HEPA (High Efficiency Particulate Air) is a technical definition that refers to a filter "that will remove not less than 99.97 percent of 0.3 micron diameter particles or larger from the air that passes through it."
Over the years HEPA filtration has become more commonplace, and today is used in many high-tech, critical manufacturing and clean-room applications such as pharmaceutical, photographic, computers, optical, health care, food processing and aerospace - as well as in vacuum cleaners.
HEPA-filtered vacuum cleaners are especially useful in healthcare applications where dust can contain microbes like Aspergillus, a mold that can cause infection in patients with weakened immune systems. Schools and hotels are also a promising market for HEPA vacuums since Asthma - often triggered or aggravated by dust - is the No.1 cause of chronic absenteeism in schools, and has reached near-epidemic proportions in the general population. In the workplace, it's been documented that indoor air quality not only impacts worker health but also productivity and product quality.
HEPA has not been without its critics, however. HEPA filters are far more expensive than standard filters and, because of the finer mesh and greater resistance to airflow, may reduce a vacuum cleaner's suction and performance. There are, or course, exceptions. And in many uncontrolled and non-critical environments, the benefits of HEPA filtration are quickly negated by particles entering the room air from other sources, e.g. an open door or window.
Generally speaking, though, HEPA filtration is a very useful technology, as it enables keeping dust inside vacuum cleaners rather than spreading it throughout the indoor environment. HEPA achieves its remarkable particle capture ability through layering and brain-like folds or corrugations - all in a very compact package.
Other Filter Definitions
True HEPA means the entire vacuum system in which the HEPA filter is mounted is airtight (except where air properly enters and exits the vacuum), thus preventing dust-laden air from escaping through gaps in the vacuum body or around the filter seal. Theoretically, it also means the airflow of the vacuum has been properly proportioned for the HEPA filter media, so that what exits the filter meets the 99.97 percent at 0.3-micron efficiency benchmark during use.
Similarly, sealed HEPA refers to a vacuum system that is sealed so that all the air being drawn into the vacuum goes through the HEPA filter. In some cases the term refers to only the filter itself being sealed at the filter mount.
Near-HEPA describes filtration systems that filter near, but do not meet the level of, the particle capture required for HEPA filtration. In some cases the trade-off is worth it, since near-HEPA filters cost only a fraction of what genuine HEPA costs.
ULPA (Ultra High Efficiency Particulate Air) is a technical definition that refers to a filter that will remove "not less than 99.999 percent of 0.12 micron diameter particles or larger from the air that passes through it."
Multi-stage filtration describes a system that uses multiple layers of filter material to remove particles of soil. These can be used alone or as a pre-filter for HEPA, ULPA or other filtration systems.
HEPA is HEPA...Or is It?
From a practical and technical standpoint, if the air leaving a vacuum cleaner contains more than .03 percent of 0.3-micron diameter particles, you do not have HEPA filtration. In reality, for genuine HEPA filtration to occur, the filter must be sized and sealed properly and contain the right media.
This gets confusing because manufacturers test their equipment and explain their results differently. For example, some companies test only the air going through the filter, but not the air coming out of the motor exhaust or through gaps in the body or around the motor housing and wheels. Some manufacturers test their vacuums in a closed chamber, which tests all the air coming from the machine. Other manufacturers use different methods to test their vacuums and filters, and it's possible some simply make marketing claims with virtually no testing at all.
Since not everyone follows the same rules, there are, in effect, many similar sounding but different claims made by manufacturers to market their vacuum cleaners and filtration, with very little standardization and regulatory "policing."
How can you be sure what you're buying? Research as much as you can. Ask questions, check references and require the manufacturer to provide you with copies of independent test reports that validate filtration, airflow and efficiency claims.
HEPA filters need to be replaced - or in certain cases, cleaned - when loaded. Frequency depends on the level of use, pre-filter maintenance and the manufacturer's recommendation. Vacuums with HEPA filters for critical environments-such as clean rooms where microchips are made--must be tested regularly using an established procedure.
Some companies sell portable laser particle counters for on-site monitoring, and these can be useful for checking particle counts coming from various parts of a vacuum. Costs for particle counters can range into the thousands of dollars; however, prices are falling. Remember, too, that not all filters are of the same quality. Some vacuum cleaner manufacturers claim generic filters do not fit their machines as well as their own (OEM) filters and have less-than-optimum media, thus allowing contaminated air to get out of the vacuum and into the environment.