A laminar air flow cabinet provides a sterile, particle-free working environment by passing a unidirectional airflow through a HEPA (High Efficiency Particulate Air) filter. Where a particularly high level of purity is required, an ULPA (Ultra Low Penetration Air) filter may be used.
The CEN (Comite European de Normalization) have drawn up a European standard for HEPA and ULPA filters (EN 1822-1:1998), in which filters are classified according to the Most Penetrating Particle Size, or MPPS value.
What the EN 1822 standard defines
The standard was developed specifically for high performance filtration systems, such as those used in cleanrooms and biosafety cabinets, and defines a number of parameters, which can be remembered by the acronym PESOLL:
Penetration: Calculated as a ratio of the number of particles flowing towards the filter (upstream) to the number flowing away from it (downstream).
Efficiency: The number of airborne particles trapped by the filter, compared to the number passing through it.
Superficial, or Useable Face Area: The area of the filter element through which the airflow passes, seen in cross-section.
Overall Efficiency: An average of the number of particles penetrating the Useable Face Area, under given, or controlled operating conditions.
Local Penetration: The efficiency of the Useable Face Area at a specific point, under controlled operating conditions.
Leak Threshold: The Local Penetration value which is equal to, or greater than the overall penetration efficiency of the filter.
EU classification of HEPA and ULPA filters
Using the above parameters, filters are divided into eight classes (H10 – 14 and U15 – 17), according to their overall and local percentage values of efficiency and penetration.
An H10 filter has 85% overall efficiency and 15% overall penetration (no local values specified). By comparison, a U17 ULPA filter has practically 100% overall and local efficiency, with penetration values of 0.00005% and 0.000025% respectively. This type of filter would most likely be installed in a laminar flow cabinet used for the semi-conductor industry.