[Selkirk]

Not sure about other chemicals, but carbon seems particularly hopeless. Plants produce CO2 as they respirate and consume it as they photosynthesize. So rather than think in terms of number of plants, think in terms of the weight of the plants (not the dirt). Carbon sequestration is going to have to add net weight to the plants. A human produces 2.3 pounds of CO2 per day. So, how many pounds per day will your crop of carbon scrubbing plants have to increase by in order to sequester that carbon? How much green material by weight do you need to achieve that rate of growth?

[ProllyInfamous]

This represents a nuanced understanding of most plants' carbon fixation cycles — mass captures CO2 while simple existance does not — that is technically correct.

In the application of having just a few indoor plants, I do think the air quality is fresher (just beyond CO2 levels; which indoors the effect is nil).