H-E-P-A or H-Y-P-E

Which Way Should You Spell It?

by Grant Olewiler, M.D. Manufacturing

HEPA, according to the U.S. Department of Energy, is an acronym for “High Efficiency Particulate Air” [filter]. Wikipedia expands that it can “theoretically remove at least 99.97 percent of dust, pollen, mold, bacteria, and airborne particles with a size of 0.3 µm (micrometers) at 85 liters per minute.” (3 CFM)

To give you an idea of the size of a micron, it takes 24,500 microns single file to equal one inch (2.54 cm). Conversely, 4/10,000ths of an inch equals one micron. In metric terms, a single micron is one millionth of a meter. A particle of 10 microns is invisible to the naked eye. Pollen ranges between 5-100 microns. Human hair between 70-100 microns.1

So much has been written on HEPA that it gets exhausting analyzing all the data. Most consumers are only aware of claims made by vacuum cleaners that a particular vacuum “uses HEPA filtration.” So what’s behind the science and how does it relate to our industry?

The original HEPA filter was designed in the 1940s and was used in the Manhattan Project to prevent the spread of airborne radioactive contaminants. It was commercialized in the 1950s, and the original term became a registered trademark and a generic term for highly efficient filters. Over the decades filters have evolved to satisfy the higher and higher demands for air quality in various high technology industries, such as aerospace, pharmaceutical processing, hospitals, healthcare, nuclear fuels, nuclear power, and electronic microcircuitry (computer chips).

HEPA filters are composed of a mat of randomly arranged fibers. Key metrics affecting function are fiber density and diameter, and filter thickness. The air space between HEPA filter fibers is much greater than 0.3 µm. The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Just as for membrane filters, particles so large that they are as wide as the largest opening or distance between fibers can not pass in between them at all. But HEPA filters are designed to target much smaller pollutants, and particles are mainly trapped (they stick to a fiber) by one of the following three mechanisms:

1. Interception, where particles following a line of flow in the airstream come within one radius of a fiber and adhere to it.
2. Impaction, where larger particles are unable to avoid fibers by following the curving contours of the airstream and are forced to embed in one of them directly; this increases with diminishing fiber separation and higher air flow velocity.
3. Diffusion, an enhancing mechanism which 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; this behavior is similar to Brownian motion and raises the probability that a particle will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities.

Diffusion predominates below the 0.1 µm diameter particle size. Impaction and interception predominate above 0.4 µm. In between, near the 0.3 µm, diffusion and interception predominate.2

According to Health and Safety News, AIC News, January 2002, “When selecting a vacuum, make sure you get true HEPA or genuine HEPA filtration. Avoid lesser grade look-a-like systems with names like hospital-grade HEPA, HEPA-like, or Gore-Tex microfilters. These are cheaper but in spite of similar looking ratings, they are not acceptable for conservation work. It is also important that the vacuum be designed and constructed so that all the air taken in is passed through the HEPA filter before it is exhausted. This is sometimes referred to as a ‘HEPA sealed system’.”

A number of recent articles have referred to “HEPA filter bags.” “Even if true HEPA collection bags exist, which is doubtful, they would not be acceptable to use to clean up hazardous materials. Were the bag to rip, the contaminants would spew out of the vacuum creating an extremely hazardous environment.”

Before using a HEPA vacuum, be sure to ask how to change the collection bag and maintain the intermediate filters and HEPA filter in a way that will not release contaminants. The act of opening the machine and removing the bag may create a dust cloud of exposure. Some manufacturers (Nilfisk, for example) have step-by-step instructions for changing bags and filters inside a glove bag. Others may want you to send the unit back to a service center. If you have access to local exhaust ventilation or a large lab hood, you may want to consider using this as your change area for smaller vacuum units.3

Does all this confuse you? It should. Basically portable vacuums are using the term HEPA to convince their consumers that their vacuum will be healthier to use than other vacuums. But a central vacuum circumvents the entire issue. By exhausting all germ-laden air outside the living area, there is a minimal concern for the vacuumed air contaminating the household. Where M.D. Mfg. is concerned is when manufacturers claim that their product is providing a benefit that they are not.

In our own quest to receive a “HEPA” rating on our CartMaster vacuum for hospital applications we learned something very interesting. If the initial tests of the filter do not produce acceptable results, a very simple way to increase the filter material efficiency is to reduce the speed of the air passing through it. Effectively, if you have a very small volume of air passing through a large filter, you will have better filtration. So many of the “HEPA” ratings out on the market are related to a material being tested but not at a realistic volume of air to simulate actual operational usage. The product passed the test but does not really do what the user intends it to do.

ANSI/ASME N509 section 4.3 states this: “Size (Installed Capacity) of Air Cleaning Units -- The installed capacity (cfm) of the air cleaning unit shall be no greater than the limiting installed capacity of any bank of components contained in the air cleaning unit through which the airflow must pass.”4 So the airflow of the vacuum should not be able to exceed the capacity of the rate at which it passed the test. Rarely will you ever see this figure posted by the manufacturer of the product!

The bottom line here is that central vacuum dealers need to convince their clients that HEPA is mostly a lot of HYPE. Manufacturers are using the term to market their products as healthier than they are. Oh, and by the way, our current 2-ply paper filter bag was tested at 97.3 percent efficient on .3 micron particles at a flow rate of 209 L/m2.sec.

1 http://www.wisegeek.com/what-is-a-hepa-filter.htm
2 www.Wikipedia.com Dec 11, 2006
3 Health and Safety News,” AIC News, January 2002
4 http://www.eh.doe.gov/ll/ldb/detail.CFM?Lessons__Identifier Intern=2001-RPP-HNF-IB-01-05

Reprinted from Central Vac Professional, May 2007