Whether you call them air conditioning filters or furnace filters depends if you live in a predominantly cooling or heating environment.
Indoor air pollution is among the top five environmental health risks. Air filters have to strike a delicate balance between allowing air to flow freely through your furnace, evaporator coil and ductwork while at the same time stopping all the unwanted particles in the air from being collected on your cooling coil and recirculated throughout your home.
Basically there are two categories of indoor air pollutants:
The first is Particulate Matter which encompasses dust, smoke, pollen, animal dander, cigarette smoke, particles from combustion appliances, particles associated with tiny organisms such as dust mites, viruses, bacteria and mold.
The second is Gaseous Pollutants. Generally coming from combustion processes. These include gas stoves, car exhaust, and tobacco smoke. They are also generated from building materials and furniture. Paints, glues, pesticides and cleaning supplies are also willing contributors to this category.
Believe it or not your air filter actually becomes more efficient as it becomes loaded up with solid particulate matter. There is however a fine line between a filter operating at peak efficiency and one so stopped up it’s creating a pressure drop through the system, which is never a good thing. It’s important to note that the EPA does not certify or recommend any brand or type of air filter over another. Also note some filters sold in the market place are Energy Star qualified. However EPA does not endorse any manufacturer claims of cleaner indoor air as a result of use of these filters.
Minimum Efficiency Reporting Value or MERV for short is a measurement scale designed in 1987 by the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE ) to rate the effectiveness of filters. Ratings are from 1 to 20 with 1 being next to nothing and 20 a cleanroom type application.
1. Standard one inch fiberglass. Sometimes called a grocery store filters. They are better than nothing and that’s all that can be said. Change monthly.
2. Pleated one or two inch thick. These are sold in numerous efficiencies and sizes. They can be too restrictive for some systems especially those with marginally sized equipment and ductwork. A telltale sign would be a whistling noise from your furnace. Change every one to three months
3. High efficiency media air cleaner: Sold by manufacturers like Lennox and Trane as well as aftermarket HVAC suppliers such as Honeywell and Aprilaire. 
They are 4-7 inches thick, last six to twelve months and range in efficiency from MERV 8-16. Requires special filter cabinet due to size.
4. The latest and greatest filter systems are classified as Air Purification systems. Again two of the best are made by Lennox and Trane. The Pure Air by Lennox claims it can help control everything from microscopic dust mites and mold spores to infectious bacteria and viruses. They also claim to have the only IAQ system that safely removes and destroys odors and chemical vapors. It uses a patented photo catalytic technology to filter, purify and freshen the air. Service is done once a year with the purchase of an annual maintenance kit. I’ve performed the maintenance on several and it’s a breeze.
Picture courtesy of Google Images.
