Regularly articles in the press or on the Internet praise the virtues of indoor air pollution treatment by plants.
With the general public's awareness that indoor air quality is an essential public health issue, many misleading commercial ventures have rushed into the breach and offer dozens of plants that can be grown in your home to effectively clean up your indoor air.
Most of them are relying on a study carried out in the 1980s for NASA, which seems to provide a good guarantee of the beneficial effects of plants on indoor air quality.
Can we really rely on the salespeople's speeches or is this a marketing and sales strategy to get you to buy plants?
How can plants purify the indoor environment?
Plants have several ways to absorb and capture pollutants: At the aerial level by their leaves (small holes called stomata allow gases to penetrate) as well as through their protective layer (cuticle) which can absorb deposits. In the soil, it is through the roots that the pollutants will be absorbed.
Plants therefore seem to be good candidates for absorbing ambient pollution and cleaning up indoor air.
What is the scientific evidence for the pollution control effect of plants?
The NASA study focused on very specific conditions and to address an air quality issue on a space station. The study was conducted over a short period of time, with very high doses of pollutants and an environment simulating that of an orbital station.
NASA's study was conducted in an indoor environment close to that of a space station.
More recent studies have been carried out under conditions closer to those of the indoor air in a house, apartment or office. The one produced by the French Environment and Energy Management Agency (ADEME) presents the most contrasting and interesting results.
In 2004, ADEME financed and directed the Phytair programme on the subject. For several years, indoor air quality improvement efficiency measurements have been performed with different plants under conditions as close to reality as possible by simulating the indoor air quality of a dwelling or office and not that of a spacecraft.
Three plants very common in homes and not allergenic to respiratory tracts were selected for the study: the dragon tree, the pothos and the spider plant.
Their ability to dissipate carbon monoxide, benzene and formaldehyde was assessed. These indoor air pollutants are classified as high priorities by health authorities and very often measured in scientific campaigns to study indoor air quality.
- Benzene (C6H6, CAS 71-43-2), recognized as a proven human carcinogen by the WHO, can cause genetic mutations and blood disorders, including leukaemias.
- Formaldehyde (H2CO, CAS 50-00-0), also classified as a known human carcinogen, can cause chronic respiratory diseases (respiratory tract irritation and asthma).
- Carbon monoxide (CO, CAS 630-08-0) is an odourless and colourless gas and too high a concentration in air can lead to death by asphyxiation (a frequent domestic poisoning).
The first phase of the Phytair program was carried out in aquariums of different sizes whose composition of the atmosphere is imposed and controlled. Under these conditions, plants show some potential to purify indoor air. The results obtained show that plants absorbed 80 to 100% of carbon monoxide, 16 to 83% of benzene and 70 to 100% of formaldehyde.
It would therefore be tempting to extrapolate these results to sell plants under the name "depolluting plants". However, these results were obtained under confined conditions that do not correspond to those of a human habitat.
The second phase of the programme was carried out in a test house, reproducing an indoor environment of a dwelling and as close to reality as possible. At this scale the results are less dramatic: the CO content is the same with or without the plants and the plants seem to have no significant impact on the elimination of benzene and formaldehyde. Scientists conclude that for the action of plants on these gases to be significant under real conditions, tens or even hundreds of plants would have to be installed in each room... which is not realistic.