I had to smile at the concern in her voice before explaining that the earth's plants and trees work together to provide all the oxygen we need. It's a fact that we take pretty much for granted, which makes it no less remarkable. Plants are the original air filters, taking our gaseous wastes and transforming them into clean, pure oxygen for our breathing enjoyment.

In recent years, there has been much discussion about poor indoor air quality. The use of chemicals in everything from personal care products to building supplies has put our health at risk. Fortunately for plant lovers, indoor plants may be one of our best defenses in clearing our homes of these hidden toxic fumes. How wonderful that something as aesthetically beautiful as a houseplant has this built-in ability to purify the air.

Several years ago, NASA and the ALCA (Associated Landscape Contractors of America) undertook the study of a variety of plants and their effectiveness at eliminating three specific chemicals - formaldehyde (used in many building supplies), benzene (found in inks, paints and gasoline) and trichloroethylene (found in paints, lacquers and varnishes).

The results were both impressive and interesting.

While all of the plants produced cleaner air, they were somewhat selective, each showing a preference of chemicals.* One could assume from these results that the wider the variety of plants in your home or office, the cleaner your air. Further, the NASA study concluded that effective air purification required no less than one potted plant per 100 square feet of indoor space. It's enough to send you running to the nearest greenhouse!

Need a few suggestions for your shopping list? Here are just a few of the plants included in the NASA study. All performed admirably in clearing the air.

• Spider Plant (Chlorophytum comosum) - One of the most popular houseplants, the Spider Plant is an extremely adaptable, easy keeper. Thrives in most lighting conditions, but prefers to stay out of direct sunlight. This is the perfect plant for those who are not blessed with a green thumb.

• Golden Pothos or Devil's Ivy (Epipremnum aureum syn. Scindapsus aureus) - The climbing plant, Golden Pothos, tolerates most lighting conditions, other than direct sunlight. Can be trained to grow up a pole or trail from a hanging pot.

• Gerbera Daisy (Gerbera sp.) - When grown indoors, the Gerbera Daisy will produce flowers year-round. Prefers full sun to partial shade.

• English Ivy (Hedera Helix) - English Ivy will grow in lower light, but bright light without full sun will produce a healthier plant. This plant works well in a hanging basket, or as a topiary. Warning to parents and pet owners - The leaves of English Ivy are poisonous if ingested.

• Peace Lily (Spathiphyllum) - Tolerant to most lighting conditions, the peace lily will produce more flowers in diffused bright light situations.

Be sure to properly care for your plants for the most effective results. It stands to reason that a healthy plant is the most efficient air purifier. Place the plant in the best lighting conditions available. Provide ample amounts of water, being careful not to over water. Fertilize according to each plant's needs, and keep leaves free of dust.

A healthy and varied plant population can give your home a warm and inviting look and improve the quality of the air you breathe. It's just one more advantage to being a lover of all things green.

Heather Haapoja is a freelance writer who works from her home office in beautiful northern Minnesota. To read more of her work, or for contact information, visit her home page at http://www.angelfire.com/my/haapojaworks