ASHRAE Journal - February 2014 - S13

Feature
Over the past twenty-ﬁve years, air ﬁlter test standards have
proliferated with every major nation or region establishing its
own standard. These standards are often similar but not identical.
As a result, the number of similar but not identical ﬁlter classes
has also proliferated to almost the point of being meaningless.
The measurement variability in many of these classification
schemes, especially for the lower efﬁciency ventilation ﬁlters, is
large enough for adjacent ﬁlter classes in many standards not to
be statistically different. This paper discusses ways of reducing
the number of ﬁlter classes and the potential effect on the quality
of air delivered. This discussion is not for recommending speciﬁc
new, and fewer, ﬁlter classes, but more as a challenge to all the
professionals to take a fresh look at what these ﬁlter classes really
mean to the one that matters most: the consumer. As a matter of
reference, one should keep in mind that until late 1980s, in most of
the world there were just a few ﬁlter classes as deﬁned by ASHRAE
52.1, namely 90-95%, 80-85%, 60-65% etc. In fact, these terms
are still in current conversational use in many parts of the world.
Similarly, for High Efﬁciency (HEPA) ﬁlters, one still relates to the
ﬁlter class deﬁned as 99.97% dating back over 50 years.

Table : e filter classes

ASHRAE 52.2
Current

Case 1

MERV 1
MERV 2

MERV 2

MERV 3

MERV 5

MERV 6
MERV 7
MERV 8

MERV 8

MERV 9

MERV 11

MERV 12

Filter Classes and Standards

MERV 12

For this paper, the discussions are limited mainly to European
Norms EN 779 and EN 1822, Institute of Environmental Science
and Technology, USA (IEST), and ASHRAE 52 standards, simply
because these are more widely followed around the world.
Although ﬁlters in the higher efﬁciencies are mostly speciﬁed
either according to regulations, or by unique industry speciﬁc
cleanroom requirements, lower efﬁciency HEPA ﬁlters are also
used for general ventilation and in hospitals. Thus, this article
includes discussions of ways to reduce the number of ventilation
ﬁlter classes as well as HEPA ﬁlter classes.

MERV 14

Reduced Number of Filter Classes
First, let us consider the ﬁlter classes for ventilation ﬁlters.
The case for reducing the number of ﬁlter classes of ventilation
ﬁlters can be made from several points of view, each of which we
will examine. An examination of Figure 1 shows that the range
of filter efficiencies relevant to these standards has increased

MERV 14

MERV 15
MERV 16/HEPA

MERV 2

MERV 6

MERV 8

MERV 10

MERV 10
MERV 11

Case 2

MERV 4

MERV 4
MERV 5

EN 779

HEPA

Current Case 1
G1

G2

Case 2
G2

G2
G4

G3
G4

F5
F6

F5
F6
F7

F8

F8

F8
H10

F9
H10

MERV 12

MERV 14

HEPA

from about 5 classes in the old ASHRAE 52.1, to 9 in EN 779
and to 16 Minimum Efficiency Reporting Values (MERV) in
ASHRAE 52.2. While the additional classes may be perfectly
useful in differentiating products, they may not add any more
to the comfort of the air for the user, especially in the lower
efﬁciency range where most ﬁlters are used to protect equipment
and second stage ﬁlters and not people. Hence, simply on the basis
of proliferation of classes in new versions of standards, one can
make the case that we can quite easily live with as few ﬁlter classes
as in earlier times. Although it is theoretically possible to divide
the range of ﬁlter efﬁciencies into ﬁner and ﬁner sub-ranges or
classes, because of the measurement and manufacturing variability,
differentiating such ﬁne differences is not
practical; these difﬁculties were discussed
in the authors' personal conversations with
laboratory specialists and at some technical
committees. Such practical differences have
been known to sometime cause ﬁlters of a
speciﬁed class to be classiﬁed one or more
classes different than the one specified.
Thus, if the variability in each ﬁlter class
is quantiﬁed as ± one class, the number of
ﬁlter classes in both EN 779 and ASHRAE
52.2 can be substantially reduced and the
total number of classes approaches the
number in the now obsolete ASHRAE 52.1.
The new classes are shown in Table 1.
In Table 1, two possible means for
reducing the number of ﬁlter classes are

Figure : Comparison of filter classes reprinted from the literature of lue Hea en Technolog , SA

February 2014

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S13
ASHRAE Journal - February 2014

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