I'd recently stumbled across this American Heart Association study that links increased high blood pressure due to air pollutants, which is something I should be concerned with since I live in an urban area (terrible air!), and suffer from high blood pressure (terrible diet!).
I'd followed this University of Michigan suggestion, and bought a packet of MERV 13 air filters (which cost me a few bucks on Amazon) to fit over an existing box fan I use during the summer, and produced this DIY air cleaning system (pictured below), now proudly running 24/7 in my office to help filter the air of particulate (bus fumes, pet dander, secondhand smoke). That's a generic 20"2 box fan, and the 19.5"2 filter is simply taped to the front of it, with the air being forced through the filter in the proper direction.