A Minimum Efficiency Reporting Value (MERV) rating is a measurement scale that indicates how well an air filter removes particles from the air. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) developed the MERV rating in 1987.
What does a MERV rating measure?
- MERV ratings measure how well a filter traps particles between 0.3 and 10 microns in size.
- Higher MERV ratings indicate greater capture efficiency for specific particle-size ranges.
- MERV measures particle capture; it does not measure whether a filter kills or inactivates microorganisms.
What can MERV ratings help with?
- MERV ratings can help you compare the performance of different filters.
- MERV ratings can help you choose the right filter for your home.
What types of particles can MERV ratings help remove?
- Depending on the filter’s rating, captured particles can include dust, pollen, pet dander, mold spores, smoke, and airborne particles that may carry bacteria or viruses.
What are some examples of MERV ratings?
- MERV 1–6: Basic fiberglass filters that catch lint, dust, and pollen.
- MERV 7–10: Pleated filters that catch dust mites and mold spores.
- MERV 11–12: High-efficiency filters that catch pet dander, smoke, and sneezed particles.
- MERV 13+: Higher-efficiency filters that capture smaller particles, including many airborne particles that may carry bacteria or viruses.
The challenges of filtration.
Fibrous filters have a most penetrating particle-size range, where their capture efficiency reaches its minimum. This range is often near 0.3 µm, although its exact position depends on the filter media and airflow conditions. The efficiency at this range depends on the individual filter.
Particles both larger and smaller than this range are generally captured more efficiently. Larger particles can strike or touch filter fibers, while very small particles move randomly through Brownian motion, increasing contact with fibers through diffusion. Fibrous filters therefore do not act like simple sieves that let every smaller particle pass through more easily.
HEPA filters are commonly specified to remove at least 99.97% of particles at 0.3 µm under test conditions. A particular filter’s most penetrating particle size can differ from 0.3 µm, so size-specific test data should guide comparisons. MERV ratings characterize capture over 0.3–10 µm; a MERV rating alone does not quantify performance below that tested range.
Sources.
- Centers for Disease Control and Prevention. Ventilation FAQs, question 8. Filter efficiency and respiratory particles that carry viruses. https://www.cdc.gov/niosh/ventilation/faq/index.html
- U.S. Environmental Protection Agency. Air Cleaners, HVAC Filters, and Respiratory Viruses. Filtration reduces particles containing viruses. https://www.epa.gov/indoor-air-quality-iaq/air-cleaners-hvac-filters-and-respiratory-viruses
- U.S. Environmental Protection Agency. What is a HEPA filter? HEPA efficiency, most penetrating particle size, and the MERV test range. https://www.epa.gov/indoor-air-quality-iaq/what-hepa-filter
- ASHRAE Handbook—HVAC Applications, Chapter 19: Clean Spaces. Most penetrating particle size, filter-media and airflow effects, and diffusion capture. https://handbook.ashrae.org/Handbooks/A23/SI/a23_ch19/a23_ch19_si.aspx
- American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). “Debunking Myths About MERV, Air Filtration.” Accessed February 12, 2025. https://www.ashrae.org/news/ashraejournal/debunking-myths-about-merv-air-filtration
- Environmental Protection Agency (EPA). “What is a MERV Rating?” Accessed February 12, 2025. https://www.epa.gov/indoor-air-quality-iaq/what-merv-rating
- American Society of Heating, Refrigerating and Air-Conditioning Engineers. “Filtration and Disinfection.” ASHRAE. Accessed February 12, 2025. https://www.ashrae.org/technical-resources/filtration-disinfection
- Kowalski, W.J., and W.P. Bahnfleth. 2002. “MERV Filter Models for Aerobiological Applications.” Air Media, Summer, 1. Accessed February 12, 2025. https://www.researchgate.net/figure/Composite-of-all-MERV-filter-models-based-on-initial-conditions_fig3_237558312
