Paraphrased from an article by: Jeff Fystrom, Nobles Inc. - Manufacturers of commercial cleaning equipment.

         The Truth about HEPA Filtration

         HEPA stands for High Efficiency Particulate Air (Filter).
         HEPA filters and vacuums are rated in terms of what percentage of particles
          are filtered and what size the particles are. 

         HEPA specifications that read "99.97% @ .3 microns" means that the filter will stop 
         99.97% of particles as small as .3 microns.         

        There is much misunderstanding in our industry of what HEPA filtration
          really means. Often I am asked, "is this vacuum HEPA?" The next
          few paragraphs will answer the following questions:
         · When can a filter be called a "HEPA" filter? 

         · What is the customer looking for when they ask for HEPA filtration? 
         · What is the difference between a HEPA filter in a vacuum, and
           a HEPA vacuum?
         · What our industry is not saying about filtration specifications?

         First let's start with a review of some basic terminology.

         When can a filter be called a "HEPA" filter?

         To be classified as "HEPA", a filter must stop particles as small as
          .3 microns. The lowest HEPA class is H10, with 85% of particles being
          stopped. The highest HEPA filter class is H14, which must filter 99.995%
          of particles. Therefore, HEPA is actually a rating scale that identifies a 
          filtration level. All the classes need to filter particles as small as .3 microns,
          but the difference between the classes is what percentage of particles
          the filter actually stops.

         The entire filter rating scale is listed below.

                                       Degree of Separation   
         Type   Filter Class      (%) @ 0.3 Micron
         HEPA      H 10               85
         HEPA      H 11               95
         HEPA      H 12               99.5
         HEPA      H 13               99.95
         HEPA      H 14               99.995

What is the customer looking for when they ask for HEPA filtration?

         In the vacuum industry when the word HEPA is used, it actually refers to a certain
          level of HEPA filtration. Do the following filtration levels of 99.97%
          at .1 microns, or 99.97% at .3 microns sound familiar?

         A very often-asked question is whether or not the filter in a vacuum is
          a HEPA filter. As long as 85% of the particles .3 microns or smaller,
          are stopped by the filter, it's technically a HEPA filter.
         Most customers say they want a HEPA filter in their vacuum, and assume
          that means a 99.99% efficiency rate for particles as small as .1 or .3 microns. 
          However by only asking for HEPA filter, it could have an efficiency rate as low as
          85% at .3 microns.

What is the difference between a HEPA filter in a vacuum, and a HEPA vacuum?

         A true HEPA vacuum must have NO leakage of dust around the filter. 100%
          of all working air, as well as motor cooling air, must pass through a
          HEPA filter. A HEPA vacuum must be designed properly so the cubic feet
          per minute (CFM) of airflow, does not exceed the capacity of the filter.
          This level of filtration and attention to leakage is very expensive. There
          are true HEPA vacuums on the market today, they are sometimes called critical
          air vacuums, and may cost thousands of dollars.

         A vacuum cleaner with a HEPA filter is just what it says - the vacuum has
          a HEPA grade filter in it.  But the machine may not guarantee that 100% 
          all the working and cooling air goes through the filters.  There may not be
          a HEPA filter physically located after the exhaust port.

         An incorrect assumption is made that by simply running the air
          through HEPA filters, it will make the vacuum a HEPA vacuum.
         Most vacuums we deal with simply install HEPA quality filters in machines.
          Although many of these machines have very good filtration, they are vacuums
          with HEPA filters, NOT HEPA VACUUMS.

         In most cases vacuum specifications tell you what the filtration level
          is for the filters and not the vacuum. The best way to determine the filtration
          efficiency of a vacuum is to test the entire machine. Many manufactures
          are testing entire machines but most of the results are not published.

         The most visual program to test the filtration rate of vacuums is at the 
          Carpet and Rug Institute. The Carpet and Rug Institute (CRI) has a program 
          in which vacuums are tested in a clean room to see how much particulate they
          release back into the air. The program is called Green Label, and vacuums  
          that properly filter the exhaust air are listed on their web site

What our industry is not telling you about filtration specifications?
         Vacuum manufactures are in a specification battle, attempting to make
          their vacuum filtration specification look better than the competition.
          Machines are put into the market that advertise HEPA filters with
          higher and higher levels of filtration. While all the while, not mentioning          
          that the specifications for the filter do not translate directly to the         
          filtration of the machine.

         We want vacuums that pick up dust, soil, and debris from the carpeting,
          deposit it in a collector bag and then keep it inside the machine. In 
          other words, we want the filters to stop the dust, soil and debris from
          getting back into the air we breathe. It takes more that simply putting
          a HEPA filter in a vacuum to stop dust and debris from being released
          back into the air. 

         The bottom line is that we want vacuums to keep the dirt,
          soil and dust inside the machine and not allow it to be distributed into 
          the air we breath. The best way to determine the filtration level of a 
          vacuum is to test the entire vacuum and not just the filters, whether
          they are HEPA or not.