34 | July 2020
A
s much as I've written about filters and
filtration in this blog, I have to confess
there's one aspect of filter efficiency I've
resisted learning about. Every article
I've written that mentions filter efficien-
cy talks about only one rating scale:
Minimum Efficiency Reporting Value
(MERV). A couple of other scales are in common use, too,
but I just didn't want to know anything about them because
I buy and recommend MERV rated filters. But I took the
plunge recently, and here's what you ought to know about
the three main filter rating systems.
Minimum Efficiency Reporting Value - MERV
The standard rating system for filters is called Minimum Effi-
ciency Reporting Value, or MERV. When I say it's the "stan-
dard" rating system, I mean it. This rating system is defined
in an ASHRAE standard (52.2), which specifies the perfor-
mance required to meet each MERV level and the testing
required. Higher-MERV filters remove more particles from
your air. They also remove smaller particles. A standard fi-
berglass one-inch filter is typically a MERV 2. The high end
of the scale is MERV-16. (It used to be 20, but ASHRAE
dropped it to 16 years ago.)
Before we go further, we need to define the three gener-
al size ranges of particulate matter (PM).
• Range 1: 3.0 to 10.0 microns (includes PM10)
• Range 2: 1.0 to 3.0 microns (includes PM2.5)
• Range 3: 0.3 to 1.0 micron
The words making up the MERV acronym are a bit un-
wieldy – What did you expect from engineers? – but the
"minimum efficiency" part is the most important. For a filter to
get the MERV-13 rating, for example, it must filter out a min-
imum of 90% of the Range 1 particles (PM10), a minimum
Understanding Filter Ratings: MERV, FPR, and MPR
