This NASA technical paper (1989) describes a two year study of plant phytoremediation to clean air from harmful substances. The motivation was to find a promising approach to reduce trace levels of air pollutants inside future space habitats is the use of higher plants and their associated soil microorganisms.

Even without existence of hundreds of synthetic organic chemicals off-gassing into tightly sealed environments, man’s own products would cause indoor air pollution problems. In this study, the leaves, roots, soil, and associated microorganisms of plants were evaluated as possible means of reducing indoor air pollutants. Additionally, a novel approach of using plant system with an activated carbon filter was designed, where large amounts of contaminated air was moved through the system and plant roots and their associated microorganisms then destroy the bacteria and the organic chemicals, converting them into new plant tissue.

Chemicals used in the plant tests were benzene, trichloroethylene (TCE), and formaldehyde, and 12 different species of common houseplants. Tests were done in sealed chambers with one of the selected species or soil controls without plants present, where one of the three chemicals were injected. Samples were collected immediately after chemical introduction, 6h after, and 24 h after.

Different plant species removed chemicals during a 24-h exposure:

• Benzene 21.4 % – 67.7 %
• TCE 10.0 % – 41.2 %
• Formaldehyde 47.4 % – 70 %

When the same plants and potting soil are constantly exposed to chemicals, their capacity to continuously clean the air improves: i.e. Chinese evergreen improved benzene removal from 47.6 % to 85.8 % during a six week period. The plant root-soil system appears to be the most effective for removing VOCs.

“Low-light-requiring houseplants have a potential for improving indoor air quality by  removing trace organic pollutants from the air in energy-efficient buildings. This plant system is one of the most promising means of alleviating the sick building syndrome associated with many new, energy efficient buildings.”

Microorganisms have the ability to genetically adapt, and increase their ability to utilize toxic chemicals as a food source. Therefore maximizing air exposure to the root-soil area would be the best method to build best air filtration.

The activated carbon-houseplant studies show, that when a fan is used to rapidly move large volumes of air through the roots and activated carbon filter, the filter absorbs pollutants and holds them until the plant roots and microbes can utilize them as a food source. It was also measured that no pathogenic microorganisms were found in the filter exhaust air.