Manufacturers of high-efficiency particulate air (HEPA) filters for cleanrooms and controlled environments test their products for particle removal efficiency. Equally, once the unit is installed in the user's production site, the test should be performed promptly to verify the integrity of the filter as well as the installation. Typically, HEPA filters capture up to 99.97% of particles as small as 0.3 microns.
The dispersed oil particulate (DOP) scan testing, also known as filter integrity testing, or leak testing, is one of the most quoted methods by industry standards. The test is recommended to be repeated at regular time intervals to verify the continued efficiency of the filter. During the test, the filter is challenged by introducing particulates and measuring the output.
The HEPA filter leak test is mandated in various international standards, including the FDA Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing – cGMP and the World Health Organization Technical Report Series, No. 961. For cleanrooms, the ISO 14644-3 standard provides further guidance.
The traditional aerosol photometry test method, used since the 1950s, is adequate for measurements to 0.003% by DOP scan testing. To perform an accurate filter leak test, it is necessary to make sure the challenge aerosol concentration is uniform over the entire upstream face of the filter(s) being tested; this is called spatial uniformity. Otherwise, high or low local concentration may cause the filter to incorrectly fail or pass the test. The ISO 14644-3 suggests that the variation in test aerosol concentration through time should not exceed ±15%. The uniformity of the challenge upstream of the filter should be verified.
If a cleanroom suite is the target of a test, the challenge aerosol is injected just after the air handling unit (AHU). By the time the air gets to the filters, the test particles are well mixed within the air ducts leading to an even concentration across the back of each terminal filter.
If the aerosol cannot be injected just after the AHU, it must be introduced into the ductwork system at least 15 to 20 duct diameters from the filter.
Flow rate for the test
It is important that the proper flow rate through the filters is established prior to testing.
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