Editor’s Note: This article was originally published in May 2009 and has been updated for accuracy and completeness.
The indoor pollutants that affect health are formaldehyde, Volatile Organic Compounds (benzene and trichloroethylene or TCE), airborne biological pollutants, carbon monoxide and nitrogen oxides, pesticides and disinfectants (phenols), and radon. These pollutants contribute to ‘sick building syndrome’, which causes symptoms ranging from allergies, headaches and fatigue through to nervous-system disorders, cancer and death.
Through studies conducted by NASA, scientists have identified 50 houseplants that remove many of the pollutants and gases mentioned above. NASA, with assistance from the Associated Landscape Contractors of America, conducted a two-year study directed by Dr. B.C. Wolverton, an environmental engineer from Picayune, Washington, and a research scientist for NASA for over 20 years. Dr. Wolverton’s study of the interaction between plants and air found that houseplants, when placed in sealed chambers in the presence of specific chemicals, removed those chemicals from the chambers. He concluded that plants can clean pollutants in homes, offices, factories and retail outlets.
Later, Wolverton expanded the study and assigned plants a rating from one to 10, based on a plant’s ability to remove chemical vapors or indoor air toxins, ease of growth and maintenance, resistance to insect infestation, and the rate at which water evaporates from the leaves. He summarized much of his research in his book, “How to Grow Fresh Air – 50 Houseplants that Purify Your Home or Office.”
More information on this study as well as references and details on specific chemicals can be found on Dr. Wolverton’s website.
Here are some details about the highest ranking houseplants in that study.
Top ten indoor plants for removing formaldehyde, benzene, and carbon monoxide from the air:
1. Areca Palm (Chrysalidocarpus lutescens)
In tests conducted by Dr. Wolverton, the Areca palm removed more toluene and xylene than any other plant. Toluene is one of the notorious ‘toxic trio’ (that also includes formaldehyde and dibutyl phthalate) known to be hazardous to human health. It’s most commonly found in paint thinners, nail polish remover, glues, and correction fluid. Xylene is used as a solvent in the printing, rubber, and leather industries. These chemicals make their way into our homes and offices and their long-term, cumulative impacts are unknown.
Extremely effective at removing toxins from indoor air, the Areca palm also emits large amounts of water vapor—a boon in locations with dry air. It’s tolerant of most indoor environments though prefers a humid area to avoid tip damage. (If the tips of your plant go brown, this usually indicates your air is too dry.)
Also called the “Butterfly Palm,” this upright houseplant is somewhat vase shaped with delicate, fast-growing fronds. Specimen plants can reach 10 to 12 foot in height. When selecting an Areca palm look for plants with larger trunks at the base of the plant. Plants that have pencil thin stems tend to topple over and are quite difficult to maintain.
Ensure your plant has a good, loam-based potting soil and fertilize on a regular basis, except in winter. Water enough to keep the root ball damp, and mist regularly to deter insects and to keep your plant looking fresh.
2. Lady Palm (Rhapis excelsa)
The Rhapis are some of the easiest palms to grow, but each species has its own particular environment and culture requirements. The “Lady Palm” is a durable palm species that adapts well to most interiors, preferring semi-sunlight and a temperature of 60-70 F (16-21 C). While the plant grows slowly, it can eventually reach more than 14 feet in height with broad clumps often having a diameter as wide as they are tall.
In general, the plant is very resistant to insect infestations, though occasionally spider mites will attack a Lady Palm. These can usually be removed with soapy water if caught soon enough. Other things to watch for include a location that’s too dry: this will cause the tips of the fronds to turn brown and dry out.
To care for your Lady Palm, water generously in spring and summer, increasing the volume in winter if your plant is located in a warm, dry environment. Feed monthly with liquid fertilizer and enjoy the plant’s glossy leaves on graceful, arching stems.
3. Bamboo palm (Chamaedorea seifrizii)
The bamboo palm is another plant that scored very high in its ability to remove formaldehyde from the air. This plant also effectively removed benzene—an ingredient used to make dyes, lubricants, rubbers, and detergents—and trichloroethylene, an industrial solvent. Its lacy, green fans born on clusters of slender canes add humidity to any room, along with a tropical feeling.
Also called the “reed palm,” the bamboo palm prefers bright indirect light and a temperature of 60-70 F (16-24 C). A southeast window in a moderately warm room is the perfect location. New plants will lose of some interior foliage as they acclimate to indoor settings, but this is normal. Remove dead fronds, but don’t pinch out the tip of new stalks because this will slow or eliminate new growth. In terms of a growing medium, bamboo palms prefer a little sand added to their soil mix.
While this plant likes to stay uniformly moist, it doesn’t like to be over-watered or to sit in standing water. Water generously during the spring and summer when the plant is growing actively, pouring any leftover water from the drainage tray. In winter, water just enough to keep the root ball moist.
Indoor palms may attract spider mites, which can be controlled by spraying with a soapy solution.
4. Rubber Plant (Ficus robusta)
The rubber plant is especially effective at removing formaldehyde from indoor air. A favorite since the Victorian era, the plant grows very well inside and tolerates temperatures as low as 40 F (5 C) for short periods. Its ideal temperature range is 60-80 F (16-27 C).
While rubber plants prefer semi-sunlight to semi-shade, they will tolerate dim lighting, making them extremely easy to situate. Avoid direct sunlight, however, especially in summer. In dry, centrally heated rooms, watch for scale insects, spider mites, and thrips. Growing to a height of eight feet with a spread of up to five feet, rubber plants may also need to be supported by a stake. Wear gloves when pruning, since the milky sap may irritate the skin.
During the summer months, feed regularly with diluted liquid fertilizer. Water thoroughly from mid summer to fall, allowing the soil to dry out before watering again. In winter keep slightly moist. This plant doesn’t tolerate overwatering.
5. Dracaena “Janet Craig” (Dracaena deremensis)
Rated as one of the best plants for removing trichloroethylene from the air, the Dracaena is also one of the easiest houseplants to grow. The mature plant reaches 10 feet in height with a spread of three feet, and can live for decades if properly maintained.
These plants do best in indirect sunlight coming from the east/west, or semi-shade. They can adapt to lower light levels if the watering is reduced. Keep the soil evenly moist and never let the plant get soggy. In the same way, avoid letting the root ball dry out completely. Water less frequently in the winter months. Mist frequently with warm water, removing any dead leaves. (Leaf tips will go brown if the plant is under watered but this browning may be trimmed.) If possible, avoid fluoridated water, since these plants can be sensitive to fluoride.
In spring and summer, feed with liquid fertilizer every two weeks. Refrain from fertilizing during the wintertime. Repot using a commercial potting soil every other year. The plant’s ideal temperature range is 60-70 F (16-24 C). The plant can survive in lower temperatures (as low as 50 F or 10 C), but the leaves may suffer, turning yellow. They are also susceptible to mites, scale insects, and mealybugs—more so when the air is too dry. Keep humidity above 40% and remove insects when they appear with a wash of soapy water.
6. Philodendron (Philodendron sp.)
Philodendrons come in many different varieties. Those ranking high on Dr. Wolverton’s tests include the Red Emerald (Philodendron erubescens), the Lacy Tree (Philodendron selloum), the Heart-Leaf (Philodendron oxycardium), and the Elephant Ear (Philodendron domesticum or tuxla). All are easy to grow and thrive with little attention, providing years of detoxifying for your home or office.
In general, philodendrons prefer medium intensity light but will tolerate lower light conditions. The Heart-Leaf philodendron is probably so popular because of its propensity to grow anywhere and everywhere indoors. This variety also climbs and can reach a height of six feet. To help it obtain a more bushy appearance, pinch out some growing tips regularly. Direct sun will burn the leaves of most philodendrons and stunt plant growth.
One thing to keep in mind when growing philodendrons indoors is that they need to be misted regularly and the leaves kept free of dust. Soil should be evenly moist, but allowed to dry between watering. Plants need less water overall during the winter months. Feed philodendrons with liquid fertilizer every two weeks during their active growing season. Large plants will need a full-strength fertilizer. The ideal temperature range for philodendrons is 60-70 F (16-21 C). These plants are less tolerant of temperature extremes.
7. Dwarf Date Palm (Phoenix roebelenii)
Scoring very high for its ability to remove airborne chemicals, the dwarf date palm is especially effective at ridding the air of xylene, a common solvent ingredient. The plant is well suited to controlled temperature environments in offices and homes and will survive for decades if situated optimally.
Place in semi-sunlight free from drafts with temperatures between 60-75 F (16-24 C). Provide adequate space, since this plant, although slow growing, will reach heights of 8-10 feet. But one caution: the dwarf date palm should not be placed near children’s play areas because it has sharp, needle-like spines arranged near the base of the leaf stem. These can easily penetrate skin and even protective clothing.
To care for your dwarf date palm, keep the root ball evenly moist, allowing the plant to dry out only in winter between waterings. Mist frequently, and feed weekly (except in winter, when a bi-weekly feeding is optimal) for best results.
8. Ficus Alii (Ficus macleilandii “Alii”)
The Ficus Alii is effective at removing chemical vapors from the air and grows easily indoors, resisting most insects. It prefers a humid environment and full or semi-sunlight, though it will tolerate lower light conditions. Its ideal temperature range is between 65 F and 85 F (16-24 C).
The Ficus Aliii should not be placed near heating or air conditioning vents, or near drafts because this could cause leaf loss. Soil should be kept moist but allowed to dry out between watering. If your plant’s leaves turn yellow, this may indicate overwatering.
During the plant’s active growing season (spring through fall), fertilize with diluted liquid fertilizer monthly (in a sunny location) or bi-monthly (in lower light). Ensure your fertilizer doesn’t contain boron, since the plant is sensitive to that element. Also be aware that the Ficus alii is poisonous and should be kept away from pets and young children.
In its tropical habitat, Ficus alii will grow into a large evergreen tree. When grown inside as a potted plant, however, the Ficus alii will reach approximately 10 feet in height.
9. Boston Fern (Nephrolepis exaltata “Bostoniensis”)
In the studies conducted by Dr. Wolverton, the Boston fern ranked the highest out of all plants tested in its ability to remove formaldehyde from the air. Since formaldehyde appears in everything from plastic garbage bags to paper towels, facial tissues, floor coverings, and adhesives—and gas stoves and tobacco smoking also release this chemical—the Boston fern is a smart addition to any household or office.
The Boston fern grows to four feet in height with a spread up to five feet. It has feathery ferns, which are best displayed as a hanging plant or in a pot placed on a pedestal. It prefers a cool environment, bright but indirect sunlight, and regular attention.
Keep the soil barely moist and mist frequently with warm water or the plant’s leaves will turn brown. This plant is prone to spider mites and whitefly, which can be controlled using a soapy water spray. Aphids may also attack Boston ferns if nearby. Inspect new plants for bugs before bringing them home.
When the plant is producing new foliage, feed weekly with diluted liquid fertilizer. During the winter, feed sparingly. Never let the root ball of a Boston fern dry out. A humus rich potting soil will support the plant’s needs and help ensure you don’t need to water daily.
Boston ferns prefer to be somewhat potbound, so only move to a bigger container when roots begin peeking through the drainage holes or if water runs directly through the pot. Repotting is best done in the spring, when the plant begins actively growing again. The ideal temperature range for a Boston fern is 65-75 F (18-24 C), dropping down to 50-65 F (10-18 C) at night.
10. Peace Lily (Spathiphyllum "Mauna Loa")
Excellent for removing alcohols, acetone, trichloroethylene, benzene, and formaldehyde, the Peace Lily also has a high transpiration rate to both purify and humidify indoor air. Its ribbed, glossy green leaves grow on stiff stalks that produce the characteristic white ‘flower,’ making it one of the few tropical plants that blooms indoors.
This hardy plant also tolerates neglect. Its ideal situation includes indirect sunlight and high humidity, but it will grow in semi-sunlight or semi-shade when placed out of drafts. For best results, the Peace Lily should be thoroughly watered, then allowed to go moderately dry between waterings. The leaves should be misted frequently with warm water and washed occasionally to prevent attack from pests like scale insects and spider mites.
Toxic to humans and pets, the plant’s leaves contain calcium oxalate and should be kept away from children. The compact plant grows to a height of three feet with a two-foot spread, preferring temperatures between 60-75 F (16-24 C) during the day, with slightly cooler temperatures at night.
How to Grow Fresh Air: 50 House Plants that Purify Your Home or Office (Penguin; First Edition edition April 1, 1997).
Wolverton Environmental Services (http://www.wolvertonenvironmental.com/air.htm), Last updated May 2009.
Dr. B.C.Wolverton’s book is available online: “How to Grow Fresh Air – 50 Houseplants that Purify Your Home or Office”.
His Latest book, Plants: Why We Can’t Live Without Them is also now available online.