Many air purifiers use High Efficiency Particulate Air filters, also known as HEPA filters, as their main filtration systems. The air purifiers that use HEPA filters reduce the amount of allergens, dust, and microscopic particles in the air. HEPA filters are known for their efficiency at capturing these airborne particles. A true HEPA air purifier is typically rated to remove 99.97% of airborne particles.
HEPA filters were invented to be used in air purifiers. Scientists in the Manhattan Project created the HEPA filtration system during the 1940s to remove radioactive particles from the air in labs. The filters have been improved to some degree over the years and are now used in purifiers in clean rooms, hospitals, and airplanes as well as homes and businesses.
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There are two levels of HEPA filtration in air purifiers. First is the true HEPA or absolute HEPA level, which catches 99.97% of all particles that are 0.03 micron or larger. In the United States, these filters must go through a testing process to determine if they have this effectiveness level in order to be certified as a true HEPA filter. True HEPA filters typically have a serial number signifying that they passed the testing process.
The second effectiveness level of a HEPA filter is the HEPA like or HEPA type filter. These either have not gone through the true HEPA testing or have not met its stringent requirements. These filters are typically 85% to 90% effective in removing air particles.
The filter in a HEPA air purifier is typically an accordion-folded sheet of HEPA quality filtration material. There are three main ways that this material in a HEPA air purifier catches particles. First, the larger particles are trapped, in much the same way that a strainer traps tea leaves but allows the liquid tea to pass through. This filtration process is called interception and it is common among most air filtration systems.
Unlike many other filters, HEPA filters do not simply trap particles larger than the filter’s smallest holes. The second manner of filtration in a HEPA air purifier is called impaction. Impaction uses the circuitous air flow in a HEPA filter to propel particles into the fibers. As air passes through the fibrous sheet, it shifts and moves to find its way. Particles follow the current and are often trapped in the process.
The third filtration process utilized by a HEPA air purifier, called diffusion, catches the smallest particles, mostly those at the 0.1 micron size. These particles are so small that they interact with the air currents on a molecular level. They are caught as they run into molecules of air, bounce off, and hit the filter fibers.