There are two reasons why "high efficiency" vacuum cleaners are not considered "HEPA" vacuums.

1. Not all vacuums marketed as "high-efficiency" contain a true HEPA filter.
2. Not all vacuums that incorporate a true HEPA filter actually filter at true HEPA levels.

Not all vacuums marketed as "high-efficiency" contain a true HEPA filter.
A HEPA (High Efficiency Particulate Air) vacuum contains a filter that consistently filters out at least 99.97% of the airborne particles that are 0.3 microns in size or larger. These filters were originally developed during WWII to protect atomic industry workers from radioactivity carried on extremely fine dust particles. HEPA vacuum cleaners have become very popular in recent years, due to increasing (and justified) public concern about whether vacuum cleaners adequately filter out allergens and other particles.

One consequence of this popularity is that some manufacturers now market vacuum cleaners using such terms as HEPA-like, HEPA-type, high-efficiency or with other wording to indicate that their filters are more efficient than older-style vacuums. Some consumers jump to the conclusion that all "high efficiency" filters or vacuums operate at true HEPA filtration levels. This is not true.

Some filters marketed this way provide significantly higher level of filtration than standard filters, especially for larger particles. Depending on the intended use of the vacuum cleaner, this level of filtration may be entirely adequate. However, for situations where a true HEPA vacuum is needed (e.g. performing lead, asbestos or mold remediation) only a vacuum cleaner certified by the manufacturer as providing HEPA filtration should be used.

Not all vacuums that incorporate a HEPA filter actually filter at true HEPA levels.
A lot of confusion with regards to filtration efficiency is a result of not understanding the difference between the efficiency of a filter and the efficiency of a machine that incorporates the filter.

For purposes of illustration, let's assume a vacuum system has a filter that is 100% efficient at removing particles .3 microns and larger from the airstream. A logical assumption would be that the vacuum cleaner as a whole is 100% efficient at .3 microns. However, what happens if only 50% of the air going through the machine actually passes through the filter? This can happen due to internal design, leakage around seams, seals and gaskets, etc. In this case, the machine is removing only 50% of the particles of this size (50% efficiency), despite the fact that it contains a 100% efficient filter.

Assume the following (all numbers are theoretical):

• A vacuum system with airflow of 100 cfm contains a true HEPA filter which removes 99.97% of the particles 0.3 micrometers and larger that pass through it.
• Air being pulled through this system averages 100,000 particles in this size range per cubic foot. (This is unusually clean air.)
• Therefore 10,000,000 particles pass through the machine each minute.

Percentage of air passing through filter	Percent filtration efficiency of vacuum system	Particles per minute in exhaust air
100.0	99.97	3,000
99.9	99.87	13,000
99.5	99.47	53,000
99.0	98.97	103,000
95.0	94.97	503,000
90.0	89.97	1,003,000

As the table illustrates, unless the vacuum cleaner is designed and built so that it is fully sealed (100% of the air pulled in through the nozzle actually passes through all filters, especially the HEPA filter), theoretical efficiency of its filters is somewhat irrelevant. Unfortunately, building a fully sealed machine is both difficult and expensive. It requires precision in manufacturing, and expensive attention to details of seals and gaskets.

Additional thoughts about HEPA vacuums
HEPA vacuums (as defined above) tend to be significantly more expensive than non-HEPA units, even those that are "high-efficiency." The HEPA filters themselves are also quite expensive. The cost for some HEPA filters can exceed several hundred dollars. For this reason, most HEPA vacuums contain a number of prefilters to catch the larger particles that can help to extend the life of the HEPA filter. All of these filters must be regularly maintained in order to function properly. You should expect maintenance of a HEPA vac to require more time and attention than a more "standard" vacuum.

A laser particle counter can be used to measure the size and number of particles in the exhaust of a HEPA vacuum. This allows the user to monitor whether the machine and its filters are functioning properly.

The HEPA filter may be located either in front of or behind the vacuum motor. Either system works, when properly engineered. One advantage of locating the filter behind the motor is that the HEPA filter will catch the small carbon and other particles given off by the motor. This can make the use a laser particle counter more efficient when attempting to determine whether the unit is filtering properly.

Any vacuum that is not initially designed to provide HEPA filtration is unlikely to achieve true HEPA performance. Some manufacturers produce vacuums that they claim filter at HEPA levels, but without using an actual HEPA filter. Instead, they use a series of bags and filters in a fully-sealed system, designed so that the system as a whole performs at HEPA filtration levels.

Wet-dry HEPA vacuums are also available. They are usually more expensive than HEPA dry-vacuums, and care must be taken to ensure that the HEPA filter remains thoroughly dry while not in use. Many HEPA filters are manufactured from materials that are excellent food sources for mold when there is enough moisture present.

Most HEPA vacuums are a tank or canister type. They are extremely versatile, and are probably the type most common used by cleaners. Some may have a motorized brush attachment that allows them to be used for vacuuming carpet. Upright vacuum cleaners that filter at HEPA levels are also available.

European vacuums may list an S-class filter, which is equivalent to a HEPA filter.

ULPA filters and vacuums
There is an even higher level of filtration available, ULPA (99.999% efficient at .12 micrometers). This level of filtration is widely used in pharmaceutical labs, electronics cleanrooms, and in other situations where the highest possible level of air cleanliness must be maintained. ULPA filters and vacuums are considerably more expensive, clog faster and are seldom necessary for abatement or remediation projects. A few HEPA vacuums can be converted to an ULPA vacuum simply by replacing the HEPA filter with an ULPA filter.

— Jim Holland