March 2nd, 2023 | Posted in Air Cleaners
We read a lot about HEPA filtration when dealing with indoor air quality concerns. That leads one to ask: “What’s so special about HEPA?” High Efficiency Particulate Air (HEPA) filters (also known as H13 or H14 filters in many countries) are media-type filters with an extraordinarily high filtration efficiency. By definition, new HEPA filters can remove airborne particles of 0.3 microns in diameter with 99.97% efficiency. The 0.3 micron criterion is selected because that particle size is the most difficult to capture with media-type filtration.
There are several mechanisms by which media filtration captures airborne particles and the smaller the particle to be captured, the “tighter” the filter. This works well until the particles become so small (about 0.3 microns) that they can pass between the fibers of even a very tight filter. The 0.3 micron particle diameter, referred to as “the most penetrating particle size (MPPS),” is so small that ordinary filtration mechanisms such as interception and impaction are ineffective at capturing them, These very small particles adhere to the filter fibers due to electrostatic and molecular forces before they pass through the filter medium.
In order to ensure that the HEPA filter medium can perform as expected, these filter media are laboratory tested and are sometimes marketed as “True HEPA.” Purchasers of HEPA filtration should be aware that so-called “HEPA-type”, HEPA- like”, or HEPA-style” filters are likely not laboratory tested and may not achieve the filtration required.
Because of the higher manufacturing costs and the cost of laboratory certification, HEPA filters are more expensive than less efficient filters. HEPA filters also tend to become overloaded or “clog up” when faced with significant particle concentrations. In view of their replacement costs, HEPA filters in air cleaning devices are used in conjunction with a pre-filter. A 95% ASHRAE dust spot filter is a commonly used pre-filter. Changing these pre-filters out on a regular basis is a good way to extend the life of the HEPA filter. That’s because when the pre-filter gets sufficiently loaded, the particle-laden air may pass around the pre-filter and then encounter the HEPA filter and deposit all of the particles in the HEPA filter, clogging it up as well. Regular change-outs of the pre-filter obviates this concern and extends HEPA filter life.
One should be aware that that there is more to a HEPA application than simply specifying the HEPA filter. In order to be effective and avoid having the particle laden air pass around the HEPA filter, the air cleaning unit must be designed for a tight seal between the HEPA filter and the unit cabinet. In the absence of an effective seal between the cabinet and the HEPA filter, the particle laden air will pass around the filter, causing the air cleaning device to become ineffective. So bear in mind that in order to get HEPA performance from the filter, it is necessary to have a cabinet designed for that level of filtration efficiency.
It is also important to recognize that it is possible to render HEPA filtration ineffective if too much air is passed through the filter medium. The higher air velocities associated with excessive air flow will interfere with the ability of the electrostatic and molecular forces that cause the 0.3 micron particles (and smaller) to cling to the filter fibers. For a 2’x 2’ filter module, 1,000 cubic feet per minute is a commonly used flow rate.
Will HEPA filters stop gases and odors? No. HEPA filters are ineffective against gas-phase airborne contaminants. Activated charcoal or potassium permanganate sorbent bed modules are used to remove gaseous contaminants.
An often-asked question is: “Can HEPA filters collect virus and bacteria?” The size of SARS-CoV-2 ranges from 0.07 microns to 0.09 micronsi. Bacterium generally range from 1 to 3 micronsii. These size ranges indicate that HEPA filters would be effective against bacteria and virus particles.
Several manufacturers offer HEPA units. Air Quality Engineering is among them and it would be prudent to contact your Air-Quality Engineering applications specialist for professional guidance before ordering a HEPA unit.
i Byung Uk Lee Minimum Sizes of Respiratory Particles Carrying SARS-CoV-2 and the Possibility of Aerosol Generation Int J Environ Res Public Health. 2020 Oct; 17(19):