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Industry standards for HEPA filters in biosafety cabinets

posted on: September 7th, 2012
Every laminar flow hood depends on one or more HEPA filters, which work in combination with the laminar air flow passing through the cabinet to maintain a sterile, particle-free environment.

Strict industry standards apply to the choice of HEPA filters for horizontal and vertical laminar flow systems. This is an important issue in biosafety cabinets, where both the operator and the environment are potentially at risk from harmful particulates. Different standards have been developed for European and American systems, although in many respects they are similar.

American HEPA filter standards

HEPA (High Efficiency Particle Air) filtration is widely used in US domestic appliances, which are rated according to their MERV, or Minimum Efficiency Reporting Value. The higher the MERV rating, the more effective the product is at filtering out airborne particles. However, the filters used in biosafety cabinets must comply to standards laid down by NSF International. The best known is the NSF-49 standard, which states a Class 2 biohazard cabinet must be equipped with a Type C filter, with a 99.99% efficiency rating at 0.3 microns (μ), on both the upflow and exhaust side.

Although the Type C filter is the standard for Class 2 booths, other filter types are available. Each is classified according to its penetration and efficiency at a set particle size, according to standards set down by the Institute of Environmental Sciences and Technology, or IEST.



European Standards for HEPA filters

Filters in Europe and the UK come under the EN1822-1 standard, which like the American system classifies HEPA and ULPA filters according to their efficiency. In order to comply with the EN 12469 standard for biosafety cabinets, booths used for the handling of infectious or hazardous substances must be equipped with a class H14 HEPA filter, as defined by the EN18221 classification.

Unlike the American system, the EN1822-1 standard does not assign a specific particle size. Instead, an MPPS (Most Penetrating Particle Size) value is used. An H14 filter has 99.995% efficiency at its most penetrating particle size.

Testing filters for efficiency

In both America and Europe, the efficiency of HEPA filters is typically tested by using an aerosol generator. This forces a dense cloud of microscopic particles of a known size through the filter. At one time, DOP (Di-Octyl Phthalate) particles were used, but following concerns over possible carcinogenic properties this was replaced by another product, and renamed as Dispersed Oil Particulate. POA (Poly Alpha Olefin) is another alternative, while ULPA filters  used in laminar air flow systems for the assembly of delicate microelectronics  are tested using a PSL (polystyrene latex spheres) aerosol. By scanning the filter with a photometer or other detection device, the tester can determine both the upstream and downstream mass of the airflow particles, and therefore determine the filters efficiency at a particular testing particle size.

To determine the MPPS value, airborne particles of a known size are propelled through the pores at the same rate as that used in a laminar flow hood. A multi-channel counter equipped to read particles of 0.1 to 0.5μ in size then determines the channel through which most particles were allowed to pass, and therefore the MPPS value.
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Contained Air Solutions Ltd, Unit 4, Greengate, Middleton Junction, Manchester, M24 1RU. Tel +44(0)161 655 8860 Fax: +44(0)161 655 8865 Email: info@containedairsolutions.co.uk