One of the most common questions we get is asked a variety of ways:
How much MERV is enough?
What efficiency filter should I use?
Isn’t 8 enough?
As is the case with most questions like this, the answer is: It depends. To come to an answer for general HVAC purposes an understanding of how the MERV filter efficiencies relate to Particulate Matter (PM). General HVAC purposes would be ventilation for office and other commercial buildings, K-12 schools, colleges and universities, retail buildings, and hotels. In these cases there typically isn’t source generation of particulate like industrial manufacturing facilities or hospitals that have more pathogen control concerns.
What is PM anyway?
PM is simply all of the particles that are suspended in the air. This can be further broken down by a maximum particle size, which often occurs because smaller particles stay suspended in the air longer and are typically more detrimental to human health as smaller particles can penetrate lung tissue and even enter the blood stream. The most common different PM subclasses are PM10 (particles <10µ), PM2.5 (particles <2.5µ) and PM1 (particles <1µ or sub-micron particles). Of greatest concern for human health is PM1 as it is those sub-micron particles that not only make up 99% of all airborne particles they are also the ones that can penetrate down into the lungs and bloodstream.
For a point of reference, a human hair is about 70µ in diameter. As seen in the graphic below if a 6’ tall average weight person were representative of 1 µ, then 2.5µ would be the size of an adult elephant, and 10µ would be the size of a blue whale. This visual is important to remember because air filters are tested on particles between 0.3µ-10µ which is a substantial chasm, couple that with the fact that under ASHRAE 52.2/MERV weighs their numerical scale toward capturing a higher percentage of larger particle, but 99% of particles are under 1µ - which is a bit backwards and is why ISO 16890will eventually replace ASHRAE 52.2
Now that we have a basic understanding that most particles in the air are sub-micron in size and these are also the most harmful particles to our health – it would make sense that the amount of filtration that should be used will be based on the air filter’s ability to remove these small particles from the air. What makes this difficult is there is no reference to the distribution of particles in atmospheric air anywhere inthe ASHRAE 52.2 standard. This omission makes it important to understand how fractional efficiency curves relate to the particles size concentrations that make PM. Another difficulty is that the three particle size ranges reported (E1, E2, and E3) average a filter’s efficiency across the entire range of particle sizes without regard to how many of those particles are in the air. For example a filter rated at 13 needs to average 50% removal or better for the E1 range which reports the average of particles with a median size of 0.35µ, 0.475µ, 0.625µ, and 0.85µ. So a filter could capture no particles 0.35µ as long as it captured enough in the larger sizes to average 50%, even though in atmospheric air those 0.35µ particle outnumber 0.85µ particles by a margin of 7-to-1, meaning the real world impact of that filter is questionable at best. (A competing test standard ISO 16890solves this problem by using a mass particle capture percentage based on the atmospheric air particle distribution)
Looking at filters rated at 11, they only require 20% removal for E1 particles. Somehow that is defined as a “high-efficiency” filter, but mating that data against the particles that are actually in the air 11 isn’t very efficient. As a matter of fact in countries that use a test standard other than ASHRAE 52.2 that would be considered a medium efficiency pre-filter.
When you mate the MERV fractional efficiency against the distribution of ambient air particles you get this:
With the current test standard it is advised that the minimum efficiency filter that should be used in order to provide clean, safe air to building occupants is MERV 13. These filters generally take out over half of all particles in the air which will give a true air cleaning effect, especially when multi-pass or re-circulated air is used. If you still have questions, please contact us – we’re here to help with any air filtration questions!