The NASA Clean Air Study was originated a long time ago, and no idea if there is more up-to-date info on this, but it does note that all plants remove carbon dioxide from room air, and release oxygen. It seems to me that a more in-depth study would tell you how many plants it takes to purify and oxygenate the air in a given space. But it's safe to assume (and of course we know) that you can't have too many plants.
Here's a link to a study on reducing ozone (primary air pollutant) in indoor air that concludes all the plants they tested did effectively remove ozone from an enclosed space. If you live in a city that suffers air inversions, and has bad air quality it seems you'd want to have lots of BIG plants!
http://www.sciencedaily.com/re...
