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, surpassing even 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 "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 from our botanical friends
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
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.
Nature's bio-cleaning machines
In the past, houseplants were sought only for their beauty and
psychological value. Thanks to NASA research findings, houseplants now
have a third 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. The ability of
houseplants to improve indoor air quality and one's health is no
longer a matter of conjecture - it's scientific fact.
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
We are just now beginning to understand some of the mechanisms that
create these symbiotic relationships. Approximately 42 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 indoor air quality.
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. This is basis of 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 arid environments.
Tropical plants need aggressive microbes that can rapidly recycle
jungle debris. Because rainforests are dark, warm and humid, mold and
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, benzene 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 indoor
air quality is established.
Humidity: the basics
Plants use two well known processes to move chemicals in the air to
their roots: Leaves absorb certain chemicals in the air and transport
them inside plant tissue down to the roots, and plants pull air down
around their roots when moisture is emitted from leaves during
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.
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, findings proved 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 to 60 percent. Plants produce healthy, microbial-free
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 to 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 effect predicted by
Ironically, some doctors advise their allergy patients to avoid house
plants. House plants 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 house plants. 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.
Healthy air for your home
As a general guide, two or more medium to large plants (14"-16"
containers) per 100 square feet of area are recommended. Of course,
more plants and larger plants would certainly increase effectiveness.
Plants alone may not be the total solution when serious indoor air
quality problems exist. Proper source management (allowing building
materials and furnishings to vent volatile fumes before installation),
complete air distribution and preventive maintenance are all
components of a healthy building.
Fan-assisted planter/air filters
Fan-assisted planter/filters may be needed to rapidly remove
pollutants from the air. 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.
We should all breathe easier knowing our beautiful house plants are
working so hard to keep us healthy!
Written by: B.C. "Bill" Wolverton, Ph.D. Bill Wolverton is president
of Wolverton Environmental Services in Picayune, Mississippi, and
author of the book How To Grow Fresh Air. A retired NASA scientist, he
has studied plants as a way of cleaning the air for many years.This
article first appeared at Judy Barrett's Homegrown: Good Sense Organic
Gardening for Texas.