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”
“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.
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.
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 23was 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.
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?
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