What is a MERV rating?

As air moves through a building’s HVAC system, air filters trap and collect large and small particles such as dust, allergens and microorganisms. According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), this filtration helps provide healthier indoor air quality. A filter's MERV number indicates how it's rated to remove these particles. But what is MERV rating, exactly?

An air filter’s minimum efficiency reporting value (MERV) rating measures how effectively the filter stops dust and other contaminants from passing through the filter and into the air stream. Filters with higher MERV ratings trap small particles more effectively than filters with lower MERV ratings.

In general, filters with a rating of MERV 16 or below are considered to be HVAC-system-grade filters for residential, commercial and general hospital use. MERV 17 through MERV 20 filters are typically used in surgical operating rooms, clean rooms and other contexts that require absolute cleanliness.

MERV Filter Ratings and Efficiency

When selecting the right filter for your application, higher is not always better. Using an air filter with a MERV rating higher than what your furnace or air conditioner manufacturer recommends can actually impair its performance.

The smaller pores in more highly rated air filters create resistance to air flow, and if the filter is used in an HVAC system that is not designed to handle this resistance, it can lower the system's efficiency, decrease indoor air quality, and put strain on the system's fan.

MERV Filter Ratings and Air Quality

A position paper published by ASHRAE looked at the claim that particle filtration has health benefits. According to this paper, there's a well established connection between higher concentrations of particles in outdoor air and poor health outcomes, so it would make sense that filtering these particles out of indoor air could lead to better health outcomes. 

The ASHRAE paper reviewed studies suggesting that:

  • Particle filtration can be "modestly effective" against allergy and asthma symptoms.
  • Particle filtration can lower the concentration of infectious airborne particles that spread many communicable illnesses, which is why some models suggest that it can "substantially decrease the portion of disease transmission caused by these small particles."
  • The health benefits of particle filtration are proportional to "the reduction in total exposure to particles less than 2.5 µm in diameter."
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