Research is not always right

Pushpa Girimaji

Inspired by a number of WhatsApp messages on how we can purify the air inside our homes by keeping plants, I decided to buy some, but found many of the suggested plants to be quite expensive. My question is will plants really rid the air inside my house of atmospheric pollutants?

Well, I have looked at some of the published work in this area and I would say that there is evidence of the plants’ ability to absorb toxins in indoor air in laboratory conditions. However, there is not much scientific evidence to back that they can do so as effectively outside the laboratory, in real homes. Besides, most of the research has been on volatile hydrocarbons as indoor toxins and not on atmospheric air pollutants such as PM 10, PM 2.5, nitrogen dioxide, sulphur dioxide, that we are dealing with.

Can you please give reasons for coming to this conclusion?

Interior Landscape Plants for Indoor Air Pollution Abatement is perhaps the first published (1989) work in the area. Supported by NASA (National Aeronautics and Space Administration, USA) office of Commercial Programmes and the Associated Landscape Contractors of America, the two-year research in controlled laboratory environment evaluated the leaves, roots, soil and the associated microorganisms for their ability to reduce indoor air pollutants. The research confirmed the varying capacities of the 12 plants used in the study to absorb toxins in the air, but the toxins studied were three volatile organic compounds (VOC) — benzene, trichloroethylene and formaldehyde — released by paints, modern furnishings, particle boards, high-tech equipment, common cleaning agents, etc. in insulated, modern buildings with reduced fresh air exchange.

In fact, most of the research in the area has been with reference to volatile organic compounds. Another published work, Screening indoor plants for volatile organic pollutant removal efficiency (American Society for Horticultural Science, 2009) refers to the high rate of contaminant removal of five out of 28 house plants tested. But here too, the pollutants studied were VOCs.

Interestingly, while suggesting the need for further research on the topic, the scientists here pointed out the difficulties in translating this work into real-life situations, because each species of plant had a different pollution absorbing capacity and the toxins that they absorbed also varied. This meant that for every indoor location, you would require a large variety of plants tailored to the type of VOCs present and their rates of emanation at each specific location, they said.

In fact, several reviews of the in the area have pointed out that while in closed test chambers, plants purified the air, in real buildings, with significant air exchange with the outside, the results have not been so encouraging, raising questions on whether a few plants can really purify the air in homes or offices. Scientists also point out that in laboratory studies, plants are exposed to adequate light; this maximises photosynthesis and enhances their capacity to absorbs toxins. But the results could vary in homes with inadequate light. Besides, different plants have been found to absorb different pollutants and so you would need a large number of a wide variety of plants to clean the indoor air, not really practical.

In fact, a study on a real building, A pilot study on the effectiveness of indoor plants in removing the VOCs in a seven storey office building (Journal o Epidemiology, 2011) concluded that “there appears to be no strong evidence” that the plants play a special role in cleaning the indoor air. On the other hand, a nine-week study done in a primary school in Aveiro, Portugal, using six plants hung from the roof, concluded: “The presence of potted plants likely favoured a decrease of approximately 30 per cent in PM10 concentrations.” (Journal of Toxicology and Environmental Health, 2012)

I must also mention that a review of work in this area, published in 2015 in the American Society for Horticultural Science (Phytoremediation of Indoor Air) observed that the existing published phytoremediation rates determined using small test chambers are far higher than can be achieved with static potted plants in buildings.

Another review, published on November 6 this year in the Journal of Exposure Science and Environmental Epidemiology, has also questioned the effectiveness of plants as indoor purifiers. It read: “Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies.”