Publications

Air Filter MERV Ratings: What Do They Mean?

by Teresa Rainey

April 20, 2020

The common heating, ventilation, and air conditioning (HVAC) systems serving our former workplaces and our new workplaces (our homes) are recirculating air systems. We can improve the indoor air quality of our work environments by selecting the proper filter. To make the best selection, you need to understand the filter’s MERV rating.  MERV, which stands for Minimum Efficiency Reporting Value, is a rating based on the filter’s ability to remove contaminants from the air stream. Air contaminants include particulates such as dust, pollen, and smoke and biologicals such as mold, spores, bacteria and viruses. The chart below demonstrates the relative size of these contaminants.

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Figure 1: Common air contaminants and their relative sizes [DHHS (NIOSH) Publication No. 2003-136, Hinds, 1982]

ASHRAE Standard 52.2 Standard for Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size is the testing standard for MERV ratings. The test is based on three size ranges of particles: Range 1 is 0.3 to 1.0 microns; Range 2, 1.0 to 3.0 microns; and Range 3, 3.0 to 10.0 microns.

The chart below provides a summary of the MERV performance data. 

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In addition to particle size removal efficiency, it is important to pay attention to the filter’s associated pressure drop. Pressure drop is a measurement of the resistance across the filter media. Much like your own blood pressure, a higher blood pressure means your heart is working harder to circulate blood. An increase in pressure drop means the fan needs to work harder to distribute the same amount of airflow. ASHRAE Standard 52.2 includes test procedures to determine the filter’s resistance to airflow performance characteristics. While not universal, in some cases, a higher performing filter may have a higher associated pressure drop. Your existing HVAC fan may not be sized to overcome the additional pressure drop. The result may be bypass air around the filter through gaps and reduced fan airflow. Selecting a filter that removes targeted air contaminants and meets your system pressure drop limitations will provide a workplace environment with enhanced indoor air quality.

Want to know more? For further information, reference “Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks” published by Department of Health and Human Services, Centers for Disease Control and Prevention and National Institute for Occupational Safety and Health, April 2003. 

Teresa Rainey

Teresa Rainey, PE, LEED Fellow

Director of Engineering