Providing a comfortable and healthy environment for building occupants is critical, as people spend more than 90% of their time indoors (Moschandreas, 1981). In the United States, buildings consume 40% of primary energy annually, and conditioning of residential spaces consumes nearly 50% of that energy (Energy Information Administration, 2009). During heating season, HVAC systems use energy to increase indoor air temperature while subsequently reducing RH. Without supplemental humidification, indoor RH may be as low as 10% in cold climates (Kalamees et al., 2009; Nordström et al., 1994; Reinikainen et al., 1991). Acceptable building humidity levels are governed by American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 55; upper limits on humidity are imposed to prevent mold and structural damage, but there are currently no building humidity lower limits. Therefore, many commercial and most residential buildings do not control humidities in the low humidity range (<40% RH) during heating season.
Low humidities can have adverse health consequences on building occupants (Sterling et al., 1985), such as increased eye irritation (McCulley et al., 2006) and skin dryness (Sunwoo et al., 2006). The survival of many pathogens and their transmission are sensitive to humidity levels, with increased transmission at lower humidities (Lowen et al., 2007; Shaman and Kohn, 2009; Yang and Marr, 2011).
Due to the impact of low humidity on building occupants and the desire to reduce building energy consumption, passive or low-energy humidification approaches are of increasing interest. Plants can influence the humidity of an interior environment through transpiration (water movement through a plant and water vapor emission from foliage) and evaporation (conversion of water to vapor at the surface of root medium), the combination of which is evapotranspiration. Many low-light plant species can grow indoors under existing lighting conditions (DelPrince, 2013; Manaker, 1997; Pennisi and van Iersel, 2012), reducing or eliminating the need for additional energy for lighting. Light can also be supplemented in interior environments to optimize plant longevity (DelPrince, 2013).
Researchers have examined the humidification effects of plants on indoor office environments (Lohr, 1992; Lohr and Pearson-Mims, 1996; Wolverton and Wolverton, 1996; Wood et al., 2006). Lohr (1992) studied the effects on plants in two offices with room temperatures of 22 °C: one office contained a variety of plants, including varieties of Peperomia (Piperaceae family) and chinese evergreen (Aglaonema sp.), and one control office contained no plants. The plants increased office humidity to 30% compared with 25% RH in the control office. Further work examined the effects of potted plants in a computer laboratory; humidity was not significantly altered, but particulate matter accumulation decreased by as much as 20% in the presence of plants (Lohr and Pearson-Mims, 1996). In a residential study, the presence of plants in a home increased RH by more than 10% and decreased airborne microbes by more than 30% (Wolverton and Wolverton, 1996). Peace lily (Spathiphyllum ‘Sweet Chico’) and dracaena (Dracaena ‘Janet Craig’) plants were field-tested in Australian offices during the heating season (Wood et al., 2006). The plants did not significantly influence room humidity but they reduced total volatile organic compounds levels.
Plant-based systems show potential for humidifying, removing indoor pollutants, and providing psychological benefits (Dravigne et al., 2008; Fjeld, 2000; Laviana et al., 1983). The majority of previous plant-based humidification research was field tests conducted in offices or residences. This study quantifies evapotranspiration rates from two common indoor plant species with different photosynthetic pathways in a controlled environmental chamber. The research objectives include 1) quantification of evaporation rates from a root medium, and evapotranspiration rates from variegated spider plants and green jade plants, at two indoor humidity levels; 2) evaluating the impact of the diurnal light cycle on transpiration for two plant species with different photosynthetic pathways; and 3) calculation of the plants’ potential impact on RH of a single interior bedroom, such as in a skilled nursing facility (SNF).
American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) 1999 Standard 62-1999, Ventilation for acceptable indoor air quality. Amer. Soc. Heating Refrigerating Air-Conditioning Eng., Atlanta, GA
Cobos, D.R. & Chambers, C. 2010 Calibrating ECH2O soil moisture sensors. 14 Oct. 2016.<http://manuals.decagon.com/Application%20Notes/13393_Calibrating%20ECH2O%20Probes_Print.pdf>
DelPrince, J.M. 2013 Interior plantscaping: Principles and practices. Cengage Learning, Delmar, NY
Dravigne, A., Waliczek, T.M., Lineberger, R.D. & Zajicek, J.M. 2008 The effect of live plants and window views of green spaces on employee perceptions of job satisfaction HortScience 43 183 187
Energy Information Administration 2009 Residential energy consumption survey. 14 Oct. 2016.<http://www.eia.gov/consumption/residential/>
Kalamees, T., Korpi, M., Vinha, J. & Kurnitski, J. 2009 The effects of ventilation systems and building fabric on the stability of indoor temperature and humidity in Finnish detached houses Build. Environ. 44 1643 1650
Laviana, J.E., Mattson, R.H. & Rohles, F.H.J. 1983 Plants as enhancers of the indoor society. Proc. Human Factors Ergonomics Soc. Annu. Mtg. p. 738–741
Lohr, V.I. 1992 The contribution of interior plants to relative humidity in an office, p. 117–119. In: D. Relf (ed.). The role of horticulture in human well-being and social development. Timber Press, Portland, OR
Lohr, V.I. & Pearson-Mims, C.H. 1996 Particulate matter accumulation on horizontal surfaces in interiors: Influence of foliage plants Atmos. Environ. 30 2565 2568
Lowen, A.C., Mubareka, S., Steel, J. & Palese, P. 2007 Influenza virus transmission is dependent on relative humidity and temperature PLoS Pathog. 3 e151
Manaker, G.H. 1997 Interior plantscapes: Installation, maintenance and management. 3rd ed. Prentice-Hall, Upper Saddle River, NJ
McCulley, J.P., Aronowicz, J.D., Uchiyama, E., Shine, W.E. & Butovich, I.A. 2006 Correlations in a change in aqueous tear evaporation with a change in relative humidity and the impact Amer. J. Ophthalmol. 141 758 760
Ninomorua, P.T. & Cohen, M.H. 1999 IAQ in nursing homes. Amer. Soc. Heating Refrigerating Air-Conditioning Eng. J. 41:34–38
Nordström, K., Norbäck, D. & Akselsson, R. 1994 Effect of air humidification on the sick building syndrome and perceived indoor air quality in hospitals: A four month longitudinal study Occup. Environ. Med. 51 683 688
Pennisi, S.V. & van Iersel, M.W. 2012 Quantification of carbon assimilation of plants in simulated and in situ environments HortScience 47 468 476
Reinikainen, L.M., Jaakkola, J.J. & Heinonen, O.P. 1991 The effect of air humidification on different symptoms in office workers: An epidemiologic study Environ. Intl. 17 243 250
Roelfsema, M., Konrad, K.R., Marten, H., Psaras, G.K., Hartung, W. & Hedrich, R. 2006 Guard cells in albino leaf patches do not respond to photosynthetically active radiation, but are sensitive to blue light, CO2 and abscisic acid Plant Cell Environ. 29 1595 1605
Sterling, E., Arundel, A. & Sterling, T. 1985 Criteria for human exposure to humidity in occupied buildings Amer. Soc. Heating Regrigerating Air-conditioning Engineers Trans. 91 611 622
Sunwoo, Y., Chou, C., Takeshita, J., Murakami, M. & Tochihara, Y. 2006 Physiological and subjective responses to low relative humidity J. Physiol. Anthropol. 25 7 14
Taiz, L. & Zeiger, E. 2006 Plant physiology. 4th ed. Sinauer Assoc., Sunderland, MA
Thimijan, R.W. & Heins, R.D. 1983 Photometric, radiometric, and quantum light units of measure: A review of procedures for interconversion HortScience 18 818 822
Wolverton, B. & Wolverton, J.D. 1996 Interior plants: Their influence on airborne microbes inside energy-efficient buildings J. Miss. Acad. Sci. 41 99 105
Wood, R.A., Burchett, M.D., Alquezar, R., Orwell, R.L., Tarran, J. & Torpy, F. 2006 The potted-plant microcosm substantially reduces indoor air VOC pollution: I. Office field-study Water Air Soil Pollut. 175 163 180