The numbers in a HEPA specification indicate the filter's effectiveness at removing particles greater than 0.3 microns in size from room air. The most common effectiveness rating quoted is 99.97 percent, but there are categories of HEPA filters starting at 85 percent effectiveness.
Most HEPA filters available at retail are rated H13, which indicates 99.97 percent effectiveness. H14 filters remove 99.997 percent of contaminants, and U15, U16, and U17 filters are even more effective. These filters require a sealed air delivery system to reach this level of purity, however, and are generally not available at the retail level. E10 is the lowest classification of HEPA filter, with an 85 percent effectiveness rating.