Ensuring Filter Integrity: An Overview of Filter Integrity Testing

Producers of high-efficiency particulate air (HEPA) filters designed for cleanrooms and controlled environments conduct assessments of their products’ particle removal efficiency. Similarly, upon installation at the user’s production site, it is essential to promptly execute a test to validate both the filter’s integrity and the installation. Generally, HEPA filters are capable of capturing as much as 99.97% of particles as small as 0.3 microns. 

The dispersed oil particulate (DOP) scan testing or PAO(Poly Alpha Olefin) 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. 

Flow rate for the test

It is important that the proper flow rate through the filters is established prior to testing. When checking for filter leaks onsite, the system should be checked following tests described in ISO 14644-3 to verify that airflow volume, velocity tests balancing and, if appropriate, the uniformity of these parameters are within specified limits. These tests should precede the challenge uniformity test and leak test.

The standard ISO 14664-3 suggests a penetration of 0.01% of the test challenge concentration but allows alternative criteria to be agreed between customer and supplier. The FDA Guidance, however, indicates that 0.01% penetration is a leak. 

Selecting the aerosol:

There are three types of aerosol that can be used for testing filter installations in a cleanroom. These are:

• Cold generated aerosol
• Hot generated aerosol
• Microspheres

The first two are formed from oil-type liquids. Polystyrene latex (PSL) spheres are most commonly used in microelectronic applications, while Poly Alpha Olefin(PAO) in life sciences.

What is a leak?

A leak in a filter refers to an unintended opening or gap in the filter medium, housing, frame, or seal, which allows particles or substances to bypass the filtration system. In air filters like HEPA (High Efficiency Particulate Air a leak compromises the filter’s ability to effectively capture and retain particles, reducing its overall efficiency and potentially leading to contamination of the filtered air. 

Scanning for leaks

Typically, there is a gap between the HEPA filter and its housing, with the gasket positioned at the rear. To identify gasket leaks in the space between the filter frame and housing, a probe is inserted into that region, and a thorough scan is conducted.

In the case of a gasket leak, particles disperse and fill the gap between the filter and the housing During the scan for a gasket leak, the tester may encounter a concentrated particle presence at a distance from the actual leak, potentially leading to a mistaken identification of a leak. In such instances, it is advisable to withdraw the probe from the sample tube and conduct a scan with the considerably smaller tube area to pinpoint the location of the highest particle penetration reading and thus determine the gasket leak accurately.

Fixing a leak

A final word of wisdom: if a leak in the HEPA filter is found, the best way to fix it is by following the recommended fill patching procedure or the replacement