HEPA Filter described with references
How and Why HEPA cleans BETTER than 0.3 micron!
DEFINITION of HEPA by JSArmour
HEPA, or HEPA filter: An air filter that must remove a minimum of 99.97% of 0.3 micron (0.3 µ) particles that pass through the filter. Particles of this size are the Most Penetrating Particle Size (MPPS), which is the most difficult size to filter.
Here is a visual representation - a “graph” - that shows the “rating” is actually the lowest efficiency!
WHAT DOES THIS MEAN?
This means particles both larger and smaller than 0.3 microns will be captured at efficiencies the same or better than 99.97% efficiency, and have been shown to exceed 99.99%. Thus, a HEPA is sufficiently effective for those particles commonly referred to as ‘ultra-fine’ by the mold industry.
For respirator filters, NIOSH designates a HEPA filter as filter class “100” with three categories of resistance to filter efficiency degradation due to the presence of airborne oil droplets, i.e., N100, R100, P100. (N is not resistant, R is resistant but limited for single shift; P is oil-“proof" and can be used more than one work shift. HE is a HEPA filter with the same efficiency and degradation category as N100 but are made for PAPR only.)
Micron (µ), micrometer (µm) (syn.); one-millionth of a meter; also written variously as 0.000001 meter, 10-6 m, or 1/1,000,000th of a meter. Micron it is sometimes written using the greek letter µ (“mu”) and sometimes µm. Or µm for micrometer.
NOTE: a centimeter, cm, is only one-hundredth of a meter; millimeter, mm, is one-thousandth of a meter, m.
A micron is one thousand times SMALLER than a millimeter.
A micron is ten thousand times SMALLER than a centimeter.
A micron is one million times SMALLER than a meter (which is just a few inches longer than one yard!) - this means an American Football Field is 100 MILLION MICRONS long.
TECHNICAL and SCIENTIFIC REFERENCES for HEPA
NIOSH
“The 0.3-micrometer diameter used in the certification testing is approximately the most penetrating particle size for particulate filters. Although it seems contrary to expectation, smaller particles do not penetrate as readily as 0.3-micrometer particles. Therefore, these respirators will filter all other particle sizes at least as well as the certified efficiency level."
NIOSH Guide to the Selection and Use of Particulate Respirators Certified Under 42 CFR 84, Department of Health and Human Services (DHHS) The National Institute for Occupational Safety and Health (NIOSH) Publication No. 96-101, the Centers for Disease Control and Prevention (CDC).
NIOSH Guide to the Selection and Use of Particulate Respirators Appendix E: Commonly Asked Questions and Answers About Part 84 Respirators, The National Institute for Occupational Safety and Health (NIOSH). Centers for Disease Control and Prevention. 6 June 2014.
OSHA
OSHA Technical Manual Section VIII: Chapter 2 Respiratory Protection,
Appendix VIII: 2-1. Glossary
“High efficiency particulate air (HEPA) filter a filter that is at least 99.97% efficient in removing monodisperse particles of 0.3 micrometers in diameter and larger. The equivalent NIOSH 42 CFR part 84 particulate filters are the N100, R100, and P100 filters.
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EPA
Indoor Air Quality (IAQ) https://www.epa.gov/indoor-air-quality-iaq/what-hepa-filter
“HEPA is a type of pleated mechanical air filter. It is an acronym for "high efficiency particulate air [filter]" (as officially defined by the U.S. Dept. of Energy). This type of air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns (µm). The diameter specification of 0.3 microns corresponds to the worst case; the most penetrating particle size (MPPS). Particles that are larger or smaller are trapped with even higher efficiency.
Department Of Energy regulations
“HEPA filters…capture 99.97% of 0.3 micron particles”
U.S. Code of Federal Regulations
CFR 42 Part 84 (2020)
Biological Safety Association (professional organization behind bio-safety cabinets and other HEPA equipped safety equipment and lab clean rooms)
“HEPA filter papers acceptable for biological safety installations routinely give collection efficiencies greater than 99.99% when tested with 0.3 mm diameter DOP. By increasing the fraction of fine glass fibers that are less than 0.25 mm in diameter in the paper, it is possible to obtain efficiencies greatly in excess of 99.999% for 0.1-0.3 mm particles with a modest increase in filter resistance, typically about 25%. These papers are often referred to as ultra low penetration aerosol filters (ULPA). [From, “HEPA Filters”, 1998 Melvin First HARVARD J of Am Biological Safety Association]
Lawrence Livermore Laboratory
“The HEPA filter leak test was implemented in 1960 in the U.S. to verify that the installed filtration systems did not have leaks.(s) This test represented a second- best choice at that time since it was not possible to conduct in-place penetration tests using the available test equipment. The problem was that the particle measuring instruments at that time could not distinguish between particle sizes, and monodisperse 0.3 Lm aerosol generators were not portable. The available light , scattering photometers were portable but could not distinguish between different particle sizes. To measure HEPAfalter penetration at 0.3 ~m diameter, it was necessary to have a monodisperse 0.3 Urndiameter generator, which were not portable. [from “In-Place HEPA Filter Penetration Test” , Lawrence Livermore Lab, July15-18, 1996]
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The Capture PROCESS =====
"HEPA filters capture particles through three interrelated processes:
The first is interception. This occurs when a particle is following the flow of the air and comes within very close range of a fiber. The particle then sticks to the fiber.
The second is impaction. This occurs when larger particles hit the fibers directly, embedding into them.
The final is diffusion. As the smallest particles flow through the filter, they can collide with gas molecules. When this happens, the particles slow down or stop, which increases the chance the filter will capture them via interception or impaction.
