Formaldehyde is a major contaminant in indoor air that originates from particle board, plywood, carpet, curtain, paper products, tobacco smoke, certain adhesives, and other sources (Salthammer, 1999; Spengler and Sexton, 1983). Formaldehyde concentrations in new houses are often several times higher than that in older homes (Marco et al., 1995). Indoor volatile organic compounds (VOCs) such as formaldehyde can result in “multiple chemical sensitivity” and “sick building syndrome” (Shinohara et al., 2004) and several other physical symptoms for those exposed (e.g., allergies, asthma, headaches) (Jones, 1999; Kostiaineh, 1995). The World Health Organization estimates that undesirable indoor volatiles represent a serious health problem that is responsible for more than 1.6 million deaths per year and 2.7% of the global burden of disease (WHO, 2002). As a result of its undesirable effect on health, 0.17 μL·L−1 has been established as the upper limit for the concentration of formaldehyde in the indoor air of new houses in Korea (Ministry of Environment, Republic of Korea, 2006).
Plants are known to absorb and metabolize gaseous formaldehyde. The volatile enters the leaves through stomata and the cuticle and is more readily absorbed by the abaxial surface and younger leaves (Giese et al., 1994; Ugrekhelidze et al., 1997). Once absorbed by the leaves, it generally enters the Calvin cycle after a two-step enzymatic oxidation to CO2 (Schmitz, 1995). The amount of formaldehyde removed by indoor plants does not significantly increase with light intensities across the range commonly encountered within homes; however, there are considerable differences between light and dark conditions (Kil et al., 2008b). Approximately 60% to 90% of 14C-formaldehyde was recovered from the plants (Giese et al., 1994; Schmitz, 1995) and it was assimilated approximately five times faster in the light than in the dark (Schmitz, 1995). Some of the formaldehyde is converted to S-methylmethionine and translocated in the phloem to various organs (e.g., seed, roots) (Hanson and Roje, 2001).
Assessing indoor plants for phytoremediation efficiency involves comparing the purification capacity among species under standard conditions. Comparing a cross-section of orchids, the formaldehyde removal efficiency of Sedirea japonicum was the highest, whereas Cymbidium spp. was the lowest of the species tested (Kim and Lee, 2008). The half-life (time required for 50% removal) is considered a good indicator of the purification capacity of a plant and allows comparing the efficiency among species under standardized conditions (Kim et al., 2008; Orwell et al., 2006; Oyabu et al., 2003). Likewise, expression of VOC removal based on leaf area allows comparing plants of varying size (Kim and Kim, 2008) and is essential for determining the number of plants needed for specific indoor environments.
Certain microorganisms found in the growing media of indoor plants are also involved in the removal of VOCs as illustrated by the fact that when the plant(s) are removed from the media, the VOC concentration continues to decrease (Godish and Guindon, 1989; Wolverton et al., 1989; Wood et al., 2002). The root zone eliminates a substantial amount of formaldehyde during both the day and night. The ratio of removal by aerial plant parts versus the root-zone was ≈1:1 during the day and 1:11 at night (Kim et al., 2008). Likewise, the removal efficiency of the media increases (≈7% to 16%) with increased exposure frequency (Kil et al., 2008a) suggesting an apparent stimulation of the organism(s). A number of soil microorganisms are capable of degrading toxic chemicals (Darlington et al., 2000; Wolverton et al., 1989), although few of the microbes that are directly associated with formaldehyde removal has been identified.
Plants excrete into the root zone significant amounts of carbon that stimulate the development of microorganisms in the rhizosphere (Kraffczyk et al., 1984; Schwab et al., 1998). The phyllosphere is also colonized by a diverse array of microorganisms (Mercier and Lindow, 2000). Therefore, rhizospheric and phyllospheric microorganisms as well as stomate-mediated absorption provide a means of biofiltration of VOCs from indoor air. As a consequence, phytoremediation of indoor air is seen as a potentially viable means of removing volatile pollutants in homes and offices (Darlington et al., 1998; Giese et al., 1994; Kempeneer et al., 2004; Kim et al., 2009; Salt et al., 1998; Wolverton et al., 1989; Wood et al., 2002). As a result of the importance of formaldehyde as an indoor air pollutant, we determined the formaldehyde removal efficiency of a diverse cross-section of indoor plants.
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