January / February 2000

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Houseplants for Healthy Living by B.C. "Bill" Wolverton, Ph.D. Science is now catching up with what gardeners have known for decades. That is, growing plants can relieve stress, while helping to clean the environment. Gardening has become the number one leisure activity in the United States and Canada, even surpassing sports. A growing body of research shows that cultivating plants indoors and outdoors may be the best medicine available for improving mental and physical well-being, at any age. Although many events helped popularize indoor gardening, nothing compares to the impact in the United States of the large group of "Baby Boomers." Baby boomer is a term used to describe the population phenomena of post-World War II. The boomers are now the most significant consumer group in the United States. They have become the plant lovers of the ’90s. As baby boomers grow older, they begin to move their passion for growing plants indoors. Houseplants have become mainstream because people find them rewarding in a calm and attractive way. As more people become concerned about the environment, their health, and crime in the outside world, greening of indoor environments will continue to expand. Although the term "green building" is becoming an attractive concept to building managers and building occupants, the use of living plants is not part of the present concept. Architects and engineers are beginning to design buildings with an eye toward low-emitting carpets, paints and furniture. This is good but should only be the first step. A further step should include the design of houseplants into each building, mimicking the earth’s natural processes. Benefits derived from our botanical friends include a wide range of psychological and physiological effects. Studies conducted on plant-people interactions have provided overwhelming evidence that plants do indeed have a measurable beneficial effect on people and the spaces they inhabit. Plants not only add beauty to a room, but also make it a friendly, inviting place to live or work. Plants symbolize friendship and appear to have a calming, spiritual effect on most people. This perhaps explains why plants play such an important role in human events such as weddings, funerals, holidays, hospital stays and birthdays. Plants are also used as background props for most important events such as television addresses, commercials, etc. People feel relaxed when they are near or tending to living plants. Corporations install interior landscaping to increase worker productivity and decrease absenteeism. Elite hotels, restaurants and other businesses use plants to help entice customers to their establishments. During early manned space flights, NASA astronauts and Soviet cosmonauts expressed a desire to have plants on board their space vehicles. Plants can help reduce stressful conditions inside cramped space capsules during long-duration flights. In the past, houseplants were sought only for their beauty and psychological value. Thanks to NASA research findings, houseplants now have a third dimensional value. Studies, conducted in the early 1980s at the John C. Stennis Space Center in Mississippi, provided evidence that houseplants can also improve indoor air quality (IAQ). The ability of houseplants to improve IAQ and one’s health is no longer a matter of conjecture, but scientific fact. Plants: Nature’s Bio-Cleaning Machines Plants and their root microbes are nature’s "biological cleaning machines." It is commonly understood that plants purify and revitalize the earth’s air and water. In general, we know that the animal/plant/microbial world is harmoniously balanced so that each benefits from the other. We are dependent upon these interactions for our existence. However, we are just now beginning to understand some of the mechanisms that create these symbiotic relationships. Approximately forty-two species of interior plants have been evaluated for their ability to remove various indoor air contaminants from sealed chambers. Hundreds of experiments have been conducted and technical reports published that seek to answer legitimate concerns about placing plants in buildings for the specific purpose of improving IAQ . After more than ten years of extensive research, both laboratory and "real-world," we now have a basic understanding of how plants function to remove indoor pollutants. Research, conducted by Wolverton Environmental Services, Inc. and supported by the Plants for Clean Air Council in Mitchellville, Maryland, continues to expand on the research begun at NASA. Specifically, we are trying to understand how plants clean and revitalize the air and how to use this knowledge to improve indoor air quality. Plants use ingenious methods to obtain food and protect themselves from would-be enemies. Each plant has the ability to culture microbes on and around its roots specific for its needs. These microbes biodegrade and mineralize (compost) dead leaves, animal waste, tannic and humic acids, and other debris to provide nutrients for the microbes and their host plant . . . the basis for organic gardening. Geographic locations and environmental conditions of the plant’s origin determine which microbes it cultures. For example, the microbes associated with plants that evolved underneath the canopy of tropical rainforests (most houseplants) differ from those in an arid environment. Tropical plants need aggressive microbes that can rapidly recycle jungle debris. Because rainforests are dark, warm and humid, mold and bacteria thrive. However, tropical plants excrete substances that protect their leaves from airborne molds and mildew. When these plant species are placed in an indoor environment, they continue to suppress airborne mold spores. Because chemical pollutants commonly found indoors, such as formaldehyde, benezene and xylene, have structures similar to components found in tannic and humic acids, microbes adapt to biodegrade these chemicals also. Thus, the basis for plants’ ability to improve IAQ is established. Plants use two well-known processes to move chemicals in the air to their roots: (1) Leaves absorb certain chemicals in the air and transport them inside plant tissue down to the roots, and (2) Plants pull air down around their roots when moisture is emitted from leaves during transpiration. Plants with high transpiration rates are able to move greater amounts of air. Therefore, the more efficient air cleaners are plants with high transpiration rates. Plant transpiration rates are controlled by humidity. Plants attempt to balance humidity levels for their optimum well-being by controlled release of moisture from their leaves. When humidity is high, plants emit less moisture into the air then when humidity is low. (See chart.) Early critics complained that too many plants in buildings would cause the humidity levels to rise and support the growth of mold and mildew. However, these findings prove otherwise. Low humidity, most prevalent during winter months, dries the respiratory system and makes one more susceptible to colds, viruses and allergens. Ideally, humidity should range between 40-60 percent. Plants produce healthy, microbial-free moisture. On the other hand, mechanical humidifiers, when not properly maintained, can become a source of mold and mildew. When plants transpire, they not only add moisture to the air, but also emit substances that help suppress airborne mold spores and bacteria. Although these substances are yet to be identified, we do understand their function. Recent findings show that plant-filled rooms contained 50-60 percent fewer airborne mold and bacteria than rooms with no plants. Interestingly, air in the plant-filled rooms had fewer microbes, even when temperature and humidity levels were raised—the exact opposite as predicted by some critics. Ironically, some doctors advise their allergy patients to avoid houseplants. Houseplants have been falsely accused of harboring mold spores. The real problem is usually overwatering and the growth of mold on wet carpeting. To avoid these problems, use hydroponic (soil-less) methods in water-tight plants to grow houseplants. If potting soil is used, cover it with aquarium gravel and feed and water from the bottom to keep the surface dry. There are also many commercial sub-irrigation systems available. When large planters are used, the need for frequent watering can be eliminated. The Best Plants The top five plants were rated using the following criteria: 1) ability to remove formaldehyde from sealed chambers, 2) easy of growth and maintenance, 3) resistance to insects, and 4) transpiration rates. The top rated plants may change from time to time as newer research becomes available. As a general guide, two or more medium to large plants (14"-16" containers) per 100 square feet area are recommended. Of course, more plants and larger plants would certainly increase effectiveness. Plants alone may not be the total solution when serious IAQ problems exist. Proper source management (allowing building materials and furnishings to vent volatile fumes before installation), complete air distribution, and preventative maintenance are all components of a healthy building. Also, fan-assisted planter/filters may be needed to rapidly remove pollutants from the air. Fan-Assisted Planter/Air Filters Once the biological mechanisms of plants were understood, it was only natural to merge man and nature’s technologies. By combining the most effective air filtering media, mechanical air flow devices, and living plants. WES, Inc. has developed a family of enhanced plant/air filters. These aesthetically designed, patented planter units not only increase the air purification capacity of houseplants by as much as 200 times, but also help maintain healthy indoor humidity levels. One of the unique properties of this natural air purification is that under normal operating conditions, the filtering media is bioregenerated (self-cleaned) by the plant’s root microbes. Therefore, the filter media does not require periodic replacement as is the case with other commercial air filters. The first enhanced planter filter is currently marketed under the name "Nature’s Air Filter." A completely redesigned and vastly improved model will soon be on the market. Built-in versions are also available for large installations. Summary Research has identified at least three functions that houseplants perform to improve indoor air quality: 1) reduce chemical vapors emitted by synthetic materials, 2) add healthy, microbial-free humidity to dry indoor air, and 3) reduce airborne mold spores and bacteria. We should all breathe easier knowing our beautiful houseplants are working so hard to keep up healthy! Bill Wolverton is president of Wolverton Environmental Services, Inc. in Picayune, Mississippi and author of the book, "How To Grow Fresh Air."