The answer is a lot more complex than you might imagine.
filters remove up to 90 percent of mold spores and pet dander, which are in the range of 3 to 10 microns. Filters works like a magnet to grab even smaller particles, such as bacteria, the particles that carry viruses, and the soot in smoke and smog.
Manufacturers typically recommend that the high-efficiency filters be changed every three months. The Health House program adds a suggestion to look at the filters monthly. If filters are obviously dark and clogged, go ahead and replace them. Not everyone's house and habits are the same. If you have three or four cats, or if someone in your house smokes, you should change the filter more often.
With the old-style filters, if you forget to change the filter after a month, it's really no big deal. The dirt trapped on the fibers actually increases the filter's ability to block small particles, and plenty of air still gets through for the furnace to run relatively efficiently. The newer filters, however, significantly restrict air flow once they become somewhat clogged. This prevents your furnace from moving as much air as it was designed to do, which causes it to run longer to heat your house, which in turn adds to your gas or electric bill.
Our advice? Focus on reducing the amount of dust that is stirred up inside your house by encouraging everyone to leave their shoes at the door; keeping pets and smokers out of the house; vacuuming regularly and thoroughly with a central vacuum system or a portable vacuum equipped with a HEPA (high-efficiency particulate air) filter; and "using as effective a furnace filter as the homeowner's budget allows, changing or cleaning the filter as often as the manufacturer recommends.
Or ...use the washable filter that came with the furnace. "It's just mesh, not very fine, vacuum it off every month and put it back in..