The original HEPA filter was designed in the 1940's to retain airborne radioactive particles. Today they are widely used to clean the air by retaining bacterial and viral organisms. By preventing the spread of these organisms, it also holds true that the possibility of illness may also be significantly reduced. HEPA filters are designed to retain at least 99.97% of particulates 0.3 microns and larger.
HEPA filters work in one of three ways:
- Interception, where particles following a line of flow in the airstream come within one radius of a fibre and adhere to it.
- Impaction, where larger particles are unable to avoid fibres by following the curving contours of the airstream and are forced to embed in one of them directly; this increases with diminishing fibre separation and higher air flow velocity.
- Diffusion, an enhancing mechanism 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.