Given the situation with the oil spill in the Gulf of Mexico, I thought it might be timely to post the following excerpt from my article Clean Your Air and Brighten Your Day with Houseplants to list some indoor plants which have been found to be most effective at removing toxins from indoor air:

In the late 1980s, a study by NASA and the Associated Landscape Contractors of America (ALCA) resulted in excellent news for homeowners and office workers everywhere. The study concluded that common decorative houseplants such as bamboo palms and spider plants not only make indoor spaces more attractive, they also help to purify the air. While it`s a well known fact that plants convert carbon dioxide into oxygen through photosynthesis, the NASA/ALCA study showed that many houseplants also remove harmful elements such as trichloroethylene, benzene, and formaldehyde from the air. With our ultra modern homes and offices that are virtually sealed off from the outside environment, this study is just as important now as when it was first published.

The NASA study, Foliage Plants for Removing Indoor Air Pollutants from Energy Efficient Homes, was conducted by Dr. B.C. Wolverton, Anne Johnson, and Keith Bounds in 1989. While it was originally intended to find ways to purify the air for extended stays in orbiting space stations, the study proved to have implications on Earth as well.

Under controlled conditions, NASA and ALCA spent two years testing 19 different common house plants for their ability to remove these common pollutants from the air. Of the 19 plants they studied, 17 are considered true houseplants, and two, Gerbera Daisies and Chrysanthemums, are more commonly used indoors as seasonal decorations.

The advantage that houseplants have over other plants is that they are adapted to tropical areas where they grow beneath dense tropical canopies and must survive in areas of low light. These plants are thus ultra-efficient at capturing light, which also means that they must be very efficient in processing the gasses necessary for photosynthesis. Because of this fact, they have greater potential to absorb other gases, including potentially harmful ones.

NASA found that some of the plants were better than others for absorbing pollutants, with certain houseplants found to remove as much as 87 percent of indoor air pollutants within 24 hours. However, all of the plants had properties that were useful in improving overall indoor air quality. NASA also noted that some plants are better than others in treating certain chemicals. For example, English Ivy worked better than some other plants for treating air contaminated with Benzene. The Peace Lily was very effective in treating Trichloroethylene and the Bamboo Palm worked well for filtering Formaldehyde.

After conducting the study, NASA and ALCA came up with the following list of plants most recommended for treating air pollution. Note that all the plants in the list are easily available from your local nursery.

Heartleaf Philodendron - Philodendron scandens `oxycardium`
Elephant Ear Philodendron - Philodendron domesticum
Cornstalk Dracaena - Dracaena fragrans `Massangeana`
English Ivy - Hedera helix,
Spider Plant - Chlorophytum comosum
Janet Craig Dracaena - Dracaena deremensis `Janet Craig`
Warneck Dracaena - Dracaena deremensis `Warneckii`
Weeping Fig - Ficus Benjamina
Golden Pothos - Epipiremnum aureum
Peace Lily - Spathiphyllum `Mauna Loa`
Selloum Philodendron - Philodendron selloum
Chinese Evergreen - Aglaonema modestum
Bamboo or Reed Palm - Chamaedorea sefritzii
Snake Plant - Sansevieria trifasciata
Red-edged Dracaena - Dracaena marginata

Here is a list of the pollutants NASA tested for and the plants they found that worked the best for each one:

Trichloroethylene (TCE)

Trichloroethylene is primarily used in the metal degreasing and dry cleaning industries; also in printing inks, paints, lacquers, varnishes, adhesives. In 1975 the National Cancer Institute reported that an unusually high incidence of hepatocellular carcinomas was observed in mice given TCE by gastric intubation and now considers this chemical a potent liver carcinogen.

The NASA study found that the best plants for removing trichloroethylene are the Gerbera Daisy, Chrysanthemum, Peace lily, Warneckei, Dracaena marginata

Benzene

Benzene is a very commonly used solvent and is also present in many common items including inks, oils, paints, dyes, plastics, rubber, dyes, detergents, gasoline, pharmaceutical, tobacco smoke, synthetic fibers. In addition it is used in the manufacture of explosives.

Benzene has long been known to irritate the skin and eyes. In addition, it has been shown to be mutagenic to bacterial cell culture and has shown embryotoxic activity and carcinogenicity in some tests. Evidence also exists that benzene may be a contributing factor in chromosomal aberrations and leukemia in humans. Repeated skin contact with benzene will cause drying, inflammation, blistering and dermatitis.

Acute inhalation of high levels of benzene has been reported to cause dizziness, weakness, euphoria, headache, nausea, blurred vision, respiratory diseases, tremors, irregular heartbeat, liver and kidney damage, paralysis and unconsciousness. In animal tests inhalation of benzene led to cataract formation and diseases of the blood and lymphatic systems. Chronic exposure to even relatively low levels causes headaches, loss of appetite, drowsiness, nervousness, psychological disturbances and diseases of the blood system, including anemia and bone marrow diseases.

The best plants for removing benzene were determined to be English Ivy, Dracaena marginata, Janet Craig, Warneckei, Chrysanthemum, Gerbera Daisy, Peace lily

Formaldehyde

Formaldehyde is a ubiquitous chemical found in virtually all indoor environments. The major sources which have been reported and publicized include urea-formaldehyde foam insulation (UFFI) and particle board or pressed wood products used in manufacturing of the office furniture bought today. It is used in consumer paper products which have been treated with UF resins, including grocery bags, waxed papers, facial tissues and paper towels. Many common household cleaning agents contain formaldehyde. UF resins are used as stiffeners, wrinkle resisters, water repellents, fire retardants and adhesive binders in floor coverings, carpet backings and permanent-press clothes. Other sources of formaldehyde include heating and cooking fuels like natural gas, kerosene, and cigarette smoke.

Formaldehyde irritates the mucous membranes of the eyes, nose and throat. It is also a highly reactive chemical which combines with protein and can cause allergic contact dermatitis. The most widely reported symptoms from exposure to high levels of this chemical include irritation of the eyes and headaches. Until recently, the most serious of the diseases attributed to formaldehyde exposure was asthma. However, the Environmental Protection Agency (EPA) has recently conducted research which has caused formaldehyde to be strongly suspected of causing a rare type of throat cancer in long-term occupants of mobile homes, where particle board and other sources of formaldehyde are used extensively.

When it comes to removing formaldehyde, the best plant choices are Azalea, Philodendron, Spider plant, Golden Pothos, Bamboo palm, Corn plant, Chrysanthemum, Mother-in-law`s tongue.

Many people feel that the use of houseplants is not needed if they are using an air purifier. However, research has shown that even if you use a HEPA air filter or an ionic air purifier, there is a lot of difference that can still be made by many common house plants, which act as a living air purifier. With the exception of an activated carbon filter, common indoor air filters, such as HEPA or ionizers, will totally miss the toxic household gases, such as carbon monoxide, formaldehyde, and volatile organic gases (VOG).

According to NASA, one plant should be used for every 100 to 120 square feet of office or living space and the plants should be in at least six inch containers with nothing covering the potting soil. In addition to removing pollutants in your home or office, plants also make for a more pleasant place to live and work - where people feel better, perform better, and enjoy life more. Here are some of the other benefits from having indoor plants:

Plants are a Source of Oxygen - Plants take in carbon dioxide and produce oxygen through the process known as photosynthesis. The more plants you have, the more oxygen you will receive.

Plants Make You Happy - House plants make people feel calmer and more optimistic, says Bruno Cortis, M.D., a Chicago cardiologist. Interestingly, he says that studies have shown that hospital patients who face a window with a garden view recovered more quickly than those who had to look at a wall.

Plants Fight Fatigue and Colds - According to a University of Agriculture in Norway study, indoor plants can reduce fatigue, coughs, sore throats and other cold-related illnesses by more than 30 percent, partially by increasing humidity levels and decreasing dust.

Plants at the Office - Major corporations and work environments are beginning to catch on that adding plants indoors does wonders for employee health and morale. According to one study published in Rehabilitation Literature, a manufacturing company integrated plants into its office so that no employee would be more than 45 feet from greenery. The result? Company administrators said they noticed enhanced creativity and increased productivity in employees. One popular plant to use in offices is the Dragon Tree plant. Besides being one of the most effective in removing harmful impurities from the air, it`s exotic looks adds character to any room.

It is important to note though that not all plants are good as indoor air cleaners and that some plants are poisonous and should be handled with care, or not at all if you have small children. Some examples of toxic plants include: Nightshade, Creeping Charlie, Foxglove, Oleander, Sago Plant, Privet, Rhododendron, Umbrella Plant, Ivy, Ripple ivy, Sweet Pea, Vinca, Spider Mum, and Poinsettia. Consumers looking for houseplants that purify air should probably steer away from the above named plants if they have young children and indoor pets, as these plants can sometimes be fatal if consumed.

Resources for more information:

http://ntrs.nasa.gov/archive/nasa/ssctrs.ssc.nasa.gov/journal_mas/journal_mas... 
http://www.ssc.nasa.gov/environmental/docforms/water_research/water_research.... 
http://ntrs.nasa.gov/archive/nasa/ssctrs.ssc.nasa.gov/foliage_air/foliage_air... 

Other sources included:

http://www.coopext.colostate.edu/4DMG/Plants/clean.htm 
www.cleanairgardening.com/houseplants.html 
http://www.zone10.com/technology/nasa-study-house-plants-clean-air.html 
http://www.buzzle.com/editorials/6-22-2006-100164.asp