Air Purifying Plants : Facts and Myths
Do Indoor Plants Remove Toxins?
All of a sudden there are thousands of articles, blogs nursery advertisements about air purifying indoor plants. Green hobbyists have always liked to decorate interiors with these evergreens. Keeping indoor plants is nothing new for them but the way to look at these plants changed all of sudden. Daily tens of articles keep popping up on my social media pages and number varies from 3-20 best plants for air purification. Each of these quote a single source- ‘NASA’… without giving any reference to the original article. It will be difficult to trace back the article which started this now but we can surely go through the original report to check if plants really suck toxins from indoor air. (One can find the original report here)
Time and Type of Study-
One of the reason no article actually gives any reference to original article is to make it a recent and more relevant study. Infact it’s almost a 3 decade old study published on 15-September 1989.
It’s a 30 page report of a simple experiment of putting a plant in a chamber and introducing a known amount of toxin. Measuring the concentration of toxins after a fixed time (here 24 hours) to see if the plant absorbs the toxins. Experimental setup and other details can be found in the report. Clearly the chamber doesn’t represent a room. Chamber leaking was also observed to be negligible before the experiments were started. Main points and observation from the study are-
- Plants were maintained in original pots and kept in greenhouse in between tests. Fertilizers too were used.
- The number of plants studied were 12 (not 15 or 21). 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.
- Only three toxins were tested- Benzene, Trichloroethylene and Formaldehyde( there are hosts of other toxins around)
-Initially it was observed that plants cleaned the toxins in large amounts within 24 hours creating doubt about the fast metabolism at such low lighting.
-Then it was observed that plants without leaves and only soil with microbes too clean air when only pot of soil and only plant root system was tested. Microbe’s numbers grew after repeated exposures showing that microbes feed on these toxins and multiply.
-This prompted the scientists to use activated carbon in place of soil and a small pump to suck air through the pot. This was shown to remove large amounts of pollutants from the room. This system was proposed for cleaning indoor air. (Check report)
Which means that plants don’t contribute much to removal of pollutants but work along with soil ,microbes and carbon.
The actual conclusion from the report says it-
“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”
Lots of facts have blossomed lately referring to this report. Here are some of the false facts or myths
Plants clean 90% of chemicals in 24 hours
This is a huge number and the study looked at only 3 pollutants. . For those pollutants also it was not 100%. One must keep in mind that amount of pollution in a room is not a onetime case. The degassing of paints, furniture is continuous. . The house can never be pollution free unless one equals the rate of generation to rate of removal of chemicals. Nothing was done in the study to check this. Looking at the rate of removal of onetime pollutant one can calculate to see that a large number of plants will be needed to remove continuous degassing by household items(check next paragraph)..
1 plant per 100 sq feet of home for most effective air purification.
Study doesn’t provide any information regarding number of plants needed for average household. One can try and back calculate considering fixed amount of pollution in the room. The experiment used two different sized chambers, 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.This means if we go by the experimental results will need 50 small plants or 25 large plants for each 100 sq ft room. Where will we stay???? Where did this number 1 came from ????
The best 10, 15, 17 or 20 plants are listed by name
One comes up with titles like “15-18-21 Best air-filtering houseplants according to NASA” .Well the study looked only at 12 and even in that had to give up 5 as their contribution was negligible. They just keep on adding various plants without any scientific data. One post lists Aloe Vera as the top plant–but the NASA study showed it was one of the worst in removing formaldehyde–it was dropped from the main part of the study because it was so ineffective. Most of the articles just change the order of plants according to their wish. There is no single plant which can be called best for all the toxins.
Are Plants Responsible For Removing The Pollutants?
The fact is that plants are not able to remove most of the pollutants can be tested by this simple experiment. Testing the full plant first than removing the leaves, testing the soil alone and leaking from the chamber. This was done for many plants but data for one plant represents for all.
Combination of benzene and 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%
With errors involved in experiments 58% and 50% are almost statistically same. Even if its 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.
Also a plant exposed to benzene for a 6 week period had % removal and the bacterial count in the soil increased, Clearly The benzene was fed the bacteria, and so they multiplied.
Rest of the data is found in table complied from the multiple tables of the report.
| Plant systems(Valued of full plant and foliage removed plants almost same) | Benzene(% removed) | trichloroethylene(%removed) |
| English ivy | 89.8 | 10.9 |
| Janet Craig | 77.6 | 17.5 |
| Golden Pothos | 73.2 | 9.2 |
| Peace lily | 79.5 | 23 |
| Warneckei | 70 | 20.2 |
| Marginata | 79 | 13.2 |
| Mother-in-law’s tongue | 52.6 | 13.4 |
| Leak Control | 5.3 | <1.0 |
| Soil Control(fresh potting soil) | 20.1 | 9.2 |
| Chinese evergreen | 47.6 | – |
One can safely say that most of the articles found are far removed from the facts and data observed in this study. Are mainly biased and cherry picking to catch reader’s eye.
It doesn’t mean that one should not have plants. They are great decoration, give oxygen during daytime and some do attract dust to collect on them but don’t depend on them to clean air of toxins.
A 1500 sq ft home would need around 400 large plants to remove most of the tested chemicals wont it be better to avoid using volatile chemicals in the first place. Clean air enthusiast Kamal Meattle has large number of plants in a sunny rooftop green house and a air purifying system for his office building.
Plants do their job of giving us our breath but one must not overburden them.
References
1) Plants Remove Air Pollutants: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930073077_1993073077.pdf
2) How Well Do House plants Clean Air?: http://www.buildingecology.com/articles/critical-review-how-well-do-house-plants-perform-as-indoor-air-cleaners/
3) Can House Plants Solve Indoor Air Quality Problems: http://www.practicalasthma.net/pages/topics/aaplants.htm
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