HEPA stands for High-Efficiency Particulate Air.
HEPA Filters are designed to remove 99.97% of all particles down to 0.3 microns in size. For comparrison, pollen is 5-100 microns and human hair is 70-100 microns.
HEPA filters are composed of randomly arranged fibers on a mat-like surface that allows for more fibers to be trapped within them.
HEPA Filters use 3 methods for capturing particles:
- Impaction: Larger particles are simply unable to avoid capture by the fibers and are therefor forcefully imbedded into the filter. This action increases with higher air flow and smaller fiber seperation (the reason why some HEPA air cleaners are significantly more expensive than others).
- Interception: Particles, following the flow of air, come within close proximity to a fiber and adhere to it, thereby removing from the surrounding air.
- Diffusion: An enhancing mechanism that is the result of the impact with gas molecules by the smallest particles which are then obstructed from their course and captured in the filter. This process thus increases the chances that a particle will be stopped by either of the above mentioned methods of capture. This becomes the central process in lower air-flow velocities.
HEPA filters were originally designed in the 1940's to prevent the spread of airborne radioactive contaminants.
Over the years, they have evolved to accommodate the constant demand for better air quality in hospitals, aerospace, pharmaceutical companies, and nuclear facilities who know how critical they are in preventing the spread of bacterial and viral organisms.
The American Lung Association claims they "are the most efficient mechanical filters for removing small particles which can be breathed deep into the lungs. The highest efficiency air cleaners, widely available today, remove 0.3 micron sized particles at a minimum of 99.97% efficiency."
Examples of HEPA Air Purifiers: