I just did a little cursory research on air quality and health, here is what I have found.  (If you know more about this, please comment!)

* If you feel worse in the summer than winter, suspect air quality : air quality is lower in the summer.  Both particulates and ozone are higher.  Plus more forced air usage (from air conditioning).

* Put a higher quality filter in your central air handler.  Find as high a MERV rating filter as you can.  Examples.  Surely this works better than buying an air filter for one room, and is cheaper!

* If you want to go crazy, get this (which is external to your air handler and a more significant investment).

* Ozone (which is bad for you) is highest during hot times — during the day in the warm part of the year.  Perhaps one strategy in the summer is open a window at night to get fresh air, and keep them closed during the day (what are other solutions?)  Consider working out early in the morning when concentrations are low (although before rush hour when particulates and CO are bad).