It’s that time of year again, winter is coming. We close the windows and doors of our home to keep out out the cold, and in the process we lock in all kinds of chemical pollutants. Never fear, a few air purifying plants will help with that problem. This must work since all kinds of home magazines, newspapers and web sites tell you it works. Headlines like “the 10 best plants for removing pollutants in your home” appear daily this time of year. How well do plants work at cleaning the air in our home?
Air Purifying Plants Do Remove Some Pollutants
There is no doubt that plants do absorb chemicals and particulate matter from the air. Our outdoor environment is improved by the presence of plants.
It only makes sense that having plants indoors would have the same effect and in fact this is quite true. House plants do remove chemicals from the air in our home. They do remove CO2 and they remove some other pollutants. The real question that needs to be asked is the following:
Do house plants reduce the amount of pollutants in the air inside the home?
These might seem like the same statements but they are not. Plants can and do remove pollutants, but do they remove enough of them to make a difference to the amount of pollutants in our air? If you had two identical homes and one had a few plants growing and the other did not, would you see a difference in the air quality? That is the important question.
Air Pollutants in the Home
So what is the big deal? What kind of air pollutants do we have in our home? Why is it suddenly a problem?
Years ago homes in developed countries were quite drafty. Clean air from outside was constantly being blown through our homes, exchanging polluted air with fresh air. Things were good, except that heating bills were very high. Along came the better sealed homes–remember the R-2000 homes? Homes being built today are so much more air tight that they prevent fresh air from coming in.
At the same time, more and more products in the home are man-made and many of these products give off a constant low level of polluting chemicals. The plastic carpet, the fabric on the chairs, and all of our electronics are polluting our air. You might remember formaldehyde insulation? And those air fresheners, scented products and spray cans are some of the worst polluters.
So while our homes have become more air tight, the stuff in our homes are producing more pollutants than ever before. The result is a poor quality of air in our homes, especially in winter.
NASA Tests Plants
In order to better understand this issue, we need to go back in time and see where the scientific evidence started. Some early studies showed that plants absorb VOCs (volatile organic carbons), such as benzene, toluene, octane, and trichloroethylene . This lead NASA to do some research on air purifying plants, which they reported in 1989. The results showed that in a lab situation, plants did in fact absorb VOCs from the air.
That started an avalanche of recommendations in the popular press. “House plants purify the air in the home”. People started recommending “the best air purifying plants” for the job. What a relief–buy a few house plants and we are all saved from air pollution.
By now I am sure you have guessed that this is all a myth.
The Birth of a Myth
Before looking at the reality of the situation it is educational to understand how it all went wrong. This myth was born, like many others, as a result of two main mistakes.
- Results obtained in a controlled lab situation were extrapolated to a field environment–our homes.
- Actual data was exaggerated and misinterpreted by the media.
Both of these mistakes happen all the time with almost all topics, and they happen a lot with gardening advice.
The early testing done on plants used small lab chambers, that were highly controlled, and which were crammed full of plants. They did not simulate the situation in our homes and the researchers doing the work never claimed their work showed house plants would reduce pollution in our homes.
The media took the results, selected the numbers they liked best, jumped to unfounded conclusions and announced “air purifying plants work to clean our air”.
To have a closer look at the details of these mistakes have a look at A Myth is Born – Plants Don’t Purify Air.
Plants Don’t Improve Our Indoor Air
More recently a few researchers have studied the effect of plants in real world situations. These studies were done in offices, but this is fairly similar to our homes.
These studies show that the level of pollutants do NOT go down over time.
Why is there such a difference in the studies? The main reason for the difference was that the initial lab work was done with a lot of plants in a small space. To establish this in our homes we would have to have a 100 house plants in every room. That’s not practical–it would be great to have all these plants–but not practical. Having a plant or two for each room in your home will just not make much of a difference.
The second reason for the difference is that in our homes the material in them is constantly giving off pollutants. These are produced faster than plants can remove them. In the lab test chambers, the pollutants were injected once and then the air was monitored to see a reduction in pollutants. These are two very different situations.
Here is what the EPA has to say on their FAQ page (ref #2): “The only available study of the use of plants to control indoor air pollutants in an actual building could not determine any benefit from the use of plants“.
Best Plants for Reducing Pollutants
You will find lots of lists recommending the best plants for removing home pollutants. I am not sure where this information comes from. Most of the studies show that there is little difference between one plant and another. The size of plant and the number of plants is more important than the variety.
Plants vs Microbes
If plants removed pollutants from the air, then more leaves should remove more pollutants. In one study NASA measured the ability of certain plants to remove pollutants–this gave them a base line value. They then removed some leaves. It seems pretty obvious that if you remove leaves, the plant will remove less pollutants–right? Wrong. What they found was that removing leaves improved the plants ability to remove pollutants.
Long story short, and after many more tests, they were able to show that it was the microbes in the soil that were removing most of the pollutants. The microbes were much more efficient at removing pollutants than the plants–surprise!
It seems quite obvious to me that if we went back to dirt floors we would solve our home pollution problem, but the popular press has not picked up on this idea!
Air Purifying Plants Will NOT Clean Your Air
House plants are a great addition to your home, but they have no real effect on the quality of your air, unless you have hundreds of them. The microbes in the soil are more valuable than the plants as far as cleaning air goes, but even they have little effect on the quality of your air.
The best thing you can do is to allow fresh air into your home, and reduce the number of cleaners, and perfumed products you use. Fewer renovations, and fewer electronic devices will also help. Stop using products sold in spray cans.
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.
New Study on Office Buildings
Added October 2016.
A new field study has been brought to my attention and is reviewed here. The study is called The Potted-plant Microcosm Substantially Reduces Indoor Air VOC pollution: I. Office Field Study by Ronald A. Wood et al.
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 but not in the building at large. There was no difference between using 3 or 6 plants and the reduction was only seen when the offices had high levels (>100 ppb) of VOCs present. These high levels are above that found in a normal home.
In conclusion, the study indicates that adding such plants would have a limited effect, if any, in a home with average VOC levels.
A more detailed analysis can be found in the discussion section of A Garden Myth is Born – Plants Don’t Purify Air.
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) How Well do Air Plants Perform as Indoor Air Cleaners?: http://www.buildingecology.com/articles/critical-review-how-well-do-house-plants-perform-as-indoor-air-cleaners/
2) Indoor Air Pollution: http://www.epa.gov/iaq/pubs/hpguide.html#faq7