HEPA Efficiency:
By definition, a HEPA filter
must be capable of removing a minimum of 99.97 percent of all
airborne particulate matter 0.3 micron (micrometer) in diameter
and larger. To ensure this fractional efficiency, manufacturers
individually test each filter with a generated aerosol which
allows the efficiency on 0.3 microns to be determined. One test
known as Mil-Std-282 or the "hot" or "thermal"
DOP test, is performed with sophisticated laboratory equipment
requiring constant monitoring and calibration to maintain aerosol
size, concentration and efficiency measurements. Mil-Std-282
is one of the most widely accepted industry standards for testing
0.3 micron efficiency and is required for compliance with most
nuclear, military and U.L. specifications. IEST-RP-CC-007 is
another efficiency test which uses sophisticated particle counters
which are calibrated to count a specific particle size such as
the 0.3 microns required for HEPA filters. Efficiency tests must
not be confused with the "cold" DOP test or leak tests
which are not intended for measuring overall filter efficiency
(for more information go to Testing).
Recently, much confusion has
been created with the proliferation of filters labeled "HEPA-type",
HEPA-like", HEPA-media" or "hospital-grade HEPA".
These filters typically range between only 90 to 99 percent efficient
on 0.3 micron. While being an increase in efficiency over regular
filters, they
are not individually tested and do not meet the HEPA specification.
Similarly, efficiency phrases
such as "as small as" or "at" mean that if
all particles were that small, the filter would still have that
efficiency and should not be confused with the phrase "down
to" which may mean a mixture of particle sizes for the stated
efficiency.
ULPA Efficiency:
The efficiency of an ULPA filter
(ultra low penetration air) is 99.999% on 0.12 microns. The IEST-RP-CC-007
test standard mentioned above is also used for ULPA efficiency
testing.
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