Furnace filter ratings are intended to provide a way to determine which filters are the best value. The problem is that you may need to know how well the furnace filter does with certain types of particles. This is especially true for people that have allergies or respiratory problems.
There is a simple concept with furnace filters that is directly related to the filter's performance. The tighter or denser media means smaller particles that are captured. Therefore, for people that need better air quality, you need a filter that will capture small particles.
Need to change your furnace filter? See the article 'Replacing a Furnace Filter', for more information.
Understanding Dust and Bacteria Particle Size
How big is big? Or better yet, how small is small? Particles are sized in microns. Furnace filter rating systems will be based on the size of particle (in microns) that can be captured.
A micron is 1/25,000th of an inch. So capturing a particle that is "1" micron in size is a pretty big deal. In fact, your best residential filters are designed to stop that size particle (see the rating information below).
Is one micron the smallest particle that you need to worry about. In a sense yes. There are smaller particles, but if you put a filter in that would stop them, your furnace would not run. It would not have the power required to pull the air through the filter. So the compromise is going after all the particles up to one micron in size.
If you get the one micron dust, you will also get all the dust and bacteria that are larger than that.
There are some rating systems for furnace filters. Not all rating systems are completely unbiased. In most cases they give you relative performance criteria.
Furnace Filter MERV Ratings
The MERV rating, (Minimum Efficiency Reporting Value) has a numbering range from 1 to 20. This rating system is sponsored by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) and is intended for comparison use. This rating system has been in use since 1987. The generally accepted range for residential use would be between 5 and 12.
The MERV rating is based on tests that determine the filters ability to stop particles from .3 to 10 microns in size. This covers a wide range of mold, bacteria, pollen and dust. As a rule, the higher the number on the MERV rating, the better. You need to keep in mind that a filter that stops particles, also stops air flow. Filters with MERV ratings above 12 are generally for commercial environments where the hvac equipment can handle the resistance.
For a residence, the types of furnace filters you choose should have a MERV rating between 5 and 12. The likely case would be between 7 - 9, a filter in this range provides decent protection without unduly restricting air flow.
For a graphical chart of what types of particles are stopped by a particular MERV rated filter, you can see this 'Particle Size Chart'.
MPR Rating System
Another system that you may run across is the MPR Rating System (Micro Particle Performance Rating). This system is sponsored by 3M for their Filtrete filters. It is a similar system to the MERV rating. It focuses on smaller particles. The MPR system uses numbers up to 1500, with the higher numbers being better.
The range for the MPR system that most homeowners would want to be in would be 800 to 1,250. The filters that it applies to are pleated disposable filters.
Other Filter Terms
Cheaper types of furnace filters may try to deceive you with numbers that have little meaning, but sound good. 'Arrestance' is a term that has to with a filter's ability to remove larger dust particles. Even a cheap filter may claim an 'Arrestance' rating of 80 - 90%. This may sound good, but it tells you nothing of how well it removes other contaminants. It is possible for a filter to have an Arrestance rating of 80% and have a MERV rating below 4.
Another term you may run across is 'Dust Spot Efficiency'. This measures the filter's ability to remove particles that can stain. Although it will not be an exact alignment, a filter with a higher MERV rating will also have a higher 'Dust Spot Efficiency' rating.