The MERV rating indicates the percentage of particles the filter will remove from the air passing through it. In general, a MERV rating of 6 indicates that the filter will capture up to half of the particles in the air; a filter with a rating of 8 will trap 70 to 85 percent of air-born particles it encounters; and a rating of 11 or higher means that the air passing through the filter is up to 95 percent cleaner when it comes out of the filter than it was when it went into it [source: Lowe's].

A properly sized, installed and functioning heating and air system circulates the air in your home every hour. In the process, it pulls that air through the filter. Just how much the filter cleans the air depends on the MERV rating of the filter.

MERV stands for Minimum Efficiency Reporting Value. MERV ratings indicate the size of particles that a filter can remove from the air passing through it. MERVs range from 1 to 16, with a higher number indicating a higher cleaning efficiency because it can filter smaller particles out of the air. The cheap, 1-inch (2.54-centimeter) thick, disposable filters made of jumbled fiberglass or natural fiber strands typically have a MERV rating of 1 but can go up to 4. Pleated filters made of nonwoven, disposable fabric have smaller pores, and the pleats increase the surface area of the filter so it can hold more particles than a flat surface can. These filters have MERV ratings of 3 and higher depending on the density of the fabric and the number of pleats [source: INDA]. Some are charged with static electricity to attract and hold air-born allergens.

The particle catching efficiency actually goes up as the filter gets dirty; buildup on the fibers shrinks the openings the air passes through and allows the filter to capture more particles. This is good only up to a point. The particle-removing efficiency of the filter is inversely related to the energy efficiency of your heating and air system. A by-product of cleaning the air is a restriction of the air-flow through the system.

Think about times you've worn a dust mask while you worked on a project. It's harder to breathe through the mask than it is if you aren't wearing one. If you upgrade to the HEPA (high-efficiency particulate air) mask that filters out particles as small as 0.3 microns with up to 97 percent efficiency, it gets even harder to pull in enough air to keep you going [source: Howarth and Reid]. The same is true of your heating and air system. The denser the filter, the harder the system has to work to pull in air. A clogged filter may cause your system to run continuously, driving up your heating or cooling bills. That's why it's important to check your filters at least monthly and change them when they get dirty.

What should I do if my air filter looks clean when it's time to change it?

According to Freddie Williams, an instructor of air-conditioning technology at Lanier Technical College in Oakwood, Ga., high-grade filters are the most efficient way to clean the air in your home. But what do they look like when they've done their job? When it's time to change your filter -- anywhere from 1 to 3 months after you installed a fresh one -- it should look dirty.

"A build-up of dust is usually apparent," Williams said. "There should be gray, ashy-looking material on the duct side of the filter."

If your filter looks clean after it's been in place for the recommended time, here are some things you should check:

1. Does the filter fit properly into the holder? If the filter is loose or too small for the space, the air can circulate around it instead of going through it. Measure the filter space and purchase a filter that fits snugly.
2. Is the filter installed upside down? There is a correct air-flow direction for most air filters. Look for arrows on the filter frame, and install the filter so that the arrows point toward the fan.
3. Is the filter you're using right for the job you want it to do? If you're using a low-end filter, it's not going to catch much dust. Upgrade to a filter with a higher MERV rating to increase the air cleaning efficiency.
4. Check your rate of air exchange. According to Williams, if your system is functioning properly, it should run for about 15 minutes per cycle, with a cycle rate of not more than three in an hour. If it runs shorter cycles, it isn't creating the desired rate of air exchange. Call a professional and get your system checked.

Your home environment and how often you run the heat or air can also affect how quickly your air filter gets dirty. If your home is well sealed, you have no pets, no dust-prone furnishings like carpet and fabric-covered furniture, and you dust and vacuum every day, your air filters will have fewer air-born particles to collect. Also, the system only filters the air when it's running. If you install a new filter, but don't turn on the heat or air conditioning until a month or two later, the filter should still be relatively clean since the system hasn't been forcing air through it.