Introduction
Research studies
Recommended plants for specific pollutants
"Breathing walls"
Problems with plants

Introduction

Plants have long been known for their ability to remove carbon dioxide from the air, but their ability to remove other types of pollutants is being examined. Over the past few years, there has been some publicity suggesting that houseplants have been shown to reduce levels of some chemicals in laboratory experiments conducted by the National Aeronautics and Space Administration (NASA). This research showed that some plants are more effective than others at filtering certain pollutants. Though NASA's research was to see if plants could be used for space habitation, this is another example of the new field of phytoremediation, which is the use of trees and plants to clean up contaminated environments.¹ It is one of the latest things to be considered in the fight against indoor air pollution. Currently, however, there is little evidence that a reasonable number of houseplants remove significant quantities of pollutants from homes, offices or schools.

Research Studies About Plants

The NASA experiment evaluated various plants as to how effective they were in removing pollutants such as formaldehyde, carbon monoxide (CO), and nitrogen dioxide (NO₂). For example in this project, spider plants were placed in closed chambers with 120-ppm carbon monoxide or 50-ppm nitrogen oxide (NO₂). After 24 hours, spider plants removed 96 percent carbon monoxide and 99 percent nitrogen dioxide. After 24 hours, Golden pothos removed 75 percent of the carbon monoxide. NASA said the results of these tests suggest:

However, there were unique features to the NASA experimental set-up such as activated carbon soil, air blown through the soil, single contaminant release, and a closed chamber climate controlled environment. These tests were not conducted in a real-world setting though the work has been widely translated into recommendations to place foliage plants in offices and other workplaces to improve the quality of the indoor air. There are studies being done that evaluate the effect of plants in offices. In one study, attitudes and productivity were compared between three offices (12 m²). One office had zero plants, one office had 10 plants, and the third office had 22 plants. Although the people in the offices rated their comfort, attractiveness of the office, the experience, and their performance higher with more plants in the room, the results showed that their productivity actually fell by 12 percent.² This was contrary to a study at Washington State University that found that participants working in a controlled office setting with plants were 12 percent more productive and less stressed than those in a work environment without plants. The study also showed that test scores of those around plants increased and the reaction time for those working near plants was faster.³

In terms of decreased amounts of pollutants, one study looked at formaldehyde concentrations. With 20 plants in each office, average formaldehyde levels only fell about 12 percent. The researchers said this demonstrates that indoor plants are not an effective way to clean indoor air and create a healthful environment.² Many plants need light in order to covert carbon dioxide into oxygen. Some researchers in one hospital evaluated several plants that do not need light to see which one was most effective in converting carbon dioxide into oxygen at night when there is no sunlight. They found the Apicra deltoidea to be the most efficient followed by the Sedum pachyphyllum.⁴

Another study measured the amount of dust that settled in rooms both in the presence and absence of plants. Researchers found in a computer lab, where about two percent of the room was filled with plants, that there was less dust than when the room did not have the plants. The same thing happened in an office space they evaluated. The researchers had thought that since the plants increased the humidity in the room that airborne particles would increase in weight from the moisture and settle out at a greater rate resulting in more particulate matter, but this was not the case. As in nature, the leaves actually trap some of the dust floating around in the room. In particular, vegetation with rough surfaces from fine hairs or raised veins is more efficient in intercepting particulate matter than smooth vegetation.⁵

Here are the plants they examined and what pollutants are recommended they be used for: ^6,3

Pollutant	Found In	Effective Plants
Benzene	Gasoline Inks Oils Paints Plastics Rubber Dyes Tobacco Smoke Synthetic Fibers Detergents Pharmaceuticals	African Daisy Chrysanthemum Bamboo Palm Corn Plant Dracaena Janet Craig Dracaena warneckii Dragon Plant English Ivy Gerbera Daisy Marginata Peace Lily Snake Plant Spathiphyllum
Formaldehyde	Tobacco Smoke Plywood Foam Insulation Room Deodorizers Waxed Paper Facial Tissues Paper Towels Permanent Press Fabrics Fiberboard	The most effective plants to use in removing pollutants like formaldehyde are those with a large leaf surface area. Azalea Aloe Vera Bamboo Palm Boston ferns Corn Plant Chinese evergreen Chrysanthemum Date Palm Dieffenbachia Golden Pothos Mini-Schefflera Peace lily Peperomia Mother-in-law's tongue Philodendron (Heart-leaf, Lacy tree, or Elephant ear) Poinsettia Snake Plant Spider Plant
Trichloroethylene	Inks Paints Lacquers Varnishes Adhesives Dry Cleaning	Chrysanthemum Dracaena warneckii Dragon Plant English Ivy Gerbera Daisy Marginata Parlor Palms Peace Lily Pothos

"Breathing Walls"

One way that introducing plants to reduce indoor pollutants has shown promise has been a project with a "breathing wall." This is a large biofilter that includes hundreds of species of plants, water, rocks, frogs, fish, insects, earthworms, and a "scrubber" made of fiber glass panels covered with porous lava rock. This scrubber is the heart of the system. Its panels are wetted by circulating water and covered with moss and maidenhair ferns. Unlike plants just sitting around in a room, the 80-foot tall, 15-foot long "breathing wall" is "active" which means it is attached to the ventilation system. Fans suck air through this wall. The water from that area circulates into a terrestrial zone, into the aquarium, and then to the top of the wall where the ventilation system picks up the filtered air and distributes it throughout the system. Spores, in particular, are attracted to the system so they do not float around.⁷

This system also differs from just setting a bunch of plants around a room because this is its own ecosystem that has the potential for developing its own response to pollutants in the air. In addition, this system is water-based, or hydroponic, which increases the air's contact with the bioscrubber. When researchers tested the effectiveness of the breathing wall by releasing a variety of VOCs, they found that it worked well in breaking down formaldehyde, which breaks down easily in water and can be eaten by bacteria and plants. Toluene does not break down in water as easily and not as much of it is removed. Trichloroethylene (TCE), which may be emitted by laser printers and photocopiers, does not break down easily in water so even less is removed. A single pass through the wall removed 80 percent of formaldehyde, 50 percent of toluene, and 10 percent of TCE.³

To maintain proper humidity levels, there is a computer attached to the "breathing wall" that monitors the level of humidity as well as the number of people entering and leaving the room. It takes about 20 minutes for the computer to make the appropriate adjustments to bring the humidity levels to within tolerable limits.

One of the benefits of this concept is that it uses a total environment approach. It also uses a combination of many purifying plants to cover a wide range of chemical pollutants.

Both the "breathing wall" and NASA are looking at having contaminants brought in through the roots. That is what is usually done in phytoremediation when trees like poplars are being planted to help draw up toxins buried in the ground. There are purifying planters available that use root purification. These planters have a small, quiet fan built into the base. The air is drawn down, first through a charcoal filter for odors, through an air manifold to soil/clay growing media near the roots and out to the room.⁸

Problems With Plants

Careful selection of indoor plants is necessary if anyone suffers from exposure to molds, pollen, odors or dust. Mold can grow in the soil of the plants and release spores into the air. Remember also, houseplants add moisture to the environment that can foster the growth of mold and dust mites. All the water used on the plants goes into the air. This is a plus in dry parts of the country or during dry times of the year; but in humid areas each gallon of water added for watering plants can require about 3.5 kWh of electricity to remove. On top of that, there is currently no evidence that a reasonable number of houseplants remove significant quantities of pollutants in homes and offices.

Plants: Pros and Cons