Indoor plants improve air quality?

Well, sounds like the answer is, "not really in any meaningful way" according to this study at least:

Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies

From what I can gather, the study says: yes plants improve air quality, but you're better off just opening a window or using some other ventilation system.

If you can't open the windows for some reason or just like having plants, then indoor plants will help. But you'd need a lot of plants.

The study estimates you'd need:

10–1000 plants/m2 of a building’s floor space for the combined VOC-removing ability by potted plants to achieve the same removal rate that outdoor-to-indoor air exchange already provides in typical buildings

So even at the lowest end, you'd need 10 plants per m2! Not impossible, but it's a lot (even for wall mounted plants).

But hey, you could always have plants and open the window.