A HEPA (high efficiency particulate air) filter is an air filter that has been tested and found to comply with a specific U.S. military standard. Filters rated as HEPA can remove particles as small as 0.3 microns with 99.97 percent efficiency. Filters rated HEPA may still be ineffective at actually cleaning air in a room, so some consumer information refers to "true HEPA" filters, although this is not a technical term.
Some filters for HVAC systems are marketed as "HEPA-like" or just "high-efficiency" because they have a higher efficiency, or handle smaller particles, than ordinary panel filters. These more expensive filters should not be mistaken for true HEPA filters. Before installing a HEPA filter screen on most existing HVAC systems, the system would require an upgrade, because HEPA causes air to flow differently than it does in conventional filters. A "HEPA-like" filter is pleated like a true HEPA filter in order to increase its surface area, which reduces air resistance and makes it possible for the higher-efficiency filter to be used without upgrading the HVAC system. These high-efficiency filters do work better than panel filters, but not as well as true HEPA filters.
Efficiency and Effectiveness
In air purification, efficiency and effectiveness measure different things. A highly efficient filter may be ineffective at purifying air in a given setting. "Efficiency" describes the percentage of particles a device removes from the air that passes through it. True HEPA filters are tested for efficiency by being fed a cloud of 0.3 micron particles. The density of the particles is measured before and after passing through the filter, and the efficiency is calculated from these values. Effectiveness, on the other hand, refers to how well a device cleans the air in a room. A filter that is too small or otherwise inadequate may be ineffective in a situation, even if it has high efficiency, whether true HEPA or HEPA-like.
A filter can only clean the air that passes through the filter media. The media, which in HEPA filters looks like coarse paper, provides significant resistance to air passing through it, so air will also pass through any part of the filter that is not airtight. This leaked air bypasses the filter media and is not cleaned at all. A true HEPA filter must not only have a filter that passes the HEPA standard, it must also be free of leaks. A common point of leakage is the frame around the filter itself, but leaks may also be present along any passageway leading to the filter in a device like a portable air purifier or vacuum cleaner.