Air Filters Demystified

By Jeremy Cook

A typical home air filter.
Often taken for granted and sometimes neglected, air filters perform an important function in your house.

Home air filters are something that you rarely think about. They produce no immediately visible effect, and simply exist to allow air to pass through them. On the other hand, for both human and machine health, they are extremely important both at home and in the workplace. Without these filters, your heating and air conditioning (HVAC) equipment would be negatively affected, and the air coming out of your air circulation vents wouldn’t be as clean.

Cheaper air filters are made out of fiberglass and consist of a loose mesh of fibers. Though they perform the function of cleaning the air well enough for an HVAC system, they do little to take out mold, pet dander, dust mites and other tiny particles that affect human health.

You may want to consider a pleated filter instead. They fit in the same space as a fiberglass filter, but contain a cloth-like material that is folded in such a way that there is more surface area than if it was stretched straight from one end of the square to another.

Understanding Ratings

Though these are the main two types of air filters available in a very broad sense, how well they clean the air varies greatly. There are several different standards that tell how good a filter is, making selection less than straightforward at times. These include:

  • MERV (Minimum Efficiency Reporting Value): This is the scale designed by the American Society of Heating, Refrigeration and Air-Conditioning Engineers, or ASHRAE, to indicate the effectiveness of air filters. Though ratings vary between one and 20, high-quality home air filters will generally be rated between six and 13.
  • MPR (MicroParticle Performance Rating): This rating system was developed by 3M, and rates a filter’s ability to capture particles smaller than one micron.
  • FPR (Filter Performance Rating): This system was developed by Home Depot and rates filters between four and 10, depending on performance.
  • HEPA (high-efficiency particulate air): If you think HEPA-rated filters signify very high-quality filtration, you are correct. This type of filter was developed for the nuclear industry and, depending on its rating, can trap 99.97 percent of dust particles 0.3 microns in diameter. (For scale, you can compare this to the MPR rating system which measures ability to collect particles of one micron). These filters are generally of higher quality than needed for home HVAC filter use, but come up from time to time, so are mentioned here for completeness.

The good news is that these rating systems can be correlated roughly by the debris they eliminate from the air as shown here:

  • Lint, dust, pollen: MERV6, MPR 300, FPR 4
  • Lint, dust, pollen, dust mite debris, mold spores: MERV8, MPR 600, FPR 7
  • Lint, dust, pollen, dust mite debris, mold spores, pet dander, smoke, smog, sneeze/cough: MERV 11, MPR 1200, FPR 9
  • Lint, dust, pollen, dust mite debris, mold spores, pet dander, smoke, smog, sneeze/cough, bacteria, virus carriers: MERV 13, MPR 2200, FPR 10

Note that there may be some variation between the different systems, but this should give you a general apples-to-apples idea of what’s available.

How Good Is Good Enough?

Perhaps you could simply remember what type of pollutants you’d like to eliminate and buy the minimum filter required. On the other hand, why wouldn’t you simply buy MERV 13, MPR 2200 or FPR 10 filters and forget about it? There are, of course, a few other considerations.

The most noticeable difference is filter price. A standard-grade 20 x 20-inch fiberglass filter, for example, can cost less than a dollar. Pleated filters of the same size range from around $5 for those that are pretty good to premium-rated filters that may set you back about $20 each. One alternative to spending money on quality would be to use washable filters, but you may not want to deal with the hassle and potential health effects of handling dirty filters. Plus, if it means you’ll change them less often, the benefits may be negated.

Another factor is that the better a filter is at absorbing tiny particles, the harder it is for air to get through. This can increase the load on your HVAC system, potentially causing undue wear and expensive repairs. On the other hand, if your filters are causing too much resistance, this could mean that your system may pull an excessive amount of air in from the cracks around the air filter and through any other gaps in your system, decreasing filtration efficiency further.

Changing Air Filters

Perhaps this article has convinced you to give a better filter a try, or that it’s time to change your filters around the house. Here are a few tips to help you get the most out of your filter change.

If you’ve never changed your house’s filters before, make sure you know where all of them are. Hopefully, they are placed in obvious spots on the ceilings or walls, but it’s possible that there is a filter tucked away in a closet or crawl space that you don’t even realize is there.

Write the change date on a filter to remember when it needs replacing.
Physically noting the filter’s change date can serve as a quick reference.

Once you’ve obtained your filter or filters of choice, note down the suggested change interval and add a reminder to your online or paper calendar to change it. If you’re not great with keeping track of a calendar, a great alternative (or supplement) is to write the date of the filter change on the filter itself.

Clean the edges of the air filter.
Note any dust and grime around the edges of your filter and wipe clean.

Next, remove the old air filter, taking care to throw it immediately in a trash bag to avoid getting dust around your dwelling. Check out where the filter was previously placed in the duct system, and note any dust buildup around the edges. Wipe it clean with a rag, or if it’s excessive, consider taking steps to fix these gaps. Finally, insert the new filter, paying attention to the designated air flow direction, then close everything up.

The mesh is usually on the back of the air filter, which faces inside the ducts.

The front of the filter is usually completely covered in the filtering medium.
Air flow direction will generally be indicated by an arrow on the filter itself. Note that there is a difference between the two filter surfaces.

After your designated interval, repeat the process. If the filter looks extremely dirty, try changing it earlier next time or check it intermittently. Depending on your house’s construction, you may have more or less air going through each filter than the recommended interval is designed for, or you may prefer to keep your house’s temperature at a level that requires a lot of air flow. Don’t be afraid to experiment to get the change interval you need for proper indoor air quality. You may even have to adjust as needed depending on the season.


Jeremy Cook is an engineer with over 10 years of factory automation experience who also writes for The Home Depot. He has a passion for experiments, DIYs and projects like testing and replacing differently rated air filters.

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