Indoor Environment Notebook:

Indoor Environment Notebook: re: Do houseplants improve the quality of indoor air? Thad Godish, Ph.D., cites Dr. B.C. Wolverton's research only to dispute it, claiming that botanical air cleaning may work under laboratory conditions, but in the real world the effect is minor. He does, however, praise Wolverton's work in using plants for wastewater treatment.

My take is that even if the effects are minor, that's no reason to not use them how you can, as long as you don't go overboard and raise the humidity beyond healthy levels and start growing mold. But then, I like houseplants in and of themselves...