ASHRAE Journal - November 2019 - 42

ASHRAE - CELEBRATING 125 YEARS

History of HEPA Filters
In addition to the pioneering work of Willis Whitﬁeld
and his laminar ﬂow cleanroom, cleanrooms would not
have been practical without the invention of "absolute,"
"super-interception," and "super-efﬁciency" air ﬁlters.
The development of absolute ﬁlters dates back to preWorld War II Germany. Germany had developed ﬁlter
paper for use in gas masks using ﬁnely ground asbestos
dispersed in esparto grass. A captured German gas mask
canister was sent to the U.S. Army Chemical Warfare
laboratory where additional research was performed. The
German paper was studied, and the mixture of asbestos
and esparto grass had unusually high particle retention
characteristics, acceptable resistance to airﬂow, good dust
storage and resistance to plugging from oil-type screening smokes. Hollingsworth and Vose Company (H&V) of
Massachusetts manufactured ﬁlter paper using conventional papermaking machinery.
In the 1950s the U.S. Atomic Energy Commission (AEC)
wanted to create super-interceptor ﬁlter media using

42

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N O V E M B E R 2 0 19

domestic materials rather than relying on imported
asbestos and esparto grass. Johns Manville and Owens
Corning developed sub-micron diameter glass ﬁbers,
and in 1951 an all-glass-ﬁber paper made partly from
superﬁne glass ﬁbers with diameters substantially less
than 1.0 µm was produced. In 1953 Walter Smith, working with Arthur D. Little, developed the "absolute" ﬁlter
for the AEC. By the end of the 1950s ,multiple companies
were producing absolute ﬁlters, and in 1961 the generic
acronym "HEPA ﬁlter" was coined by Humphrey Gilbert,
a former Manhattan Project safety engineer. It came
from the title of a 1961 AEC report called "High-Efﬁciency
Particulate Air Filter Units, Inspection, Handling,
Installation." A HEPA ﬁlter was deﬁned as a throwaway,
dry-type ﬁlter with a minimum particle removal efﬁciency of 99.95% (which was later raised to 99.97%) for
a 0.3-µm monodisperse particle cloud. In later years,
as advances in ﬁlter media continued, new ﬁlters with
removal efﬁciencies greater than 99.99% were developed.
These new ﬁlters were referred to as ultra low penetrating
air ﬁlters, or ULPA ﬁlters.27

ASHRAE Journal - November 2019

Table of Contents for the Digital Edition of ASHRAE Journal - November 2019

Contents
ASHRAE Journal - November 2019 - Intro
ASHRAE Journal - November 2019 - Cover1
ASHRAE Journal - November 2019 - Cover2
ASHRAE Journal - November 2019 - 1
ASHRAE Journal - November 2019 - Contents
ASHRAE Journal - November 2019 - 3
ASHRAE Journal - November 2019 - 4
ASHRAE Journal - November 2019 - 5
ASHRAE Journal - November 2019 - 6
ASHRAE Journal - November 2019 - 7
ASHRAE Journal - November 2019 - 8
ASHRAE Journal - November 2019 - 9
ASHRAE Journal - November 2019 - 10
ASHRAE Journal - November 2019 - 11
ASHRAE Journal - November 2019 - 12
ASHRAE Journal - November 2019 - 13
ASHRAE Journal - November 2019 - 14
ASHRAE Journal - November 2019 - 15
ASHRAE Journal - November 2019 - 16
ASHRAE Journal - November 2019 - 17
ASHRAE Journal - November 2019 - 18
ASHRAE Journal - November 2019 - 19
ASHRAE Journal - November 2019 - 20
ASHRAE Journal - November 2019 - 21
ASHRAE Journal - November 2019 - 22
ASHRAE Journal - November 2019 - 23
ASHRAE Journal - November 2019 - 24
ASHRAE Journal - November 2019 - 25
ASHRAE Journal - November 2019 - 26
ASHRAE Journal - November 2019 - 27
ASHRAE Journal - November 2019 - 28
ASHRAE Journal - November 2019 - 29
ASHRAE Journal - November 2019 - 30
ASHRAE Journal - November 2019 - 31
ASHRAE Journal - November 2019 - 32
ASHRAE Journal - November 2019 - 33
ASHRAE Journal - November 2019 - 34
ASHRAE Journal - November 2019 - 35
ASHRAE Journal - November 2019 - 36
ASHRAE Journal - November 2019 - 37
ASHRAE Journal - November 2019 - 38
ASHRAE Journal - November 2019 - 39
ASHRAE Journal - November 2019 - 40
ASHRAE Journal - November 2019 - 41
ASHRAE Journal - November 2019 - 42
ASHRAE Journal - November 2019 - 43
ASHRAE Journal - November 2019 - 44
ASHRAE Journal - November 2019 - 45
ASHRAE Journal - November 2019 - 46
ASHRAE Journal - November 2019 - 47
ASHRAE Journal - November 2019 - 48
ASHRAE Journal - November 2019 - 49
ASHRAE Journal - November 2019 - 50
ASHRAE Journal - November 2019 - 51
ASHRAE Journal - November 2019 - 52
ASHRAE Journal - November 2019 - 53
ASHRAE Journal - November 2019 - 54
ASHRAE Journal - November 2019 - 55
ASHRAE Journal - November 2019 - 56
ASHRAE Journal - November 2019 - 57
ASHRAE Journal - November 2019 - 58
ASHRAE Journal - November 2019 - 59
ASHRAE Journal - November 2019 - 60
ASHRAE Journal - November 2019 - 61
ASHRAE Journal - November 2019 - 62
ASHRAE Journal - November 2019 - 63
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ASHRAE Journal - November 2019 - 65
ASHRAE Journal - November 2019 - 66
ASHRAE Journal - November 2019 - 67
ASHRAE Journal - November 2019 - 68
ASHRAE Journal - November 2019 - 69
ASHRAE Journal - November 2019 - 70
ASHRAE Journal - November 2019 - 71
ASHRAE Journal - November 2019 - 72
ASHRAE Journal - November 2019 - 73
ASHRAE Journal - November 2019 - 74
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ASHRAE Journal - November 2019 - 76
ASHRAE Journal - November 2019 - 77
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ASHRAE Journal - November 2019 - Cover3
ASHRAE Journal - November 2019 - Cover4
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