MERV Ratings, Explained

In this post:

• What are MERV ratings?
• What types of particles do MERV-rated filters trap?
• What MERV rating is best for your HVAC system?

What is a MERV Rating?

Two common questions we receive are: "What is the definition of MERV and what does MERV mean when buying an air filter?".

What is the definition of MERV?

MERV stands for Minimum Efficiency Reporting Value. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) established the rating system in 1987. The MERV Efficiency Rating Scale ranges from 1 to 16, with 1 being the lowest efficiency and 16 describing the highest efficiency. The particle size range addressed by the MERV scale is 0.3 to 10 microns. 

What does MERV mean when buying an air filter?

Air filter efficiency refers to the relative ability of a filter to remove particles of a given size or size range from air passing through the filter. As a general rule, the higher the filter rating will yield higher air filtration capabilities. The higher the MERV rating, the better the filter is at trapping specific types of particles. We offer air filters with MERV ratings of 8 and above to offer effective air filtration and particulate removal. Most building HVAC systems use filters with MERV ratings of 8 and up.

What particles do MERV-rated filters trap?

The chart below shows common allergens and irritants MERV filters rated 8 and up can provide resistance to. MERV 8 filters trap the most common allergens, from dust mites and pollen to lint and pet dander. A filter with a MERV rating of 13 and up also provides resistance to bacteria and viruses, greatly improving IAQ, and offering healthier air to building occupants.

What MERV filter is recommended for preventing the transmission of airborne viruses such as COVID-19?

Although ASHRAE does not recommend a specific MERV filter rating for preventing airborne COVID-19 transmission, the organization suggests that upgrading HVAC systems in non-healthcare facilities to MERV-13 or the highest achievable level can be a worthwhile step in your buildings infrastructure improvement plans. It should be noted that each building's HVAC system can only support certain MERV-rated filters. The higher the rating, the harder your HVAC system will have to work because they can lead to pressure drop, which can potentially reduce airflow and diminish the system's ability to maintain humidity and temperature set points.

What's a HEPA Filter, and How Does It Relate to MERV Rated Filters?

Per the EPA, HEPA is a type of pleated mechanical air filter. It is an acronym for high efficiency particulate air (filter), as officially defined by the U.S. Dept. of Energy. This type of air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns (µm). The diameter specification of 0.3 microns responds to the worst case; the most penetrating particle size (MPPS). Particles that are larger or smaller are trapped with even higher efficiency. Using the worst-case particle size results in the worst-case efficiency rating (i.e. 99.97% or better for all particle sizes). While effective in capturing bacteria and viruses, the problem is that a HEPA is too fine of a filter media to implement into existing HVAC systems, and the only way to add one into a facility is through a standalone or portable HEPA air filtration system with its own dedicated fan designed for the increased resistance of this filter type.

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