What is a High-Efficiency Particulate Air (HEPA) filter

What is a High-Efficiency Particulate Air (HEPA) filter? How HEPA filters work,  its efficacy test and test for integrity, and Penetration,Most Penetrating Particle Size (MPPS),  and standards for a HEPA filter

It is a type of filter which has high efficiency to filter out about 99.997% that is why they are termed as High efficiency particulate air filter,HEPA filters are made up of a mesh of fiber glass fibers of thickness ranging from 0.5 to 2 micrometer and are closely packed with each other with as low clearance as 0.3μm which may be bit greater.

How HEPA filters work.
Particles in air are trapped in to HEPA filter fibers when air caring these particles is blown or passed through these fibers, by mechanism of interception (particles adhere to fibers), impaction (particles embed in to the gap) and diffusion and then blocking, particles with lower particle size than the gap between these fibers are also trapped as the result above mechanisms fiber diameter, filter thickness, and face velocity are the factors which affect the efficacy of the HEPA filter.

Importance of HEPA filter in pharmaceutical industry very great, HEPA filters are used in industry where ever it is require to make the environment free of particulate matter as well as free from microbial contaminants (bacterial ,viruses etc) in clean rooms
A HEPA Filter of efficiency of 99.97% is usually used in aseptic manufacturing filing and packing, parentral dosage manufacturing.These filters are mounted in Laminar AIR flow bench which are then used as clean area for actual aseptic manipulations or aseptic filling where usually a clean aria of class 100 are required, such laminar AIR flow bench are typically found within a clean aria of class 1000, which again are maintained by HEPA filters installed in a HVAC system.

For High Efficiency Particulate Air filter , A velocity of 0.45 meters/second (90 feet per minute) has generally been established, with a range of plus or minus 20 percent around the setpoint. Higher velocities may be appropriate in operations generating high levels of particulates.

High-Efficiency Particulate Air (HEPA) efficacy test and test for integrity.
There is a major difference between filter leak testing and efficiency testing. An efficiency test is a general test used to determine the rating of the filter. An intact HEPA filter should be capable of retaining at least 99.97 percent of particulates greater than 0.3 nm in diameter. The filters maximum resistance to airflow, or pressure drop, at its nominal flow rate is usually specified around 300 Pa.

High-Efficiency Particulate Air (HEPA) filter integrity should be maintained to ensure aseptic conditions. Leak testing should be performed at installation to detect integrity breaches around the sealing gaskets, through the frames, or through various points on the filter media. Thereafter, leak tests should be performed at suitable time intervals for HEPA filters in the aseptic processing facility. For example, such testing should be performed twice a year for the aseptic processing room. Additional testing may be appropriate when air quality is found to be unacceptable, facility renovations might be the cause of disturbances to ceiling or wall structures, or as part of an investigation into a media fill or drug product sterility failure. Among the filters that should be leak tested are those installed in dry heat depyrogenation tunnels and ovens commonly used to depyrogenate glass vials. Where justified, alternate methods can be used to test HEPA filters in the hot zones of these tunnels and ovens.

Any aerosol used for challenging a HEPA filter should meet specifications for critical physicochemical attributes such as viscosity. Dioctylphthalate (DOP) and poly-alpha-olefin (PAO) are examples of appropriate leak testing aerosols. Some aerosols are problematic because they pose the risk of microbial contamination of the environment being tested. Accordingly, the evaluation of any alternative aerosol involves ensuring it does not promote microbial growth.

The purpose of performing regularly scheduled leak tests, on the other hand, is to detect leaks from the filter media, filter frame, or seal. The challenge involves use of a polydispersed aerosol usually composed of particles with a light-scattering mean droplet diameter in the submicron size range, including a sufficient number of particles at approximately 0.3 nm. Performing a leak test without introducing a sufficient upstream challenge of particles of known size upstream of the filter is ineffective for detecting leaks. It is important to introduce an aerosol upstream of the filter in a concentration that is appropriate for the accuracy of the aerosol photometer. The leak test should be done in place, and the filter face scanned on the downstream side with an appropriate photometer probe, at a sampling rate of at least one cubic foot per minute. The downstream leakage measured by the probe should then be calculated as a percent of the upstream challenge. An appropriate scan should be conducted on the entire filter face and frame, at a position about one to two inches from the face of the filter. This comprehensive scanning of HEPA filters should be fully documented.
A single probe reading equivalent to 0.01 percent of the upstream challenge would be considered as indicative of a significant leak and calls for replacement of the HEPA filter or, when appropriate, repair in a limited area. A subsequent confirmatory retest should be performed in the area of any repair.
HEPA filter leak testing alone is insufficient to monitor filter performance. It is important to conduct periodic monitoring of filter attributes such as uniformity of velocity across the filter (and relative to adjacent filters). Variations in velocity can cause turbulence that increases the possibility of contamination. Velocities of unidirectional air should be measured 6 inches from the filter face and at a defined distance proximal to the work surface for HEPA filters in the critical area. Velocity monitoring at suitable intervals can provide useful data on the critical area in which aseptic processing is performed. The measurements should correlate to the velocity range established at the time of in situ air pattern analysis studies. HEPA filters should be replaced when nonuniformity of air velocity across an area of the filter is detected or airflow patterns may be adversely affected.
Although contractors often provide these services, drug manufacturers are responsible for ensuring that equipment specifications, test methods, and acceptance criteria are defined, and that these essential certification activities are conducted satisfactorily.

Chemicals used in integrity testing of HEPA filters?
Dioctyl phthalate aerosols which is , Di (2-ethylhexyl) phthalate, di-sec octyl phthalate, DOP, or DEHP, is most commonly used used in the test for integrity of high efficiency particulate air (HEPA) filters as there are potential health effects associated with its use on the people working with DOP test aerosols which has led to a find a newer and a safer equivalent material to replace DOP.
The next material of choice is a polyalphaolefin (POA) in the 4 centistoke (4 cSt) viscosity grade called as Emery 3004 (POA) which can replace DOP in HEPA integrity testing.

What is meant terms Efficiency, Penetration,Most Penetrating Particle Size (MPPS), for a HEPA filter

Penetration :
Penetration is the ratio of the particle count downstream of the filter to the particle count upstream.

Efficiency of a HEPA filter:
Efficiency is the ratio of the number of particles retained by the filter to the number of the particles used to challenge the filter.

Most Penetrating Particle Size (MPPS): It is a characteristic of a given filter at with this particle size filter has minimum efficacy.
Classification of HEPA filters EU norms EN 1822-1 by their retention at MPPS
H10 > 85 % (Total Retention)
H11 > 80%(Total Retention)
H12 > 99.5 % (Total Retention)
H13 >99.95 (Total Retention)
H14 >99.995% (Total Retention)

HEPA filters standards.
ISO 14644 (1-9), introduced in 2000 most of European countries follow these standards
US FED STD-209E, 1992 USA.
JACA (Japan )
AS1386 (Australia)

How to clean a HEPA filter:
HEPA filters can be cleaned with a jet of air from down stream to the upstream direction so as to dislodge the accumulated particles, and then by a suction from reverse side if required (there should be a sanitisation procedure in place for suction clenear) followed by a jet of water gently, the quality of water is important in final rinsing. And the drying in a suitable place.

Decontamination of HEPA filters:
It is important to decontaminate HEPA filter when it is used in a clean room which handles vaccines , or a viruses ,bacteria.
HEPA must be decontaminated once it off line , it is decontaminated along with housing filters.
Decontamination procedure and chemicals are required to be chosen after consideration of microbial load and their resistance factors; the entire process must be validated. Integrity of a HEPA filter should be verified when it is being replaced again. Person carrying out the activity of decontamination of HEPA filter should be trained and provided a protective or a breathing apparatus so as to protect him from harmful affect of decontaminating chemicals like formaldehyde or H2O2 fumes. When HEPA filters are required to be disposed, they must be incinerated before disposal so as to avoid contamination of surrounding.