Yes.  The transpiration rates of plants are important in removing both airborne chemicals and microbes.  When plants transpire (emit) water vapor from their leaves into the air, they also pull air down to their roots.  Any airborne contaminants are also pulled down into the plant root zone.  Microbes, living on and around plant roots in an area called the rhizosphere, breakdown and destroy the chemicals.  Microbes convert these chemicals into a source of food and energy for the plant and themselves.

      Both temperature and humidity influence the transpiration rates of plants.  Test results show that plants with high transpiration rates are more effective in removing pollutants from the indoor environment.  For example, we conducted studies in a home during the winter months using an Areca palm (Chrysalidocarpus lutescens)(height: 56-inches or 142-cm)  that was growing in a pot 14-inches in diameter (36-cm).  The test results are shown below.

       The chart above shows that temperature and humidity levels influenced plant water loss (transpiration rates).

     The amount of leaf surface area influences the rate of air purification by plants.  Generally, the larger the plant leaf surface area, the higher the transpiration rate and the greater the surface area to absorb airborne chemicals.

     The basis for recommending the number of plants per room is based upon the average amount of air pollution found in public buildings that were tested by the U.S. Environmental Protection Agency (EPA).  The EPA monitored the indoor air quality in hospitals, nursing homes and office buildings for several years and published their findings in a report entitled, " Indoor Air Quality in Public Buildings: Vols. I and II, August 1988, EPA/00/6-88/009ab".

      Detected chemical levels were extracted from this report to produce the following table:

Mean Indoor Air Concentrations of Chemicals

Found in Various Buildings

Micrograms Per Liter (µg/l)

        A 100 ft² (9.3 m²) office with an 8 ft (2.4 m) ceiling contains a volume of 800 ft³ (22,640 liters) of air.  Based upon the above data, if the air contained 0.173 µg/liter of formaldehyde, then the room would contain a total of approximately 3,917 µg of formaldehyde.  Go to the table below and estimate the number of plants required to remove this amount of formaldehyde. Formaldehyde is the predominant chemical found in the test buildings.   Therefore, if sufficient numbers of plants are added to remove formaldehyde, other chemicals should also be removed.

Removal of Formaldehyde from Sealed Chambers

By Plants Grown in Potting Soil

 * Removal rate may vary with plant size and growth medium.

** The figures above are the results of sealed-chamber studies and not "real world" conditions.  In a real environment, conditions could vary significantly.  Therefore, we recommend that one should at least double the number of plants required based upon this information.

      Ions are charged particles in the air that are formed when enough energy acts on a molecule, such as water, to eject an electron.  The displaced electron attaches itself to a nearby molecule that becomes a negative ion.

      In nature, ions are formed in a variety of ways, such as UV light, airflow friction, lighting, falling water and by plants.  Plant leaves produce negative ions as they emit water vapors.  Therefore, plants that have the highest transpiration rates produce the most negative ions.  Waterfalls and tropical forests create copious amounts of negative ions.   Synthetic building materials, clothing and furniture coverings remove large numbers of negative ions from the indoor environment.   The positive static charge of plastics also consumes large quantities of negative ions.  Therefore, the negative ion count in modern buildings is often very low.

      Since high levels of negative ions are needed for good health, large numbers of indoor plants can improve our health and feeling of well-being.  Our studies have shown that large numbers of indoor plants can reduce the levels of airborne microbes.   Although we did not measure negative ion levels, the reduction in mold spores and bacteria in the air surrounding those plants was most likely due to negative ions.  Our studies were published in 1996 in the Journal of the Mississippi Academy of Sciences {41(2): 99-105}

      Dr. Lohr from Washington State University also published a paper on how houseplants can reduce human stress and increase productivity.  {“Interior plants may improve worker productivity and reduce stress in windowless environments,” Journal of Environmental Horticulture, 1996, 14(2): 97-100.}   These effects are most likely due to increased negative ion levels in offices.

      Dr. Lohr and colleagues published another paper entitled {“Particulate matter accumulation on horizontal surfaces in interiors: Influence of foliage plants,” Atmospheric Environment, 1996, 30(14): 2565-2568.   In this paper, the researchers demonstrated that houseplants could reduce the dust levels in a computer room by 20 percent.  This reduction was most likely from the production of negative ions.

      The positive health effects of negative ions have been known for almost a hundred years.  The fact that houseplants produce negative ions is a well-established fact.

    No.  In fact, foliage plants reduce airborne microbes in the ambient air provided that the soil is not exposed.   A two-inch (more or less) layer of gravel or other porous material on top of the soil will prevent mold growth.  Studies have shown that plant-filled rooms have 50 to 60 percent less airborne microbes than similar rooms without plants.  This data was published in the Journal of the Mississippi Academy of Sciences in 1996. {Reference: B. C. Wolverton and J. D. Wolverton.  "Interior Plants: Their Influence on Airborne Microbes Inside Energy-Efficient Buildings."  J. MS Acad of Sciences, 1996, 41(3):99-105.}

     Dr. Lohr and colleagues published a paper demonstrating that interior plants could reduce the dust levels in a computer room by 20 percent. {Reference: Lohr. V. I. and C. H. Pearson-Mims,  "Particulate Matter Accumulated on Horizontal Surfaces in Interiors: Influence of Foliage Plants."  Atmospheric Environment, 1996, 30(14):2565-2568.}

     Therefore, scientific research demonstrates that interior plants can reduce the levels of airborne microbes and dust in rooms whenever a sufficient number of plants are present.  Some allergy physcians continue to recommend that patients remove all plants from their homes.   However, there is no scientific basis for this recommendation.  In fact, plants should be beneficial to allergy patients provided the plants are grown in a manner to prevent mold growth on the soil surface.

     No.  Do the animals underneath the dense, tropical canopy of the rain forest die at night from lack of oxygen?  I don't think so.  It is true that some plants use a small amount of oxygen at night.  However, others such as succluents, orchids and bromeliads actually add oxygen to the air at night.  The Sanseviera (Snake Plant or Mother-In-Law's Tongue), a common easy to grow plant, also gives off oxygen at night.   The only effect one should receive from filling a bedroom with plants is a feeling of breathing healthy indoor air.  The only caveat is to make sure the soil's surface is covered to prevent mold spore growth.