Wednesday, March 22, 2017

Do Houseplants Really Purify Indoor Air?

I started this blog post out intending to figure out which of the infamous NASA air purifying plants were safe for pets. However along my search I noticed that all of my sources for which plants were best for purifying the air were terrible. They were from blog sites themselves or otherwise commercial sites. I studied biology with the intent of being a scientist for three years, so I decided to go and find that original infamous NASA study and read it myself. What I ended up finding in the process of doing so was quite surprising.

The study in question is titled “Interior Landscape Plants for Indoor Air Pollution Abatement.” It was published in 1989 and authored by B. C. Wolverton Ph.D., Anne Johnson M.S., and Keith Bounds M.S. In it Wolverton et. al. describes testing 12 different plants for their capacities to reduce the amount of Benzene, Trichloroethylene, and Formaldehyde from sealed chambers over a period of up to 24 hours of exposure to different concentrations. It also teased at testing a special plant and carbon filtration system and went into slightly further depth about whether it was the leaves or the soil that was helping to filter the air.

The way this study is treated online one could easily imagine that it had looked at a hundred different house plants. Putting aside whether such a study does exist or not, this widely cited NASA study definitely does not do that. Furthermore when you read short plant lists on blogs titled things like, “Top NASA Air Purifying Plants” realize that they’re almost always just the same plants used in the study, not even removing the ones that performed comparatively poorly.

So what’s a green-thumb who wants to breathe cleaner indoor air to do?

First, ventilation. If you’re in an area with a relatively low level of environmental air pollution then you should look at opening up your windows more often. A large reason this nearly 30 year old study was done in the first place was because indoor areas were becoming more sealed-in to reduce energy bills causing people respiration and skin problems due to the resulting higher concentration of chemicals from indoor pollutants from everyday items such as detergents, dyes, varnishes, particle-board wood, and fire-retardants.

Second, have less polluting household items. I have no idea how to do this, but I’m certainly going to be paying more attention to it than to what plants to get from now on and I recommend you do as well.

Do plants do anything at all for indoor-air quality?

Yes-ish, although quality ventilation systems will render most of their impact meaningless. However if you’re in a climate where the outdoor air pollution levels might be high or maybe the weather gets too cold or too hot to leave the windows open all the time then indoor plants might be beneficial for your indoor air quality.

Then what plants are the most beneficial?

The plants that are best for air quality will be the ones that have high transpiration rates- basically ones that require large amounts of water. As these plants soak water up from their roots through to their leaves they also pull large amounts of air down to their roots. This air interacts with the microbes in the plant’s soil which often use what we consider to be pollutants as sources of nutrients.

Ferns, palms, and tropical plants with high water needs will probably serve you best, however that’s not to say that many other plants will also be beneficial. Plants that prefer less water will be less effective. Plants without soil and therefore without soil microbes will be even further less effective (sorry air plants!).

Now what about that plant-carbon air filtration system?

In a nutshell (although it was not the main focus of this particular study) Wolverton et. al. looked at a special planter with an activated charcoal carbon part that the plant sat on top of that had a small motor in the bottom which forced air into the bottom chamber through the soil and carbon. This study found it was quite effective at removing the investigated chemicals, however again this was not the main focus of the study. I looked up where Wolverton went with this idea and found that a company was created around the invention that is still going today.

The moral of the story is be skeptical about blog claims without clear citations of their sources and going through those sources yourself.


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