The NAFA Guide to Air Filtration (1993) describes ULPA and SULPA filters.
As early as 1961, a filter with a higher efficiency than a HEPA filter was offered. It had a DOP efficiency of 99.999% and the 12 in. (304.8 mm.) deep version had a clean pressure drop of 1.1 in. w.g. (273.6 Pa) when operating at a face velocity of 250 fpm (1.27 m/s). This filter has helped meet the requirement for cleaner air in facilities needed for the manufacture of microelectronics. It is identified by the generic name ULPA (Ultra Low Penetration Air) filter.
It was previously noted that the most-penetrating size particle for a HEPA filter was calculated to be 0.3 microns. The development of the laser photometer dust particle counter has provided evidence that the cost-penetrating size is less than this and depends not only on the filter media but also on the velocity of air through it. For this reason, the new IES Recommended Practice for ULPA filter media does not reference a specific particle size. Instead, by inference from the definition of the media, an ULPA filter is one that has a minimum efficiency of 99.999% for particles in the most penetrating particle size at the specified media velocity. The most penetrating particle size is defined as that particle diameter for which penetration through the medium is a maximum.
SULPA (Super ULPA) filters are available where maximum cleanliness is required. These filters have an efficiency of 99.9999% on the same basis as ULPA filters. The low penetration expected from ULPA and SULPA filters is such that they must be totally free of even the smallest leak. The standardization of testing methods in both the United States and Europe for ULPA and SULPA filters is now in the process of being finalized. Until it is, the individual user requiring filters at this efficiency level must be certain that the tests performed on filters demonstrate their ability to meet the user-defined needs.