Air Filters & MERV Ratings: What You Need to Know

Air filters have a broad range of applications based on their MERV rating. MERV (Minimum Efficiency Reporting Value) ratings are determined by a filter’s ability to capture differently sized particles between 0.3 and 10 microns (µm). MERV ratings range from 1-20, where the lower ratings are associated with a larger particle size. When selecting a filter, it’s not always about the numbers! It is always important to use a filter rating that is appropriate for the system you’re using. If your system’s purpose is to help reduce the transmission of illnesses by reducing the concentration of illness causing particles, then your filter rating is going to be much different than if you were trying to reduce the amount of sandpaper dust in a machine shop. If a filter with a higher than recommended rating is used in a system, it will put additional strain on your HVAC system. This additional strain can negatively impact the system’s efficiency and performance.


Filters categorized as MERV 1 through MERV 4 trap large particulates such as pollen, dust mites, sanding dust, or textile fibers. Specifically, it filters down to 10.0 micron (µm) and catches less than 20% of particulates lower than 10 microns. These types of filters can commonly be found in applications such as commercial building pre-filters, residential furnaces, and window air conditioning units.


Common applications for filters ranging from MERV 5 to MERV 8 include use in commercial buildings, industrial workplaces, and paint booths. In addition to the particulates that can be trapped by MERV 1 through MERV 4 filters, mold spores, dust, lint and cement dust can also be trapped by these filters. Filters in this range can filter particles down to 3 microns in size. MERV 6 and MERV 8 are the more commonly found filters in this range. Specifically, MERV 6 filters out about 49.9% of particles between 3 and 10 microns, and MERV 8 removes about 84.8% of particles in that size range.


MERV 9 through MERV 12 filters can be found in commercial buildings, hospitals, laboratories, and as pre or final filters. These filters can trap particles down to 1.0 to 3.0 microns. The most prevalent types in this range are MERV 10 and MERV 12. MERV 10 has the ability to filter out 85% of particles 3 microns or larger and ranges in efficiency between 50% and 64.9% for particles in the 1 to 3-micron range. MERV 12 has the ability to filter out 90% of particles 3 microns or larger and ranges in efficiency between 80% and 89.9% for particles in the 1 to 3-micron range. In addition to trapping the particulates that the lower categories can, this range of filters can also collect finer particulates such as humidifier dust, lead dust, and legionella bacteria.


Filters categorized as MERV 13 through MERV 16 trap small particulates such as tobacco smoke, pet dander, bacteria, and auto fumes. Specifically, it filters down to 0.3 to 1.0 micron and can commonly be found in applications such as general surgery, hospital inpatient areas, smoking lounges and used as final filters.


MERV 17 through MERV 20 filters are commonly used for applications such as clean rooms, labs with radioactive materials, and pharmaceutical manufacturing facilities. These filters capture particles down to less than 0.3 microns.


Another commonly used filter is a HEPA filter, or high efficiency particulate air filter, which is defined a little differently than the MERV rating system. By definition, a HEPA filter removes at least 99.97% of airborne particulates with a size of 0.3 microns. They are defined by the worst-case size particle, which in this case is the most penetrating particle size (MPPS), and the worst-case efficiency rating (99.97%). This means that these filters are classified above anything on the MERV rating scale.



References

Comfort Zone Heating & Cooling. (2018, March 13). Understanding MERV Ratings. Comfort Zone Heating & Cooling. https://comfort-zonehvac.com/indoor-air-quality/understanding-merv-ratings.


Grainger Editorial Staff. (n.d.). The Complete Air Filter MERV Rating Chart - Grainger KnowHow. https://www.grainger.com/know-how/equipment-information/kh-what-is-merv-rating-air-filter-rating-chart#:~:text=An%20air%20filter's%20minimum%20efficiency,filters%20with%20lower%20MERV%20ratings.


LearnMetrics, L. M. (2021, January 26). Understanding 1-20 MERV Rating: Chart, Filters And Air Purifiers. LearnMetrics. https://learnmetrics.com/merv-rating-chart/.



Written by:


Erica Norquist

Mechanical Designer

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