MERV ratings are a scale from 1 to 16 that measure the minimum efficiency of an air filter. Filters with ratings of 9-12 can trap more than 85% of particles with a size of 3.0-10.0 microns. A Merv 9 will trap less than 50% of the 1.0-3.0 micron size particles, Merv 10 will stop up to 64%, Merv 11 will get up to 79%, and Merv 12 is capable of trapping up to 89%. ASHRAE recommends MERV 6 or higher, the U.
S. Department of Energy recommends MERV 13, and LEED recommends MERV 8 as a minimum. When it comes to air quality, the highest MERV ratings are the most effective. However, they can also damage your HVAC system if the rating is too high for the fan motor or air handler to operate. Low airflow is the result of this mismatch.
If you are susceptible to allergies, asthma or other respiratory conditions, you may want to use a filter with a MERV rating of approximately 10 to 12. E1, E2, and E3 particulates are particle range sizes used in air filter testing to determine a MERV rating. If an air filter can capture at least 20% of E1, 65% of E2, and 85% of E3 particulates, it will earn a MERV 11 rating. While ASHRAE recommends MERV 13 and 14, it's best to select a filter with the highest possible MERV rating for your specific HVAC system. In addition, a MERV 12 filter that is only 1 (one inch) thick is likely to be more restrictive than a 4 (four inch) thick MERV 12 filter. For those who are not susceptible to allergies or respiratory conditions, a lower MERV rating may be sufficient. With the lowest MERV rating (1-), the filter will continue to capture pollen, dust mites, cockroach debris, sanding dust, spray paint dust, textile fibers and carpet fibers. Some air filters, especially those with higher MERV ratings, may impede airflow due to improved air filtration.
It's important to select an appropriate filter for your HVAC system in order to maximize air quality while avoiding damage.