The phrase "High Effiecency Air Filter" has been a subjective term in the past, and has been left up to the manufacturers, not users, of air filters to determine what makes a filter high efficiency. With this responsibility on the shoulders of the air filter manufacturers the answer usually is that high efficiency air filters start at MERV 11, with seemingly no reason behind this answer. This post looks at the data to help determine when an HVAC air filter really delivers high particle removal efficiency and helps contribute to better IAQ.
Basis for a definition: Testing standards for air filters in the United States prior to the publication of ISO 16890 relied on reporting a filter's ability to remove a certain percentage of particles based on a range of particle sizes. This makes logical sense as we filter the air to removal particles so creating tests that showed us the ability of a filter to remove certain particles was a natural conclusion; however, in HVAC filtration these numbers were not meaningful for the purposes of understanding how the filter contributes (or hurts) Indoor Air Quality. In order to really understand what filters do for the presence of particulate matter (PM) in a building we must match the filter's particle removal efficiency with the particles found in ambient air, and a high efficiency air filter is one that removes a minimum of half all particle from the air.
Failure of MERV: ASHRAE 52.2 test method gave us Minimum Efficiency Reporting Value, or MERV. It is a scale that issues a number between 1-16 to communicate the filter's ability to remove particles from the air based on what the filter catches in each of three size ranges, each of these ranges is made of 4 sub-ragnes. (Have I lost you yet?) By looking at these values in a vacuum and not taking into account the particles that the filter will likely encounter while in an HVAC system provides no real value. This system created the situation where MERV-8 (halfway through the numbering system and logically implies that half of all particles are caught) only removes less than 5% of all particles in the air. With no basis of how the removal efficiency relates to real-world or operating conditions the value is meaningless and the 1-16 scale is misleading.
Enter ISO 16890: Becasue the IAQ community, including ASHRAE 62, speaks in terms of PM when talking about the relationship between particle air pollution and air quality, ISO 16890 addresses the disconnect between old filter rating systems and how professionals within the air quality industry already talk about air quality measurements. The basis for ISO 16890 is how well a filter reduces the mass of PM at three different levels PM-10, PM-2.5, and PM-1 In outdoor air 99% of all particles are the most dangerous PM-1 particles. By having a tangible basis behind the rating system ISO 16890 provides much more intuitive and meaningful information to those who specify, use, and buy air filters for their HVAC systems.
The one drawback of the ISO 16890 method is that for non-HVAC filtration, like air cleaners in manufacturing areas or other source generation areas, where particle sizes and amounts are known this relationship to the average distribution of particles in atmoshperic air is of no value. However, I would argue that the MERV/ASHRAE 52.2 system is of no value to general HVAC filtration applications because it lacks this relationship.
Where High Efficiency Air Filtration Starts: For general HVAC applications you have to go all the way to MERV-13 before at least half of all particles are removed from the air stream treated by the filter. This is particularly important in buildings that recirculate air because in most areas of the United States indoor air is more polluted than outdoor air. If you are allowing more than half the particles through the filter then you lose the air cleaning effect that a high efficiency air filter is supposed to be providing. Most air filter manufacturers will have on their "High Efficiency" pages of their websites, or in their brochures, filters as low as MERV-11 which only remove about 20-30 percent of airborne particles.
Additional Thought on IAQ: PM is just one facet of the IAQ equation and an area that still doesn't get much attention in the United States is molecular filtration of VOCs and other gaseous pollutants. This is something that building owners and facility maintenance professionals will be forced to address sooner than later as there is a growing market of wearable IAQ monitors that heavily weight their IAQ scores to the presence of VOCs and gaseous pollutants. (Take a look at one of these wearble devices here) We are not there quite yet, but the day is coming when building occupants will go from just saying they want clean air but also have the ability to test IAQ themselves.
If you want to see what your air filters are really doing (or not doing) for you, take a look at our HVAC filter audit program: