High-efficiency particulate air (HEPA) is a type of air filter. Filters meeting the HEPA standard have many applications, including use in medical facilities, automobiles, aircraft, and homes. The filter must satisfy certain standards of efficiency. To qualify as HEPA by U.S. government standards, an air filter must remove 99.97 percent of all particles greater than 0.3 micrometer (μm) from the air that passes through. A filter thais qualified as HEPA is also subject to interior classifications.
HEPA filters are composed of a mat of randomly arranged fibers. The fibers are typically composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers. Key factors affecting function are fiber diameter, filter thickness, and face velocity. The air space between HEPA filter fibers is much greater than 0.3 μm.
The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Unlike membrane filters at this pore size, where particles as wide as the largest opening or distance between fibers cannot pass in between them at all, HEPA filters are designed to target much smaller pollutants and particles.
These particles are trapped through a combination of the following three mechanisms:
Interception, where particles following a line of flow in the air stream come within one radius of a fiber and adhere to it
Impaction, where larger particles are unable to avoid fibers by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fiber separation and higher air flow velocity
Diffusion, an enhancing mechanism that is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 µm in diameter, which are thereby impeded and delayed in their path through the filter; this behavior is similar to Brownian motion and raises the probability that a particle will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities
Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the most penetrating particle size, 0.3 μm, diffusion and interception are comparatively inefficient. Because this is the weakest point in the filter’s performance, the HEPA specifications use the retention of these particles to classify the filter.
Lastly, it is important to note that HEPA filters are designed to effectively arrest very fine particles, but they do not filter out gasses and odor molecules. Circumstances requiring filtration of volatile organic compounds, chemical vapors, cigarette, pet, and/or flatulence odors call for the use of an activated carbon (charcoal) filter instead of, or in addition to, a HEPA filter.