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Air Filter Efficiency: What is a MERV Rating?

When shopping for air filtration and dust containment options you're likely to come across a number of different acronyms and terminology that could be a little bit confusing. Because dust containment have potentially hazardous consequences, it is important to understand what all this terminology means. One acronym you might see when comparing air filters are MERV ratings or just MERV. So what is a MERV rating, anyway?

What MERV Means

Although you might indeed see references to MERV ratings when comparing air filters, MERV is a standalone acronym that stands for Minimum Efficiency Reporting Value. To simplify, this means that a filter's MERV reflects how efficiently an air filter removes particles from the air. An air filter's MERV is between 1 and 20, with the higher numbers being more efficient. Filters with lower numbers may filter larger particles such as pollen and some bacteria while those with higher numbers filter bacteria and smoke.

How a Filter's MERV is Tested

In order to determine an air filter's MERV, tests are conducted in a controlled laboratory setting. A machine generated an aerosol spray that has a known particle size, and that spray is passed through the filter to determine how efficient that filter is at stopping the airborne particles. For official MERV testing, the test is repeated 12 times for 12 different particle sizes.

The Difference Between HEPA and MERV

Dust containment protocols most often require HEPA filtration. So how does HEPA approval relate to MERV? The original MERV charts as developed by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) only went up to 17 and anything above that efficiency level was classified as a HEPA or ULPA filter and therefore not included in the chart. As more filters qualified for these higher ratings, to keep things clear, the higher levels were added. All HEPA filters must meet the standards for a MERV of 17 or greater. Otherwise they may not offer true HEPA filtration.

Know Your Equipment

In sensitive environments, an air filter should always follow the standards set forth by the facility or other governing bodies which usually includes the use of HEPA filtration. However, in other environments it is important that an air filter not only meet with filtration requirements but also be compatible with the equipment in use. Just because a filter has a higher MERV does not mean it is the best choice. A filter that is more efficient than a ventilation system calls for can damage existing equipment and lead to less efficient filtration overall. Filters should also be properly maintained, which may mean changing the filter at least every few months, depending on manufacturer recommendations.

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