HEPA Filters

General

If you do a search on the Internet for HEPA filters, you will find many references to Certified HEPA filters, True HEPA filters, “HEPA like” and “HEPA type” filters.  You will also see many claims made as to their efficiency, their effectiveness and why you should or should not use them.

 Companies that sell air purification products that do not use HEPA filters usually are either not well educated as to what particle size a HEPA actually filters or they take information out of context when describing it.  Sometimes it would appear that they simply do not tell the truth, period.

 The majority of sites (usually an equipment or product manufacturer) also claim that a HEPA filter will not filter out viruses because a HEPA is 99.97% (or greater) efficient at 0.3 microns and viruses are usually smaller than 0.3 microns.  Viruses are usually less that 0.3 microns, but a true HEPA filter is efficient at removing them from the air stream through a process called “Brownian diffusion”.

 No government agency sets filter efficiency standards – the filter companies do, but, that’s another topic.  The exception is with HEPA filters.  The Atomic Energy Commission (AEC – now known as the Nuclear Regulatory Commission – NRC) needed to know what the worst case particulate flow-through was for radioactive iodine particles in HEPA filters. This new type of filter was to be used in the production of atomic bombs and these particles could contaminate workers and could be released to the outside environment through waste water.  (See the “History” section below)

 People that sell ozone generators like to point out that HEPA filters promote microbial growth.  They lump spun fiberglass (like a disposable home furnace filter) and HEPA filters (molecularly bonded fiberglass) in the same group.  By itself, fiberglass does not promote microbial growth but growth could occur on the foreign particles captured on the filter media.  HEPA filters are used to remove small particulates and pathogenic material.  Provided the HEPA has the proper prefiltration, microbial growth will rarely occur.

 Some points to remember:

q       “HEPA type” or “HEPA like” filters are not true HEPA filters and should not be used in place of a true or certified HEPA.

q       A “true HEPA” filter will be rated at => 99.97% efficient at 0.3 microns minimum.

q       A “certified HEPA” is a true HEPA that has been individually tested by the manufacturer and meets the 99.97% (or greater depending on the type) efficiency at the 0.3 microns standard (suitable for healthcare use).

 The History of HEPA Filters

The first HEPA filter (high efficiency particulate air) was designed in the 1940's by the research and development firm, Arthur D. Little, under a classified government contract as part of the Manhattan Project, where the first atomic bomb was developed during World War II.  A major advancement in air filtration technology, the filter solved a critical need to control very small particles, which had become contaminated by nuclear radioactive sources.

Considering the condensation nuclei of radioactive iodine to be most harmful, researchers focused on the ability to capture solid particles that were created through the condensation of gases and liquid aerosols into solid matter.  Having identified 0.3 micron size particles as the most penetrating particle size (mpps) and representative of the particle of concern, 0.3 microns was established as the particle size fraction at which to determine filter efficiency performance.

Because of its effectiveness, the filter was originally referred to as an "absolute filter." The generic acronym HEPA came into use some time following 1950 when the filter was commercialized and the original term became a registered trade name.

Over the next 50 years, HEPA filtration gradually evolved as technological breakthroughs in aerospace, pharmaceutical processing, photographic film manufacturing, data processing and micro-circuitry demanded higher and higher levels of air cleanliness. If not for HEPA filtration, such milestones as the lunar landing and the introduction of the silicon chip might not have been achieved and adequate control of hazardous and toxic particulate would not have been possible.

What is a HEPA filter?

Filters are generally constructed of a 0.013 inch thick glass fiber material bonded at the molecular level.   Required minimum HEPA standards include 99.97% (or greater) capture of 0.3 micrometer diameter DOP aerosol particles.  A True HEPA's D.O.P.-rated operating efficiency is also the MINIMUM STARTING efficiency. The actual operating efficiency is higher for all other particle sizes (0.001 micron and up).

The usual standard for measuring air cleaner efficiency (and for healthcare purposes) is 0.3 microns, which is the cleanroom standard D.O.P. test (Mil-Std 282 Dioctylphthalate test, or the Potassium Chloride [KCL] test).  So a True HEPA filter has the highest proven efficiency of 99.97% or greater at 0.3 microns by this cleanroom test.

The starting efficiency of any non-HEPA air cleaner is very much lower – anywhere from 5% to 60% by the D.O.P. test at 0.3 microns.  Because all non-HEPA efficiencies drop with usage, their real operating efficiencies are even lower - probably between 1% to 15% within hours or days.  HEPA filters are far more efficient for air cleaning than other types of filtration systems provided that  efficient pre-filtering is in use.

HEPA and ULPA (Ultra Low Particulate Air) filters can be rated in varying degrees of efficiency.  Some of the most common are:  90.00%, 95.00%, 97.00%, 99.97%, 99.995 and 99.9995%.  There are other efficiencies, but these are the most common.  The current MERV ratings for these filters will be from a MERV 15 to a MERV 20.

Interesting Statistics and Comparisons

Where do particles come from?

(Human beings are the main culprits))

·         A person sitting or stopped generates about 100,000 particles/ft³.

·         Sitting down or standing up generates about 2,500,000 particles/ft³.

·         Walking generates about 10,000,000 particles/ft³.

·         Horseplay generates about 30,000,000 particles/ft³.

·         Grinding, sweeping, welding adds billions of particles/ft³.

·         Two surfaces rubbing generate billions of particles/ft³.

·         Process equipment adds particles

·         Process materials add particles

·         Maintenance activity adds particles

·         Construction residue can generate massive particles throughout the life of a facility.

What’s a Micron? (1/1,000,000 of a meter)

Examples of some common air contaminants and their size in microns:

·         Human Hair ......................................... (70 - 100 microns)

·         Human Sneeze (droplet nuclei).......... (10 - 100 microns)

·         Pet Dander .......................................... (0.5 - 100 microns)

·         Pollen .................................................. (5 - 100 microns)

·         Spores from Plants ............................ (6 - 100 microns)

·         Mold...................................................... (2 - 20 microns)

·         Smoke ................................................. (.01 - 1 micron)

·         Dust Mite Debris ................................ (0.5 - 50 microns)

·         Household Dust ............................….. (.05 - 100 microns)

·         Skin Flakes ......................................... (0.4 - 10 microns)

·         Bacteria................................................ (0.35 - 10 microns)

·         Viruses ………………………………….. (.01 - .5 microns)

The information in this document has been derived from various manufacturers, educational institutions and U. S. Government agencies.