Many modern homes are built with central forced-air systems. Typically these systems have a rectangular, one-inch thick fiberglass filter that slides underneath the furnace fan, or into a wall or ceiling register where the air returns to the furnace. Often, these inexpensive filters remove less than 10% of the microscopic particles homeowners are concerned about.
To upgrade protection, consider a medium or high-efficiency filter. Medium-efficiency filters are typically pleated, woven material and reduce 20-50% of particles 0.3-10 microns in diameter. Some of these filters use static electricity created by the airflow to add to particle collection, and both disposable and washable models are available. These filters usually cost about $5 to $20.
Electret or electrostatic filters use synthetic fibers to create a static charge. Most electrostatic filters are reusable, but need to be cleaned regularly. Even with cleaning, the filters should be replaced periodically to maintain efficiency. Costing from $25-$100, some filters can be purchased as kits with a permanent frame and the filter pad, which should be replaced every couple of years.
High-efficiency filters (sometimes mistakenly called HEPA filters) are rated at 60-95% efficiency. Depending upon whether the filter is designed to be disposable or washable, costs for replacements are between $20-$130.
HEPA-type filters are not HEPA filters, but some of the pleated types are almost as effective. Ranging from 25 to 95 % effective at microscopic particle removal, it is a good idea to verify how good the filter is at blocking particles large enough to affect respiration. Usually replaced annually with continuous use, HEPA-type filters cost around $30-$45.
HEPA (high efficiency particle air filters) systems are rarely used for central air but are the most popular portable room filtering system. HEPA filters should be replaced periodically and can cost anywhere from $25-$350 to replace.