HEPA (high-efficiency particulate arrestance) filters are in a premium-quality class of filtration. To meet HEPA standards a filter typically must remove at least 99.97% of particles that have a size of 0.3 um. HEPA filters are usually used for commercial, industrial, and healthcare applications. However, a HEPA filter can be used in a portable air cleaner for people who suffer from allergies and respiratory issues.
Many people will call high efficiency pleated Furnace/AC filters HEPA filters, however they are not true HEPA filters by definition. MERV 13 efficiency and higher filters can begin to come close to HEPA standards, but even the highest efficiency MERV 16 filter will on achieve 95% efficiency for particle sizes 0.3 - 1 um.