He tested fifty plants for their ability to remove toxic gases. The book lists fifty plants, with easy charts for choosing the right type, based on their abilities and requirements for light, temperature, etc. It does seem that a lot of plants are needed to make any difference, more than most people would be comfortable having in their living room. As other texts point out, plants causes evaporation of water, and higher humidity is undesirable as it makes it easier for mites and molds to grow. Formaldehyde and other volatile chemcials also outgas more in a humid environment.
Plants can also house molds.