Do plants really improve indoor air quality?
Contrary to popular belief, the air quality in your home is no better if there are plants growing there.
The United States Environmental Protection Agency (EPA) estimates that Americans spend 90% of their time indoors and that indoor air quality is often not as good as outdoor air quality. It could be thought that this is also the case in most industrialized nations, including Canada.
In the home, the main sources of pollutants are human activity (DIY, smoking, cooking, etc.), construction materials, everyday items (cleaning products, heating, etc.) and some external contaminants that seep into houses. According to the World Health Organization, poor indoor air quality is responsible for 1.6 million deaths annually.
Impressive but misleading results
The idea of using plants to purify air in buildings developed as a result of research carried out by Dr. Wolverton for NASA. This researcher’s experiments, which were taken up and developed by several American scientists, were conducted in a laboratory. In most cases, a plant was placed under an airtight shelter made from transparent Plexiglas. A dose of pollutant was injected into it and then the quantity of agents absorbed after 24 hours was calculated.
The absorption level for pollutants such as formaldehyde, benzene, trichloroethylene and acetone (see table) was tested in about twenty different plants. At first glance, the results are quite impressive. Dracaena deremensis ‘Janet Craig’ would have an effectiveness rating of 7.8/10 for trichloroethylene. The effectiveness rating for Ficus robusta would be 8.0/10 for benzene, formaldehyde and trichloroethylene.
The results of these investigations have been widely reported in traditional and electronic media. Entire books have even been written on the subject. According to Wolverton, 15 to 18 average-sized plants (6- to 8-inch pots) is enough to purify an area of 200 m2. Few people doubt these figures, even though they are extrapolated from laboratory data. But what is the truth? Are plants as effective in buildings as in a laboratory?
From the laboratory to the real world
John R. Girman is doubtful. This EPA researcher, an indoor air quality specialist, conducted a literature review on the subject. His research shows that most research conducted in real situations has not found that plants have a significant impact on air quality. These poor results could partly be due to gaps in the experimental protocol (incorrect calculations in particular), as data obtained in a laboratory is difficult to transpose to the real world.
So, based on Wolverton’s data, Girman calculated that 680 plants would be needed to purify the atmosphere in a standard house of 139 m2 containing 340 m3 of air. This is far removed from the recommendations of the Associated Landscape Contractors of America (ALCA) who recommend using a plant measuring 9.29 m2, so 15 plants for a standard house! The difference is due to the fact that Girman’s calculations, unlike those of the ALCA, take into account the volume of air to be treated. In Wolverton’s chambers, each plant purified on average 0.5 m3 of air. Therefore, to obtain a good output in a standard house, the 340 m3 of air/0.5 m3 of air result should be used, i.e. 680 plants. If the ALCA recommendations are followed, each plant would have not 0.5 m3 of air, but 23 m3 of air (340 m3 of air/15 plants) to clear of pollution.
Furthermore, it is difficult to compare Wolverton’s results with data used to assess the efficacy of other methods of removing pollutants. To do this, the effectiveness rating would need to be converted into a unit of mass per plant per hour. For example, we know that the extraction level of a ventilation system for an airtight house is 0.25 units of mass of pollutant an hour. As a comparison, Girman calculated that a plant with an effectiveness rating of 9/10, i.e. the best result obtained by Wolverton, would have an extraction level of 0.096 units of mass of pollutant an hour. Appropriate ventilation would therefore be 2.6 times more effective than a plant.
Girman states that laboratory experiments do not take into account the impact of pollutants emitted continuously and that the effects of ventilation in field tests should be considered. For the time being, EPA specialists believe that clean materials, good maintenance of ventilation systems and regular ventilation remain the best ways of purifying air in buildings.
The role of microorganisms
In an article published in the scientific journal Practical Asthma Review, Hal Levin, a colleague of Girman at the EPA, mentions something that is rarely stated, which is the essential role of bacteria in purifying air. To begin with, most researchers assumed that air pollutants were absorbed by foliage during photosynthesis. But when it came to comparing the performance of plants whose lower leaves had been removed with control plants that had all their leaves, against all expectations the best results were obtained with plants whose leaves had been removed. It did not take long to realize that these results were linked to better contact between air and soil microorganisms in plants whose leaves had been removed.
This discovery supported what has been known for a long time: most pollutants are degraded by bacteria. It is also the purifying capacity of microorganisms that is harnessed in biological filters, which are increasingly used to treat pollutants and odours from food processing plants. Polluted air is passed through this using an organic or inorganic substrate (peat foam, coconut fibre, soil, PVC blocks, etc.) containing beneficial microorganisms. Unlike with standard filters, pollutants do not accumulate in these systems. Microorganisms degrade them into simple agents, mainly water and carbon dioxide (CO2).
The Biowall: a huge biological filter
Dr. Alan Darlington of Guelph University in Ontario was inspired by the principle of biological filters to develop the Biowall. This plant wall is a kind of vertical hydroponic system that contains plants and microorganisms with purifying properties. A nutritive solution circulates in the substrate and feeds the plants. The best results are obtained when the Biowall is combined with a ventilation system. In this way, indoor air passes through a biological filter before being redistributed in the building. In the Biowall, plants produce an environment favourable to the growth of microorganisms while absorbing certain pollutants such as CO2. Dr. Darlington founded a company (Nedlaw Living Walls) to market his system. About twenty institutions, especially universities, have installed the Biowall in halls or cafeterias. Research continues, and there is hope that the system’s effectiveness will be demonstrated from both a health and economic perspective. This won’t be the last we hear from plants!
| A few air pollutants in homes | Source |
| FORMALDEHYDE | Particle board, concrete blocks, insulating foam, carpet, textiles, glue, paint, cosmetics, smoking, photocopiers. |
| BENZENE | Fuels, smoking, DIY products, furniture, construction and decorating products. |
| TRICHLOROETHYLENE | DIY products, furniture, smoking, paint, varnish, glue, ink, carpet, insecticides, plastics, insulation, stain removers. |
| ACETONE | Nail varnish remover, glue, paint, solvents, insulation, textiles. |
Source: Table broadly based on a guide produced by the French Environment and Energy Management Agency, and provided as a reference at the end of this article.
Based on an article by Jean-Pierre Parent in Quatre-Temps magazine, Vol. 36, No. 4, Winter 2013, pp. 39-41.