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The following plants were chosen for their ease of growth and maintenance, removal of chemical vapors, resistance to insect manifestation and transpiration (humidification) rates:

• Lady Palm (Rhapis excelsa)
• Areca palm (Chrysalidocarpus lutescens)
• Ficus alii (Ficus macleilandii)
• Peace lily (Spathiphyllum sp.)
• Golden pothos (Epipremnun aureum)
• Arrowhead vine (Syngonium podophyllum)
• Bamboo palm (Chamaedorea seifritzii)
• Dwarf Date palm (Phoenix roebelenii)
• Rubber plant (Ficus robusta)
• English ivy* (Hedera helix)

* Wild English Ivy is an invasive species in the Pacific Northwest.

A book titled, "Eco-Friendly Houseplants," (Viking/Penguin Press) by B.C. "Bill" Wolverton, will be published in September of this year.

Excerpts from Living on Earth host Steve Curwood's talk with Dr. Bill Wolverton:

CURWOOD: The EPA has identified as many as 800 volatile organic chemicals swirling inside buildings. They seep from walls and ceilings; they're in our clothing, furniture, and carpets. But there's a simple and cheap way to offset many of these toxic gases: common house plants. Finding out which ones do the best job involves a good deal of research, the kind of painstaking work done by Dr. Bill Wolverton, an environmental scientist and retired senior researcher at NASA. He's been studying the cleansing effects of indoor plants for about 25 years. Now, in our homes and offices, Dr. Wolverton, what are the greatest threats to indoor air quality?

WOLVERTON: Well back about 15 years ago with the energy crisis, we started to seal our homes and offices to make them more energy efficient. And at about the same time, we started to change the composition of materials in our homes and offices. Practically everything now, the desks, the furniture is made of synthetics. Also we started to use wall to wall carpeting which makes it nice, but unfortunately it gives off certain pollutants and particles. So what we've done over the years, in sealing up our home, we've created an unhealthy gas chamber inside our homes and offices.

CURWOOD: Now what can we do to counteract these effects and improve our air quality?

WOLVERTON: Well, answer is, take nature's living air filters, which are plants, design them in buildings or add them to buildings, and they will literally suck out these pollutants, absorb them, translocate them throughout the plant, break them down, or utilize the root microbes that they culture to help break them down. So it's a complicated process that the simple, innocent looking houseplant can do to help clean the air and hopefully save your health.

CURWOOD: And the five best plants that are easy to grow are?

WOLVERTON: Well, the ones that I prefer based on working with them for many, many years are the Peace lily, Areca palm, Lady palm, Ficus alii, and the Golden pothos...they're very easy to grow. They are among the top in removing undesirable indoor air polluting substances, such as formaldehyde. They have what we call a high transpiration rate. In other words, they add healthy moisture to a room, and that is especially important in the winter time up in your part of the country where it's cold. The air gets dry and it makes you more susceptible. So these plants, house plants, do a lot of good things for us in creating healthy indoor environments.

CURWOOD: Just how many plants do I have to have in order to clean the air in my home or my office?

WOLVERTON: We recommend that you use maybe 2 or hopefully 3 nice sized plants per 100 square feet.

CURWOOD: Well, I want to thank you for taking this time for us. My guest has been former NASA scientists Dr. Bill Wolverton. He's the author of the forthcoming book "Eco-Friendly Houseplants." That will be published by Viking Penguin in the fall. Thanks so much.