Wolverton Environmental Services, Inc.


Frequently Asked Questions about Indoor Air Pollution




Indoor Air


















1. How do plants remove chemical vapor from the air?

(1)   Plant leaves can absorb certain organic chemicals and destroy these chemicals by a process called “metabolic breakdown.”  This was proven by a group of German scientists who labeled formaldehyde with a radioactive carbon 14 tag and followed its absorption and metabolic destruction inside a spider plant (Chlorophytumn comosum).   The formaldehyde was metabolized and converted into tissue products such as organic acids, sugars and amino acids as demonstrated by the radioactive carbon 14 label.  This information was published in the Plant Physiology Journal in 1994. [Martina Giese, Ulrike Bauer-Doranth, C. Langebartels, and Henrich Sanderman, Jr.  “Detoxification of formaldehyde by the spider plant (Chlorophytum comosum).   Plant Physiology, 1994, 104: 1301-1309.

 (2)   When plants transpire water vapor from their leaves, they pull air down around their roots.   This supplies their root microbes with oxygen.  The root microbes also use other substances in the room air, such as toxic chemicals, as a source of food and energy.  Microbes, such as bacteria, can rapidly adapt to a chemical contaminant by producing new colonies that are resistant to the chemical.  As a result, they become more effective the longer they are exposed to the chemical.  It is also important to remember that the efficiency of plants or a filtering device decreases as the concentration of chemicals in the air decreases.  For example, the removal rate of a chemical is much higher at 7 parts per million (ppm) exposure than at 2 ppm.

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2. Do temperature and humidity influence the ability of plants to remove airborne chemicals and microorganisms?

      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 data is shown below.


Average Water Loss

24-Hour Period


Average Room


F (C)

Average Room

Relative Humidity


900 73 (22.8) 36.7
660 73 (22.8) 52.0
675 81 (27.2) 57.0
550 76 (24.4) 62.0

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

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3. How do I determine how many plants I need?

     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.  This data can be found in an EPA report on Indoor Air Quality in Public Buildings: Vols. I and II, August 1988, EPA/00/6-88/009ab.

      Data was extracted from this report to produce the following table:


Mean Indoor Air Concentrations of Chemicals

Found in Various Buildings

Micrograms Per Liter (g/l)

Chemicals Hospitals Nursing Homes Office Buildings
Chloroform --- .004 .002
Trichloroethylene .002 .001 .005
Benzene .003 .003 .005
Xylene .013 .005 .022
Formaldehyde .106 .081 .173

        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 g/liter of formaldehyde, then the room would contain a total of approximately 3,917 g 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*


Boston fern Nephrolepis exaltata "Bostoniensis" 1863
Dwarf date palm Phoenix roebelenii 1385
Bamboo palm Chamaedorea seifrizii 1350
Janet Craig Dracaena deremensis "Janet Craig" 1328
English ivy Hedera helix 1120
Weeping fig Ficus benjamina 940
Peace lily Spathiphyllum "Clevelandii" 939
Areca palm Chrysalidocarpus lutescens 938
Corn plant Dracaena fragrans "Massangeana" 938
Lady palm Rhapis excelsa 876

 * 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.

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4. What are negative ions?

      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 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, Dr. Lohr 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.

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5. Do Houseplants Increase Dust and Mold Spores in Rooms?

    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 published a paper demonstrating that interior plants could reduce the dust levels in a computer room by 20 percent. {Reference: Virginia Lohr.  "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.

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6. Will houseplants in a bedroom remove the oxygen at night?

     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.

7. Which houseplants are best for your child's bedroom?

Some houseplants that are considered safe to have around small children include:

  • Areca palm
  • Lady palm
  • Bamboo palm
  • Snake plant
  • Swedish ivy
  • Spider plant
  • Yucca
  • Corn plant
  • Boston fern

Placing plants in a child’s bedroom that are deemed safe should prove helpful. For example, a lady palm and snake plant, which both thrive in low-light conditions, would be ideal in a child’s bedroom.
If plants are grown in hydroculture, it is best to place a screen or some other barrier over the top of the pebbles to prevent children from placing the pebbles in their mouth.

Most houseplants do consume a small amount of oxygen during the night. However, it is an insignificant amount when compared to the overall volume of air in a room. Some plants such as the snake plant, orchid and bromeliad actually produce oxygen at night.

Plants Harmful to Children

Common houseplants that can be harmful to children if they chew on the leaves are golden pothos, English ivy, dieffenbachia, philodendron and syngonium. These are some of the commonly known plants but one should consult a trusted plant grower or medical source if concerned about a particular plant.


Excerpted from article written by Dr. Wolverton for www.safbaby.com. Read the entire article at the following link: http://www.safbaby.com/purify-indoor-air-with-house-plants-expert-advice

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1. Daly, John, Margaret Burchett and F. Torpy. 'Plants in the classroom can improve student performance,' (Oct 2010). pdficon_small.jpg (747 bytes)

2. Fjeld, T., et al. 'Effect of Indoor Foliage Plants on Health and Discomfort Symptoms Among Office Workers,' Indoors + Built Environment, 1998, 7:204-206. (Norway)

3. Giese, M., U. Bauer-Doranth, C. Langebartels and H. Sandermann, Jr., 'Detoxification of Formaldehyde by the Spider Plant (Chlorophytum comosum L.) and by soybean (Glycine max L.) cell suspension cultures,' Plant Physiology, 1994, 104:1301-1309. (Germany) pdficon_small.jpg (747 bytes)

4. Lohr, V. I. 'Particulate Matter Accumulation on Horizontal Surfaces in Interiors: Influence of Foliage Plants,' Atmospheric Environment, 1996, 30:2565-2568. (U.S.)

5. Lohr, V. I., et al. 'Interior Plants May Improve Worker Productivity and Reduce Stress in a Windowless Environment,' J. Environ. Hort., 1996, 14:97-100. (U.S.

6. Nakamura, R. and E. Fujii.  'Studies of the Characteristics of the Electroencephalogram When Observing Potted Plants: Pelargonium hortorum 'Sprinter Red' and Begonia evansiana,' Technical Bulletin of the Faculty of Horticulture of Chiba University, Japan, 1990, 43:177-183. (Japan)

7. Oyabu, T., T. Onodera, H. Kimura, et al. 'Purification Ability of Interior Plants for Removing of Indoor Air Polluting Chemicals Using a Tin Oxide Gas Sensor,' J. of Japan Society for Atmospheric Environ., 2001, Vol. 34(6):319-325. (Japan)

8. Oyabu, T., et al. 'Purification Effect of Interior Plants for Indoor Air Polluting Chemicals and Environmental Preservation,' 4th Intl. Conf. on Eng. Design and Automation, 2000, (Orlando, Fl, July 30-Aug 2) pp. 876-881. (Japan)

9. Pegas, P. N., et. al, 'Can houseplants improve indoor air quality in schools?' J. Toxicol. Environ. Health A., 2012, 75:22-23. (Abstract available from http://www.ncbi.nlm.nih.gov/pubmed/23095155)

10. Ulrich, Roger S. 'Health Benefits of Gardens in Hospitals,' Plants for People Conference, Intl. Exhibition Floriade 2002, The Netherlands. (U.S.)

11. Ulrich, Roger S., et al.  'Stress Recovery During Exposure to Natural and Urban Environments,' J. of Environ. Psychology, 1991, 11:201-230. (U.S.)

12. Wolverton, B. C. and Kozaburo Takenaka. Plants: Why You Can't Live Without Them, Roli Books, New Delhi, 2010.

13. Wolverton, B. C. Eco-Friendly Houseplants, Weidenfeld & Nicolson, London, 1996. Released in U.S. as How To Grow Fresh Air, Penguin Books, New York, 1997. (U.S.)

14. Wolverton, B. C. and J. D. Wolverton, 'Interior Plants: Their Influence on Airborne Microbes Inside Energy-Efficient Buildings,' Journal of the Mississippi Academy of Sciences, 1996, 41(2): 99-105. (U.S.) pdficon_small.jpg (747 bytes)

15. Wolverton, B. C. and J. D. Wolverton, 'Plants and Soil Microorganisms -- Removal of Formaldehyde, Xylene and Ammonia from the Indoor Environment,' Journal of the Mississippi Academy of Sciences, 1993, 38(2): 11-15. (U.S.) pdficon_small.jpg (747 bytes)

16. Wolverton, B. C. and J. Wolverton, 'Bioregenerative Life Support Systems for Energy-Efficient Buildings,' Proceedings of International Conference of Life Support and Biospherics, Huntsville, Alabama, 1992. (U.S.)

17. Wolverton, B. C., A. Johnson and K. Bounds, 'Interior Landscape Plants for Indoor Air Pollution Abatement,' NASA/ALCA Final Report, Plants for Clean Air Council, Davidsonville, Maryland, 1989. (U.S.) pdficon_small.jpg (747 bytes)

18. Wolverton, B. C., R. C. McDonald and H. H. Mesick, 'Foliage Plants for the Indoor Removal of the Primary Combustion Gases Carbon Monoxide and Nitrogen Oxides,' Journal of the Mississippi Academy of Sciences, 1985, 30:1-8. (U.S.) pdficon_small.jpg (747 bytes)

19. Wolverton, B. C., R. C. McDonald and E. A. Watkins, Jr., 'Foliage Plants for Removing Indoor Air Pollution from Energy-Efficient Homes,' Economic Botany, 1984, 38(2):224-228. (U.S.) pdficon_small.png (723 bytes)

20. Wood, R. A., et al. 'Study of Absorption of VOCs by Commonly Used Indoor Plants,' Proceedings: Indoor Air '99, 1999, Vol 2:690-694. (Australia)

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