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Volatile organic compounds (VOCs) are major contaminants of indoor air, with concentrations often several times higher than outdoors. They are recognized as causative agents of "building-related illness" or "sick-building syndrome". Our previous laboratory test-chamber studies have shown that the potted-plant/root-zone microorganism microcosm can eliminate high concentrations of air-borne VOCs within 24 hours, once the removal response has been induced by an initial dose. However, the effectiveness of the potted-plant microcosm in 'real-world' indoor spaces has never previously been tested experimentally. This paper reports the results of a field-study on the effects of potted-plant presence on total VOC (TVOC) levels, measured in 60 offices (12 per treatment), over two 5-9 week periods, using three planting regimes, with two 'international indoor-plant' species. Fourteen VOCs were identified in the office air. When TVOC loads in reference offices rose above 100 ppb, large reductions, of from 50 to 75% (to <100 ppb), were found in planted offices, under all planting regimes The results indicate that air-borne TVOC levels above a threshold of about 100 ppb stimulate the graded induction of an efficient metabolic VOC-removal mechanism in the microcosm. Follow-up laboratory dose-response experiments, reported in the following paper, confirm the graded induction response, over a wide range of VOC concentrations. The findings together demonstrate that potted-plants can provide an efficient, self-regulating, low-cost, sustainable, bioremediation system for indoor air pollution, which can effectively complement engineering measures to reduce indoor air pollution, and hence improve human wellbeing and productivity.

Indoor air-borne loads of volatile organic compounds (VOCs) are usually significantly higher than those outdoors, and chronic exposures can cause health problems. Our previous laboratory studies have shown that the potted-plant microcosm, induced by an initial dose, can eliminate high airborne VOC concentrations, the primary removal agents being potting-mix microorganisms, selected and maintained in the plant/root-zone microcosm. Our office field-study, reported in the preceding paper, showed that, when total VOC (TVOC) loads in reference offices (0 plants) rose above about 100 ppb, levels were generally reduced by up to 75% (to <100 ppb) in offices with any one of three planting regimes. The results indicate the induction of the VOC removal mechanism at TVOC levels above a threshold of about 100 ppb. The aims of this laboratory dose-response study were to explore and analyse this response. Over from 5 to 9 days, doses of 0.2, 1.0, 10 and 100 ppm toluene and m-xylene were applied and replenished, singly and as mixtures, to potted-plants of the same two species used in the office study. The results confirmed the induction of the VOC removal response at the lowest test dosage, i.e in the middle of the TVOC range found in the offices, and showed that, with subsequent dosage increments, further stepwise induction occurred, with rate increases of several orders of magnitude. At each dosage, with induction, VOC concentrations could be reduced to below GC detection limits (<20 ppb) within 24 h. A synergistic interaction was found with the binary mixtures, toluene accelerating m-xylene removal, at least at lower dosages. The results of these two studies together demonstrate that the potted-plant microcosm can provide an effective, self-regulating, sustainable bioremediation or phytoremediation system for VOC pollution in indoor air.

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