The one thing we rely on to keep us comfortable during the winter is the furnace in our homes. With a simple twist of the dial, we can adjust the indoor temperature dozens of degrees warmer than the outdoor readings to keep us toasty.

Perhaps one of the most overlooked yet most valuable features on the furnace is its filter. And don’t be fooled into thinking that a filter is a filter is a filter. While they all “filter” the air circulating through the furnace and subsequently through the house, some are much more efficient and thorough than others.

So, what’s floating around in the air that needs filtering? You name it, and it’s probably there: dust, dander, mold, smoke, pollen and the like.

The diameter of these particles is measured in microns. Human hair is about 150 microns in diameter. Household dust can range in size from less than .01 micron to 50 microns. Animal dander is generally less than 10 microns. The particles in tobacco smoke, cooking grease and wood smoker are less one micron in diameter. Bacteria can be as small as 0.5 microns and viruses even smaller, less than 0.01 microns.

A micron is, well, very tiny ” about one millionth of a meter. Only those particles that are 10 microns in size or larger, such as pollen, plant spores and sneeze droplets, can be seen with the naked eye. The others, though present, are not visible to us without the use of a microscope. So, when thinking furnace filters, think about how well they trap or remove the smaller micron particles.

Most of us are familiar with the simple fiberglass filter. “Fiberglass has long been the mainstay and still the most commonly purchased,” acknowledged Hal Schump, sales representative for Air Filter Plus, Inc. in Eudora.

The upside of the fiberglass filter is that it is inexpensive; the down side is its lack of efficiency.

“It is only 1 percent efficient on 3-10 micron dirt,” Schump said. “That’s a problem right there.” Fiberglass filters aren’t picking up much. For example, the dust that collects on the computer screen or the shelf can easily remain unfiltered, even though it is 20 microns.

A more efficient filter than the fiberglass one is a pleated filter, such as one manufactured by 3M. “It is 10 times more efficient than fiberglass,” Schump said. “It is 25-30 percent efficient on 3-10 microns. What makes them efficient is the tightness of their weave.”

Another type of pleated filter is the electrostatic filter.

“They are a step up from fiberglass,” Schump noted. Still, he cautioned that any filters that have been sitting on the shelf would work less efficiently. “If you get a fresh electrostatic filter, they are fine,” he said.

Bret Kay, president of Air Filter Plus in Eudora, checks one of his company's air filters. Efficient filters help prevent dust from collecting.

Pleated filters generally run about $15.

If you are looking for a filter that is significantly better than the standard fiberglass or pleated filter, a poly media filter, such as Dustlok may be what you need. “This is a serious filter,” Schump claimed. “It’s the only one that really works. It is 88 percent efficient at 10 microns, 28 percent efficient at one micron.” On test dirt, the poly media filter had an efficiency of 92 percent. “This actually works really well on five micron range pollens,” he said.

The poly media filter is 1 1/2 inches thick and made so that the larger particles are trapped on the top of it. As smaller micron particles move through the filter, they, too, are trapped.

“It is 100 percent poly with a needled downstream surface and a wet-tack adhesive which adsorbs dirt and finest particulate matter,” Schump explained. “The adhesive is coated on every fiber.”

In addition, the fibers are coated with an antimicrobial called Sporax, which “kills mold or mildew that lands on the filter,” he said. Sporax is FDA approved and inert, so it isn’t harmful.

Because the poly media filter is so efficient, Schump stressed the importance of changing it every three months. If you don’t, the coils on the furnace can be damaged. The cost for a poly media filter is about $25, including any size holding frame and four filters- enough for one year.

So, decide what filter is best for you. If you are still considering trading off efficiency to save a few dollars, thing about Schump’s final words to me, “A filter is too cheap when you think about your lungs.”

” Carol Boncella is education coordinator at Lawrence Memorial Hospital and home and garden writer for the Journal-World.