Strictly speaking, there are two kinds of High Efficiency Particulate Air filters. Those rated H14 are most common, stopping 99.97 percent of all contaminants 0.3 microns in size or larger. H13 filters are not as efficient, only stopping 99.7 percent of similar contaminants.
In addition to the two HEPA designations, there are three Ultra Low Penetration Air filter standards. U15 stops 99.997 percent of contaminants, U16 stops 99.9997 percent and U17 stops 99.9999 percent. However, these ULPA filters require a sealed air delivery system to reach this level of efficiency, and are therefore generally only used in laboratories or hospitals in applications that require absolute air purity. The same standard that defines HEPA and ULPA also defines three classes of Efficiency Particulate Air filters, beginning at E10 with 85 percent efficiency.
In addition to true HEPA filters, many different filters on the market are advertised as "HEPA-type." These are usually more vague in their claims, however, and may not specify the size of contaminant filtered out of the air. Industry testing usually reveals that filters without the HEPA certification range in effectiveness from 25 to 95 percent.
HEPA filters require maintenance to remain at full effectiveness. They must be changed regularly according to the manufacturer's standards, as dust and particles can block the filter and reduce its ability to clean the air. Charcoal pre-filters may help lengthen the lifespan of a HEPA filter by removing larger particles that may block filtration media.