Plants can combine simplicity in technology and low resource use with strong visual appeal and effectively enhance indoor air quality: help regulate temperature, remove toxins like formaldehyde and other pollutants. The use of plants to clean up the environment is called phytoremediation.

Child-safe indoor air-purifying plants – list at Inhabitots

Wikipedia list of air, formaldehyde, xylene & toluene-filtering plants

TOP 10 plants most effective in removing: formaldehyde, benzene, and carbon monoxide from the air

Study summary: How and why potted plants really do clean indoor air – 2005 – University of Technology, Sydney, Australia (PDF)

Healthy Plants at the Workplace (European Union sponsored)

Wolverton Environmental Services, Inc. has developed a line of phytoremediation technologies using plants and their root-associated microbes for the treatment of wastewater and in the use of interior plants for improving indoor air quality.
Air Quality Solutions' patented technologies have allowed us to develop two broad classes of Naturaire Systems to improve your indoor air quality. Our systems are superior to other indoor air filters because ours not only captures the pollutants in the air but break these pollutants down into their harmless components.
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The International Journal of Phytoremediation is the first journal devoted to the publication of current laboratory and field research describing the use of plant systems to remediate contaminated environments.