What is a MERV rating?

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What is MERV?

Minimum efficiency reporting value, commonly known as MERV rating is a measurement scale designed in 1987 by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to rate the effectiveness of air filters. The scale “represents a quantum leap in the precision and accuracy of air-cleaner ratings” and allows for improved health, reduced cost, and energy efficiency in heating, ventilation, and air conditioning (HVAC) design.

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For example, a HEPA filter is often impractical in central HVAC systems due to the large pressure drop the dense filter material causes. Experiments indicate that less obstructive, medium-efficiency filters of MERV 7 to 13 are almost as effective as true HEPA filters at removing allergens, with much lower associated system and operating costs.

The scale is designed to represent the worst case performance of a filter when dealing with particles in the range of 0.3 to 10 micrometers. The MERV rating is from 1 to 16. Higher MERV ratings correspond to a greater percentage of particles captured on each pass, with a MERV 16 filter capturing more than 95% of particles over the full range.

MERV Min. particle size Typical controlled contaminant Typical Application
1–4 > 10.0 μm Pollen, dust mites, cockroach debris, sanding dust, spray paint dust, textile fibers, carpet fibers Residential window AC units
5–8 10.0–3.0 μm Mold, spores, dust mite debris, cat and dog dander, hair spray, fabric protector, dusting aids, pudding mix Better residential, general commercial, industrial workspaces
9–12 3.0–1.0 μm Legionella, Humidifier dust, Lead dust, Milled flour, Auto emission particulates, Nebulizer droplets Superior residential, better commercial, hospital laboratories
13–16 1.0–0.3 μm Bacteria, droplet nuclei (sneeze), cooking oil, most smoke and insecticide dust, most face powder, most paint pigments Hospital and general surgery
17–20 < 0.3 μm Virus, carbon dust, sea salt, smoke Electronics and pharmaceutical manufacturing clean-room

The Minimum Airflow Efficiency Rating (MERV) determines how effectively a filter prevents dust and other contaminants from passing through the filter into the air stream. Thus, filters with a high MERV rating can trap fine particles more effectively than filters that have a lower MERV rating.

Filters with a MERV rating of 16 or lower are considered HVAC class filters and are used for residential as well as commercial applications.

MERV 17 to MERV 20 filters are typically used in hospitals and other clean rooms where maximum cleanliness is required.

Parameters and efficiency of MERV filters

When you have a question about choosing a filter for your application, it is not always necessary to use filters that have a higher MERV rating.

Using an air filter with a MERV rating higher than the manufacturer of your air conditioner or ventilation system recommends can actually degrade its performance.

Air filters with higher ratings have fine pores and create resistance to airflow if the filter is used in an HVAC system that is not designed for such resistance. This situation can reduce system efficiency, degrade indoor air quality, and put a strain on the system’s fan.

MERV filter ratings and air quality

A document published by ASHRAE addresses the claim that particle filtration is good for health. According to this document, there is a clearly established link between elevated concentrations of particles in outdoor air and poor health, so it makes sense that filtering these particles from indoor air can lead to better health.

The ASHRAE paper cites studies that show this:

  • Particle filtration can be “moderately effective” in controlling allergy and asthma symptoms.
  • Particle filtration can reduce the concentration of infectious particles in the air that spread many infectious diseases, so some models suggest that it can “significantly reduce the transmission rate of diseases caused by these fine particles.”
  • The health benefits of particle filtration are proportional to “reducing overall exposure to particles less than 2.5 microns in diameter.”
David West
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