SCIENCE TO THE GROWER: Clearing the air
by Richard Evans
Indoor air pollution can be a significant problem. The more time we spend indoors, the greater the chance of negative health effects. In 2001, average Americans spent 87% of their time indoors (Klepeis and others 2001). That percentage undoubtedly is higher now that we spend much of our time sitting around staring at our Internet-connected devices. Our building materials, furniture, appliances, and consumer products introduce lots of volatile organic compounds into the air. Some of these compounds have been linked to negative health effects, like eye irritation, headaches, and nausea. Some have even been identified as potential carcinogens and mutagens.
Studies of the ability of plants to improve indoor air quality began to appear in the 1980s, soon after the introduction of energy-efficient homes that had low ventilation rates and high concentrations of pollutants like volatile organic compounds. Researchers found that potted plants, such as golden pothos (Scindapsus aureus), nephthytis (Syngonium podophyllum), and spider plant (Chlorophytum elatum var. vittatum), could remove formaldehyde from indoor air (Wolverton and others 1984 and 1989). I remember reading that report. I was skeptical, because it seemed unlikely to me that indoor plants could achieve much air filtration while they sit passively on a credenza.
But the idea hasn’t gone away. New studies keep appearing. Given the potential for a health angle that might boost plant sales, I decided to take a more serious look. What kind of plants are most effective at removing pollutants? How many are needed?
The quest to identify species that are particularly effective at removing pollutants has yielded inconsistent results. An Australian group (Orwell and others 2004) tested seven common houseplant species in chambers spiked with benzene and found that Dracaena deremensis removed the pollutant more effectively than other species in the period immediately after exposure. After luxuriating a while in the benzene-laden air, however, differences in the ability of species to remove benzene were not so clear. In fact, the substrate in which the plants were grown eventually removed volatile compounds just as well as the plants did. Researchers at the University of Georgia (Yang and others 2009) screened 28 indoor plant species by putting them in gas-tight jars and exposing them to five volatile aromatic compounds. Five species, among them asparagus fern (Asparagus densiflorus) and English ivy (Hedera helix), excelled at removing all five pollutants. Other species, such as weeping fig (Ficus benjamina), removed some volatiles, but not all. Dracaena, on the other hand, was relatively ineffective at volatile removal.
A year later, a Korean research group (Kim and others 2010) evaluated 86 species that spanned five categories: ferns, woody foliage plants, herbaceous foliage plants, Korean native plants, and herbs. The plants were placed in airtight containers and tested for their ability to remove formaldehyde from the air. Ferns and herbs had the highest removal rates, but variation among plants in the groups was substantial and the study’s authors concluded weakly that “certain species have the potential to improve interior environments.”
The inconsistent results with different species spurred further work aimed at finding a connection between pollutant removal and the leaf waxes, hairs, stomates, and other attributes of foliage. However, much of that work is of questionable value because the experiments were poorly done, or experimental conditions unrealistic, or both (Cruz and others 2014). Nearly all of the studies employed closed chambers, in which the plants served as a passive system for pollutant removal. Since air exchange was low, or nonexistent, these chambers are a far cry from the rooms we inhabit for 87% of our time. When air exchange is accounted for in these systems, pollutant removal by potted plants is negligible. Girman and others (2009) calculated that one would need to cram 680 plants into a 1500 sq. ft. house — a plant density similar to that of a commercial greenhouse! — to achieve the level of pollutant removal reported in Wolverton’s work.
Horticulturists working with engineers have developed more effective approaches to air biofiltration, such as botanical biofilters. The general idea is to force air through a vertical wall of plants growing in a highly porous substrate. Experimental results have been promising. Torpy and others (2018), for example, tested the ability of a free-standing biofiltration wall, on which 63 individual foliage plants were grown, to remove methyl ethyl ketone (also known as 2-butanone) in a single pass through the filter. They reported an impressive 57% removal efficiency in a system that achieved 1.7 room air changes per hour. Botanical biofilters generally outperform biofilters that comprise the substrate and microbes alone (Pettit and others 2017), and plant foliage and roots contribute less to biofilter air flow resistance than the substrate does (Irga and others, 2017). However, questions about the relative contribution of plants to pollutant removal, and the potential for differences in removal efficiency among plant species and growth habits, remain unresolved.
There are other unresolved questions, too. Botanical biofilters introduce a lot of moisture into the air, so relative humidity can be elevated to uncomfortable levels. The microbial populations in substrates vary, even in response to air composition (Russell and others 2014), and there is a potential for introduction of pathogenic microbes, such as Legionella. Spores released into the room from microbes in the walls could be allergens, or could encourage mold development. Cost-benefit studies are also needed, including an assessment of the maintenance costs. For now, the best solution to removing air pollutants may be using a mechanical ventilation system.
Richard Evans is UC Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.
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