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IAQ and Plants

The idea that plants clean indoor air is a sad, continuing saga fed by bad science, commercial interests, and wishful thinking.

I published an article in the Indoor Air Bulletin

on the subject in 1992 (available on this web site) that provides some details.

Take home message:

1.   Don't use plants to improve IAQ. They don't. If anything, they pose risks to good IAQ.

2.   There is no credible scientific evidence that plants improve IAQ. The planting media has been hypothesized to be responsible for pollutant removal in some studies. The planting media alone can be expected to contribute to a limited reduction in some airborne chemical concentrations.

3.   Most advocates of indoor plant use have been funded by or are themselves providers of plants or supporting systems.

4.   If plants are used indoors for aesthetic reasons, there should be extra care to avoid moisture problems or problems with fertilizers and pesticides, all known sources of indoor air quality and health problems.

If you do have plants indoors, don't do it to improve indoor air quality. The pollutant removal effect is negligible and, as far as the science has shown, is not due to the plants but is due to adsorption on the soil and, possibly, uptake by the organisms in the root area of the plant. So, you could just put the planting mix in the space and use fans to move air through it. In one study, charcoal was added to the planting mix with fans moving the air, demonstrating that it was not the plants but the planting mix that was doing the removal.

The rate of removal by plants, even if you use the data from the one NASA research project ever done on it, is smaller than the removal of pollutants through the air exchange that takes place in a very tight building due to leakage through the envelope. If you will fill a house with three layers of the plants recommended by the advocates, the removal rate would be equal to 1/10th of an air change per hour (ach). Buildings with mechanical ventilation generally have a minimum ventilation rate of 0.5 air changes per hour. Offices using typical ASHRAE design values have about 0.8 ach.

The one. often-quoted NASA research project was done in static chambers, sealed chambers with no air exchange rates. This is not a scientifically sound way to investigate the removal rate of pollutants. A dynamic test involving an air change rate equal to those in real buildings and achieving steady state conditions is a far more relevant test. In a static chamber, a test over the time period in the NASA study would be dominated by the sink effects, removal from the air by adsorption to surfaces of the chamber and the plants. This does not give any idea about the removal rate obtained by plants in a real environment or even in a chamber over a normal period of on-going occupancy.

More recently published studies have been characterized by the use of static chambers or carelessness in the measurements of the environmental parameters. A paper presented at Indoor Air 2008 in Copenhagen last month actually showed a decrease in research subjects' task performance when plants were present.

The use of plants indoors, especially the "living wall" concept or other extensive use requiring periodic addition of moisture, creates substantial risks of moisture, mold, and bacteria problems in the air. There is a substantial risk of moisture-related problems including but not limited to mold in buildings with extensive plantings. The scientific evidence points more strongly to moisture than to mold as the relevant association in buildings with higher rates of asthma or allergy among the occupants. There are also risks from the use of fertilizers and pesticides, if required, in the indoor environment. We generally try to steer people away from plantings that require frequent irrigation, fertilizer, or pest control immediate around buildings, especially if there are operable windows.

Most of the favorable publicity around the use of plants comes from folks whose business it is to provide plants and/or the systems to support them. Try to check out your sources and the sources of funding for any study that they cite.

You can read a more extended discussion of plants and indoor air quality in an article posted on this web site under articles, "Can house plants solve indoor air quality problems?" It was originally published in my old newsletter, Indoor Air Bulletin, in 1992.

Because of the financial interest providers of the plants and supporting systems have, there continue to be many individuals innocently advocating the use of plants to improve indoor air quality. This is a problem that doesn't seem to go away because of the appeal of indoor plants and the myth that everything natural is good. Remember that many chemicals found in nature are poisonous, that many plants are poisonous and even deadly (e.g., digitalis) to humans and other living beings.

Natural insecticides such as those derived from chrysanthemums (pyrethrins) are allergenic to many people and are toxic to insects and, it appears likely, to humans.

The Wikipedia listing for pyrethrin says: "In humans, pyrethrin irritates the eyes, skin, and respiratory systems, and it may cause other harmful effects. One study suggested a link between maternal pyrethrin use and autism in children.

The study indicated that mothers of autistic children were twice as likely to have washed a pet dog with a flea shampoo containing pyrethrin while they were pregnant."

By the way, I have a few plants around me as I sit here typing,  but they are mostly orchids and cacti, not intended to or expected to clean the air. I tend to underwater them and rarely fertilize them. Of course they don't bloom as often as I'd like, but that's the trade-off for ensuring better IAQ.

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