ASHRAE Journal - December 2020 - 18

TECHNICAL FEATURE

What You Can and
Can't Do With ASHRAE
Standard 52.2
BY KATHLEEN OWEN, FELLOW ASHRAE; CAROLYN (GEMMA) KERR, PH.D., LIFE MEMBER ASHRAE

ASHRAE Standard 52.2, Method of Testing General Ventilation Air-Cleaning Devices for
Removal Efficiency by Particle Size, is a filter test standard. Its standard test method gives
rise to the " MERV " found on many filters sold in the U.S. and around the world.1 The
term " MERV filter " is everywhere lately with COVID-19 feeding interest to upgrade
filters to MERV 13 or higher. This article shares some history of ASHRAE Standard
52.2, including the very recent addition of a particulate matter (PM52.2) efficiency
calculation, and explores what you can and can't do with it.
First published in 1999, Standard 52.2 brought us particle size-dependent efficiency measurements for filters
used in HVAC systems.2 This test covers the size range
from 0.3 µm to 10 µm with efficiency curves for a filter
when clean, slightly dusty and more heavily dust-loaded.
The minimum efficiency reporting value (MERV) comes
from taking the lowest efficiency value in each of 12 size
ranges across the efficiency tests, then averaging these
by small, medium and large sizes. Small (0.3 µm to 1.0)
particles' average efficiency becomes E1. Medium (1.0 µm
to 3.0 µm) are E2, and large (3.0 µm to 10 µm) particles'
efficiencies become E3. E1, E2 and E3 are used with Table
12-1 of Standard 52.2 to determine the MERV for a filter.
Table 1 is the current version of Table 12-1.

How Has the Standard Changed & How Does That Help You?
When ASHRAE Standard 52.2 was first published in

1999, the MERV table was set up based on the efficiency
curves that were known and good estimates for what
was likely to be found for filters in what the Standard
52.2 committee believed to be logical use categories.
Some filters had been tested at that time, but there was
not a lot of particle size dependent efficiency data for
HVAC filters available to base our groupings of filters
into MERV levels on before the standard was published.
As more efficiency curves became available, the committee realized some sections of the MERV table did not
reflect the differences between real filters. One issue was
medium-level filters in the MERV 8 to MERV 11 range
usually were either 8 or 11 and almost never assigned to
MERV 9 or MERV 10. Because of the curve shapes typical
in this range and the way the E3 values were assessed,
many filters switched back and forth between MERV 8
and MERV 11 with only a few tenths of a percentage point

Kathleen Owen is an air filtration consultant with Owen Air Filtration Consulting in Cary, N.C. She is the chair of ASHRAE SSPC 52.2 and is a member of the Epidemic Task Force.
Carolyn (Gemma), Kerr, Ph.D., is retired after a career of performing IAQ investigations and research. She is a voting member of SSPC 52.2 and past chair of TC 2.4.
18

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ASHRAE Journal - December 2020

Table of Contents for the Digital Edition of ASHRAE Journal - December 2020

Contents
ASHRAE Journal - December 2020 - Intro
ASHRAE Journal - December 2020 - CT1
ASHRAE Journal - December 2020 - CT2
ASHRAE Journal - December 2020 - Cover1
ASHRAE Journal - December 2020 - Cover2
ASHRAE Journal - December 2020 - 1
ASHRAE Journal - December 2020 - Contents
ASHRAE Journal - December 2020 - 3
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