regarding mold or other hazards? Those are some pretty decent numbers. We don’t have to depend on NASA from the 1960’s or whenever they did that test.
http://pennstatehershey.adam.com/content.aspx?productId=35&pid=35&gid=6015
This reference does not provide a link to the study so we do not know if it was scientifically. We don’t have any of the data or the methodology used, and it was done by a teenager. I am not convinced it means anything.
“It is good to keep them there, but they won’t improve air quality.”
It really sounds like you’re throwing the baby out with the bathwater.
It seems like when investigating as large a question as “Do plants improve air quality?”, one really needs to read more of the relevant research in order to make an informed statement/debunking.
——————
A sampling of relevant research:
https://ia800300.us.archive.org/35/items/nasa_techdoc_19930072988/19930072988.pdf
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930072972.pdf
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120003454.pdf
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080003913.pdf
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910004539.pdf
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910014740.pdf
https://www.naava.io/science/plants-and-soil-microorganisms-removal-of-formaldehyde-xylene-and-ammonia-from-the-indoor-environment
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3230460/#r10
https://www.ncbi.nlm.nih.gov/pubmed/21641719/
https://www.ncbi.nlm.nih.gov/pubmed/15325884/
http://www.sciencedirect.com/science/article/pii/1352231095004653?via%3Dihub
http://www.wolvertonenvironmental.com/Plant-Physiol-94-Giese.pdf
https://greenplantsforgreenbuildings.org/wp-content/uploads/2014/09/Formaldehyde-removal-plant-species.pdf
https://greenplantsforgreenbuildings.org/wp-content/uploads/2014/09/Screening-Indoor-Plants.pdf
http://www.sciencedirect.com/science/article/pii/1352231095004653?via%3Dihub
https://pdfs.semanticscholar.org/227b/9de7342fac9865cfde5bdf97bcb51427d3ee.pdf
https://www.researchgate.net/publication/297404016_PURIFICATION_EFFECTS_OF_PLANTS_FOR_INDOOR_AIR-POLLUTION_IN_A_UNIVERSITY_HOSPITAL
https://greenplantsforgreenbuildings.org/wp-content/uploads/2014/09/Dela-Cruz-2014-review-on-phytoremediation-with-indoor-plants-2.pdf
“The results of the reviewed literature document that plants in
general are able to remove VOCs.” – Cruz et Al.
———————————
Further, IMHO, it’s a bit of a leap to say “There’s insufficient evidence that directly applies to this particular application, therefore there is no effect/benefit.”
A ton of legitimate science is built on indirect evidence. Pretty much all of chemistry, for instance.
I, personally, find the metabolic cell culture study’s radioactivity evidence to be significant evidence that indicates the potential for plants to play a role in the oxidation of formaldehyde. That doesn’t mean I’m espousing any recommendations for air quality, but it does mean I’ll keep growing my indoor plants, and even consider adopting a few more.
Perhaps you are frustrated by the lack of intellectual honesty and science literacy among folks and how the media uses that to manipulate the worldview of large groups of people? (Me too!)
As a science educator, I really appreciate what you are trying to do on this site, but I’m also concerned about how you’re misrepresenting the actual science by failing to research (or explain the research accurately). We’re all full of bias, but you’re writing as if you’re the exception, and that’s not very intellectually honest.
Thank you for posting the list of references. Once you have collected such a list it is important to review each reference. I am not sure you did that or you would not have included all of these.
There are 18 references.
1) 9 are not papers published in scientific journals, so they have no peer reviews before being published. One is just a website.
2) Several are just reviews of past research and one is just the back ground information of one of the authors.
3) #15 is a duplicate of #11, which deals with particulate matter not chemical pollutants.
4) #10 deals with mercury, not exactly a home pollutant.
5) Three of them deal with a special NASA BioHome- any results from these would not be transferable to homes we live in.
6) #6 has nothing to do with plants or chemical pollutants?
Out of the 18 studies only one deals with an actual home or office setting – #17. As discussed in my post, you cannot extrapolate data obtained in a small closed chamber to a home or office. I never said plants can’t remove pollutants in such special situations. What I said was that they do not clean the air in the home.
What this means is that out of the 18 references – only one is relevant to the discussion.
Reference 17 looked at VOC levels in a hospital setting. If you want to prove plants clean the air in our home this is a very poor selection of site. Hospitals are full of all kinds of chemicals not normally found in homes or offices.
Figure 3 shows that VOC levels were equal to or lower before plants were added, at all locations tested. Figure 4 is for all floors, and basically shows inconclusive results. Since this study has no statistical analysis it is very hard to know what the data shows.
But lets look at the researchers own conclusion. They had used two different sensors and the differences between them is not fully understood, as reported in the conclusion. There is NO statement that concludes plants cleaned the air. In fact it concludes more studies are required, ” It will be necessary to investigate the purification capability of plants …. in more detail”.
Based on all of the references you supplied, not one supports the idea that plants clean the air in our home.
Thanks. Would you also edit https://en.wikipedia.org/wiki/NASA_Clean_Air_Study based on your knowledge? It would be helpful, I think.
I’ll look into that – good suggestion.
“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.
“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.
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.
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!
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.
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.
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.
Sorry – no idea.
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.