HVAC filter efficiency is rated on the minimum efficiency reporting value (MERV) scale. Basic filters are rated MERV 1 to 4 and can trap most particles 50 microns and larger, as well as 25 percent of particles down to three microns. Then, the most efficient filters are MERV 17 to 20, capable of trapping 95 percent or more of particles down to 0.3 microns.
Chances are you need something in between. Most homeowners benefit from a pleated filter in the MERV 5 to 8 range. Such mid-range filters trap a majority of airborne pet dander, pollen, and dust mites down to five microns, which is perfect for people with pets or mild allergies.