Vegetation and Air Disinfection

A handful of studies have investigated the use of vegetation as a means of removing or reducing levels of airborne microorganisms. This has sometimes been referred to as "growing clean air." Recently a Canadian firm developed what it called a "breathing wall," or a wall of plants and waterfalls that seems to improve air quality (ASHRAE Journal 1998, May, p58). Earlier results of studies by government agencies seemed inconclusive but recent experience suggests there may be merit to this idea.

The reasons that vegetation may reduce levels of airborne microorganisms are varied. The surface area of large amounts of vegetation may absorb or adsorb microbes or dust. The oxygen generation of the plants may have an oxidative effect on microbes. The increased humidity may have an effect on reducing some microbial species although it may favor others. The presence of symbiotic microbes such as streptomyces may cause some disinfection of the air. Natural plant defences against bacteria may operate against mammalian pathogens.

One downside to keeping large amounts of vegetation indoors is that the potting soil may include potentially allergenic fungi. The presence of moisture may also contribute to fungal problem. Clearly there is some balance to be achieved between the desirable and undesirable effects.

The use of waterfalls in conjunction with vegetation will increase local humidity. Humidity has mixed effects, as stated before, but the use of moving water may generate positive or negative ions, or may simply cause hygrophobic microbes to precipitate out of the air. The evaporative cooling effect of dripping water may chill some microbes into inactivation. Cooling coils have a similar effect. Evaporative coolers (not warm cooling towers) may have a similar effect.

A possible application might be to route building return air through a greenhouse. Not only will some filtering effects occur, but oxygen will be replenished and the solar exposure will cause some air disinfection.

Few references are available at present, but those that provide related information are listed below.

References

  1. Burroughs, H. E. B. (1997). “IAQ: An environmental factor in the indoor habitat.” HPAC, 69(2), 57-60.
  2. Cox, C. S., F.Baldwin. (1967). “The toxic effect of oxygen upon the aerosol survival of Escherichia coli.” Journal of General Microbiology, 49, 115-117.
  3. Davey, B., and Halliday, T. (1994). Human biology and health: An evolutionary approach, Open University Press, London.
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  8. Fisk, W. (1994). “The California healthy buildings study.” Center for Building Science News, Spring 1994, 7,13.
  9. Fisk, W., and Rosenfeld, A. (1997). “Improved productivity and health from better indoor environments.” Center for Building Science News, Summer, 5.
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  11. Gregory, P. H. (1973). Microbiology of the atmosphere, Leonard Hill Books, Plymouth.
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  14. Hautanen, J., T.Watanabe, T.Tuschida, Y.Koizumi, F.Tochikubo, E.Kauppinen, K.Lehtinen and J.Jokiniemi. (1995). “Brownian agglomeration of bipolarly charged aerosol particles.” Journal of Aerosol Science, 26(S1), s21-s22.
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  23. Puckorius, P. R., Thomas, P. T., and Augspurger, R. L. (1995). “Why evaporative coolers have not caused Legionnaire's Disease.” ASHRAE Journal, Jan, 29-33.
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  25. Watanabe, T., F.Tochikubo, J.Hautanen and E.I.Kauppinen. (1995). “Review of particle agglomeration.” Journal of Aerosol Science, 26(S1), s19-s20.
  26. Wise, J. A. (1997). “How nature nurtures: Buildings as habitats and their benefits to people.” HPAC, 69(2), 48.