Homes and office buildings are being built and remodeled tighter than ever today to save energy. But sealing out drafts can mean sealing in dangerous chemicals such as benzene and formaldehyde, which are off-gassed by many things, from carpet to pressed-wood furniture. Cleaning products used at home and the office also may release chemicals, including ammonia and chlorinated solvents.

Prolonged breathing of chemicals such as these can cause headaches, allergies, throat irritation, congestion and other ailments. The Environmental Protection Agency (EPA) now ranks indoor-air quality among the five top threats to human health. The problem is worse in winter, when we spend the most time sealed in our homes and offices.

Expensive air-filtration systems are not the only solution. While working at NASA, we discovered that simple houseplants can filter many dangerous chemicals out of indoor air. The plants’ leaves absorb and destroy certain volatile organic compounds, while the microbes that live around the plants’ roots convert chemicals into a source of food and energy for themselves and their host plant.

Some houseplants are much more effective at air filtration than others. Ten that do a great job cleaning the air and are easy to grow…

THE TOP FOUR — The following four plants are exceptionally effective at cleaning the air.

Areca Palm (Chrysalidocarpus lutescens) also is known as yellow palm or butterfly palm.

Care: Keep its root-ball damp, and mist the leaves with water often. Feed monthly with liquid fertilizer except in winter. Remove dead branches promptly.

Lady palm (Rhapis excelsa) is one of the most effective houseplants for improving indoor-air quality, and it is highly resistant to most insects. It has fans, six to 12 inches wide, made up of shiny green leaves.

Care: Water generously, especially during spring and summer. Feed monthly with liquid fertilizer per label instructions. Leaf tips can be trimmed with pinking shears if they turn brown.

Rubber plant (Ficus robusta) is a hearty plant with thick, leathery, dark green leaves. It will tolerate limited light and cool temperatures and is very effective at removing chemical toxins from indoor air—the best of the ficus genus yet tested.

Care: Water regularly from mid-summer through autumn, but let the soil dry slightly between watering. Water sparingly during the rest of the year—rubber trees struggle when overwatered. Feed monthly with liquid fertilizer during the summer only.

Peace Lily (Spathiphyllum sp) produces beautiful white flowers, making it one of a relatively small number of plants that bloom reliably indoors.

Care: Keep the soil evenly moist, and feed monthly with liquid fertilizer from spring through autumn. Discontinue feedings, and keep the soil slightly drier in winter. Wash the leaves occasionally with a damp cloth to reduce the odds of infestation by insects such as spider mites.

THE RUNNERS-UP – These plants work well, too, but not quite as well as the first four

Dracaena “Janet Craig” (Dracaena deremensis “Janet Craig”) is a pest-resistant leafy plant that can live for decades. It will tolerate poorly lit areas, though its growth will be slowed. Favor the “Compacta” variety, which grows to one to three feet in height, not the regular variety, which requires more care and can reach 10 feet if not pruned.

Care: In spring and summer, keep soil evenly moist so the root-ball does not dry, but never water so much that the soil becomes soggy. Feed with liquid fertilizer twice a month. In autumn and winter, water less often and do not feed. Spray with a water mist.

Golden Pothos (Epipremnum aureum) tolerates neglect and low light and is extremely resistant to insects. Golden Pothos’ name refers to the splashes of gold or cream color on its green, heart-shaped leaves. It usually is grown in hanging baskets, but it also can climb.

Care: Allow the soil to dry slightly between watering. Feed with diluted liquid fertilizer weekly in spring and summer. Clean the leaves occasionally with a damp cloth.

Corn Plant (Dracaena fragrans “Massangeana”) is so named because its leaves look like those of corn stalks, not because it actually produces corn. It tolerates low light, though it prefers bright light.

Care: Keep soil moist but not soggy from spring through autumn, then drier in winter. Feed monthly in spring and summer with liquid fertilizer. Mist often. Brown leaf tips can be trimmed.

Syngonium (Syngonium podophyllum) produces visually appealing, arrow-shaped, green-and-white or green-and-silver leaves.

Care: Keep soil evenly moist but not soggy from spring through autumn. Allow the soil to dry between watering in winter. Feed monthly with a liquid fertilizer except in winter. Syngonium appreciates frequent misting. Pinch back shoots if you want a thicker, bushier plant.

Snake Plant (Sansevieria trifasciata) is about 50% less effective at cleaning indoor air than the other plants on this list, but it is so hard to kill that it’s worth including as an option for those who struggle to keep other houseplants alive. The snake plant has visually appealing, spear-like leaves that stand upright, typically two to four feet in height. It occasionally produces small greenish-white flowers.

Care: Water sparingly, allowing the soil to dry between watering. Overwatering is one of the few ways to kill a snake plant. Feed with diluted liquid fertilizer once a month. Remove flowers if they appear, or they might release a sticky, hard-to-clean substance.

HOW MANY PLANTS?

As a rule of thumb, one to two good-sized plants from the list above per 100 square feet of interior space tend to be sufficient. There’s no health downside to having more plants than this as long as mold doesn’t develop in the potting soil.

Helpful: Our latest research suggests that growing plants in hydroculture significantly increases their ability to clean the air. With hydroculture, plants are grown in watertight containers and rooted not in potting soil, but in expanded clay pebbles sold for this purpose at some garden stores.

All of the plants listed above will grow in hydroculture, and the Peace Lily, in particular, thrives when grown this way. Hydroculture also reduces the risk that fungal spores, mold and soil-borne pests will develop on houseplants. Plants grown in hydroculture need a complete fertilizer that contains micronutients. Ask your garden store for details.

Source: B.C. Wolverton, PhD, president of Wolverton Environmental Services, Inc., an environmental consulting firm in Picayune, Mississippi. He previously spent more than 30 years working for the US military and the National Aeronautics and Space Administration (NASA) as a civilian scientist developing systems to protect against toxic chemicals and pathogenic microbes. He is author of Plants: Why You Can’t Live Without Them (Roli) and How to Grow Fresh Air (Penguin). http://www.WolvertonEnvironmental.co