Comment by schiffern

  >if you are blowing fresh air into a room, or have the windows open, the filtration effectiveness of these units approaches zero pretty rapidly

Even with an open window, if (say) the air purifier and the window have a flow of 200 CFM, then the air will still be twice as clean (ie half as many pollution particles). I would say that's still worth doing!

Most people would just leave the window open <1", which keep CO2 down yet air purifiers still work well.