HEPA filters play a significant role in completely removing airborne particulates which are more than 5µm and particulates from 0.15 µm – 0.25 µm are removed 95% - 99.9995% efficiently. Thus they are one of important factors in maintaining quality of products.
1.0 Introduction
HEPA filters which are important in pharma if applied incorrectly can cause degradation of clean air. So some tests are carried out to ensure that they work properly and the air in cleanrooms is particulate free. HEPA filters should be checked periodically so they remain free of leakage as the leakage can damage the filter which is a cautioned issue as even a correlation is found between the leakages of HEPA filters to increasing microbial counts.
1.1 Reasons for testing
Many reasons attribute for testing the filters which if not done can cause severe damage to product quality.
1.1.1 Tests are carried out to check whether they are in compliance with specifications. Quality is validated.
1.1.2 The filter is tested whether it is operating in perfect condition or not
1.1.3 The facility should be free from particulates and in accordance with the specifications as in part 1 of ISO 14644 standards which avoids the need to test HEPA filters individually.
Particle count test is done when there is no ongoing process or movement of personnel which indicates the chance of few or no particles.
1.1.4 Among 13 different tests for HEPA filters two tests were given to test integrity of the filters. The in situ test method is aerosol photometer test method where aerosol photometer is used as measuring device. This method measures 0.003% by DOP scan testing but presently there are new instruments which can produce value 10 times lower that DOP testing as 0.0003%.
Particle counting method uses discrete particle counter (DPC) which measures 0.000005% and even better. This method involves complex mathematics to make the measurement valid.
Both particle counter method and DOP scan test mandates aerosol on the filter is distributed evenly and should be validated when used for first time in system.
1.1.5 When done for the first time it should be ensured that sufficient aerosol mixing takes place. The concentration upstream aerosol should not change more than ±15% of the measured average value. Higher concentrations than average increase the sensitivity of the test to small leaks and lower concentrations than average will decrease the sensitivity of test to small leaks
1.2 Aerosol filter method can be improved by the following ways:
1.2.1 Sufficient distance between injection point and filters to be tested, if there is no proper distance then sparge pipe or mixing manifold can be used. This manifold should be dedicated and installed temporarily or permanently for the test.
1.2.2 For the introduction of aerosol specific injection port is installed or designed
1.2.3 Installation of upstream aerosol sample port
1.2.4 The filter on the downstream side should be able to accessed for scanning, if inaccessible then longitudinal scanning probe can be used as permanent installation.
1.2.5 The SOP should reference the ports which are used and which are off the test to allow for even distribution.
When HEPA filters are used and maintained properly then there would be few failures and clean air can be supplied without hindrance. Perfect usage of HEPA filter occurs when they involve less cost in their maintenance and removing the filters which report issues of leaking.
Hepa filter testing, particle count test, aerosol filter method, hepa filters
