True or False? You should change the filter in your central heating and-or air-conditioning system at least four times a year? If you answered true, you probably are enjoying fresher, cleaner air than your neighbors.

For persons with allergies it is wise to clean or change most filters at least once a month when pollen is prevalent and twice a month the rest of the year. There are other reasons to clean or change your forced-air unit filter more often. A few examples:

1) if you live in a sandy community near a beach or in the desert
2) if you live close to a major roadway
3) if you are near a construction project in progress.

We all have a pretty good idea why filters are important. They clean whatever it is they are designed to filter. Oil filters clean oil and water filters clean water. But, furnace filters don't clean furnaces. Instead, they clean the air that is heated or cooled, and then distributed by the furnace. Because it is not called an "air" filter some think that it filters debris to protect furnace parts instead of the air that we breathe.

When shopping for a filter it is a good idea to know a little bit about a measurement called the micron. A micron is 1/1000th of a millimeter. Many pollens and spores are only a few microns in size and therefore are very difficult to filter. To effectively filter such small particles the size of the holes in the filter would have to be so small that air would have difficulty getting through. This would affect the ability of the system to move air freely.

To allow reasonably free air movement while filtering microscopic particles, electrostatic filtration is used. The particles are fed through a mesh that electrically charges them. The charged particles are then sent through another filter that contains the opposite charge. Since opposite charges attract each other, the charged particles are drawn to and adhere to the second layer of filtration material. We aren't sure whether this should be called filtration or magnetism. In any event, this is how minute particles are removed from the air in a central heating or cooling system.

Some filters use electricity to create the magnetic field and others use static electricity. The types that use conventional electricity are permanent, and once installed are only removed for cleaning. The mesh in a permanent filter is made of metal, and with proper use, literally will last a lifetime.

The filters that rely on static electricity are constructed from polyester, a plastic-like substance. If they are cleaned every month they will last for several years. Considering purchase and operating costs we suggest the latter as a first line of defense. If the polyester filter doesn't do the job well enough to solve a special allergy problem then we would recommend the electrically operated permanent type.

In any event, we strongly recommend quarterly filter changes or cleaning electrostatic or not. As homes become more and more energy efficient, higher pressures can build up. This can cause movement of particles smaller than were ever moved before. It's ironic how one technology leads to the need for another.

And remember, as a filter in a central system begins to clog up it produces increased back pressure on the blower motor. Left unattended a dirty filter actually can shorten the life of the motor an expensive item compared to the cost of a filter.