You may have come across the NASA clean air study and this 4 minute TED talk by Kamal Meattle, an Indian businessman and environmental activist. Short version: you need 4 Areca Palm plants per person to convert CO2 to O2 during the day, 6-8 snake plants per person to convert CO2 to O2 during the night, and then a devil's ivy to remove other chemicals from the air. He rattles off a list of measured impacts for working in an office building with an appropriate number of plants that also seem pretty convincing.

It's not clear why he didn't go with the ones that looked best on the NASA study, the Peace Lily (also recommended by Nicholas Angel) or the Chrysanthemum (also recommended by the Emperor of Japan). But this also the first place where I saw someone discussing the relevant conversion rate (i.e. three plants per person wouldn't be enough); the NASA study synopsis on Wikipedia, at least, only mentions number of plants per area and doesn't seem to take the leaf surface area of the plant into consideration. So one suspects he's taken cost and efficiency into account.