We tend to think of air pollution as something that happens
outside—but that's not always the case. Even inside your home, there
are things like dust mites and dirt trodden in from outdoors to worry
about. Indoor air pollution from sources like this can irritate your
lungs and contribute to
allergies and asthma.
The trouble is, if you use an ordinary vacuum
cleaner, you might simply be "rearranging the dirt": your cleaner
will trap some of the dust inside the bag or cyclone filter but let the rest
pass straight back into the room. If you suffer from asthma or another
breathing difficulty, you may find a HEPA
(sometimes defined as "high-efficiency particulate air," sometimes as "high-energy particulate arresting")
air purifier (or a vacuum with a HEPA filter) well worth the investment.
Let's take a look at how they work.
Photo: A typical HEPA filter from a household
vacuum cleaner. You can see the white, paper-like, fiber material folded and wrapped
around like a concertina under the orange plastic top and bottom case.
The folding greatly increases the area of the filter in contact with
the airstream—effectively improving the filtration without reducing the
Photo: The same HEPA filter with its gray outer gauze
filter attached. The air moves from the outside to the inside. You can see that the
gauze works here like a sieve to stop larger bits of dirt getting anywhere near the filter inside. However, the inner HEPA filter works in a completely different way. The outer gauze "pre-filter" can greatly extend the
life of the inner HEPA filter.
The simplest kind of filter is a sieve: something with holes that
are big enough to trap some particles and small enough to let others
through. Some vacuum cleaners do use filters like this to stop bigger
particles of dust and dirt—but how do you catch smaller dirt
particles as well? You could make a very fine filter or you could put
several filters on top of one another, but in a vacuum cleaner they
would clog up very quickly and stop the machine from working. Not only
that, you'd need a more powerful electric motor
to pull the air through the machine, and it would use more energy and cost more to run.
HEPA filters in vacuum cleaners tend to use two quite different
mechanisms to clean the airstream. First, there are one or more outer
filters that work like sieves to stop the larger particles of dirt,
dust, and hair. Inside those filters, there is a concertina of what
looks like folded paper designed to trap the smaller particles. The
paper is actually a mat of very dense glass fibers and, unlike the gauze,
it doesn't simply filter out small dust particles like a sieve.
Instead, it uses three different mechanisms to catch dust particles as
they pass through in the moving airstream. At high air speeds, some
particles are caught and trapped as they smash directly into the
fibers, while others snag on the fibers as they try to brush by. At
lower air speeds, dust particles tend to wander about more randomly
through the filter (a process known as Brownian motion after its
discoverer, Scottish botanist Robert Brown) and may stick to its fibers
as they do so. Together, these three mechanisms allow HEPA filters to catch
particles that are both larger and smaller
than a certain target size.
Artwork: The fibers in HEPA filters (shown here as gray
bars) trap dust and dirt particles in three ways. Some particles crash
into filter fibers and are absorbed by impact. Some are caught as
they flow along in the moving airstream, move too close to a fiber and
are trapped by interception. At lower air speeds, some are trapped by
diffusion (when randomly moving dust and air particles crash into one another and some
are pushed into the filter fibers).
As we'll see in a moment, HEPA was originally developed by nuclear scientists to help clean up dangerous, radioactive particles. Fortunately, most of us
don't have to deal with such things—but HEPA filtration is still
very useful and important in factories and workplaces, especially in
environments where dust is produced as part of the manufacturing
Types of HEPA filters
According to NIOSH (the US National Institute for Occupational Safety and Health),
a true HEPA filter is one that can trap 99.97 percent of dust
particles that are 0.3 microns in diameter (where a micron is one millionth of a meter).
There's nothing particularly significant about particles 0.3 microns
in diameter: they are simply the ones most likely to get through the
filter, and smaller and larger particles than this are trapped even more
That might sound a bit confusing, so let's just unpack what it means.
First, and most importantly, it does not mean that HEPA filters
can't trap things smaller than 0.3 microns.
Remember that HEPA filters aren't mechanical sieves: they're removing
particles by a combination of three different trapping mechanisms.
Diffusion, one of these mechanisms, is particularly effective
with very small particles, while larger particles are more likely to be trapped by impact and interception.
So relatively small and relatively large particles are both efficiently trapped in different ways,
while particles in the middle (say, 0.3 microns) are less effectively trapped by any of the mechanisms and more
likely to zip straight through. That's where the 0.3 micron measurement comes in.
It's really saying "even when we consider the least well-filtered particles,"
some 99.97 percent are still captured. 
To put 0.3 microns in perspective, it's worth
remembering that a typical human hair is roughly 50–150 microns
in diameter, so a HEPA filter is trapping dust several hundred times
thinner. The coarse, sooty particulates that spew from diesel car exhausts
are generally described as PM10 (particles less than 10 microns) or PM2.5 (less than 2.5 microns),
so you can see that HEPA filters are working at a scale at least 10 times smaller.
A genuine HEPA filter is much more hygienic than an ordinary one because it will stop mold spores and even some bacteria and viruses. It's cleaning at the microscopic—and indeed nanoscopic—scale.
By comparison, the fibers in a HEPA filter are roughly 0.002–0.5 microns (2–500 nanometers, nm)
in diameter. 
Artwork: HEPA filters screen out truly tiny particles, even smaller than 0.3 microns (blue line, bottom), which is 10–30 times smaller than PM10 and PM2.5 soot pollution (black/gray, middle), and hundreds of times thinner than a typical human hair (brown, top).
When it comes to filters used in respiratory equipment, NIOSH recognizes nine different grades, based on three different levels of efficiency (95, 99, and 99.97 percent) and three levels of
resistance to filter degradation (N, R, and P). N means not resistant to
oil, R is oil resistant, and P is oil proof. So you might see a filter labeled
N95 (95 percent efficient and not resistant to oil) or P100 (99.97 percent efficient and
You might also see HEPA filters classified using the five letters A through E, based on how well
they capture particles and resist airflow. Type A are the least effective that still meet the
basic criteria for HEPA, while type E (at the opposite end of the scale) are military grade
filters capable of coping with chemical, radiological, or biological particles. There's also
a classification based on whether filters are fire resistant (type 1) or semi-combustible (type 2).
Photo: The two HEPA filters in a Dyson vacuum cleaner (one above and one below the dirt cylinder) help to stop dust returning to the room.
Things to consider before buying
Even if you're an allergy sufferer, these sorts of classification probably aren't going to be relevant if all you're doing is buying a simple air purifier or vacuum cleaner. For most people, the key thing is to
make sure what you're buying has genuine HEPA
filtration (remember, that's 99.97 percent of particles at 0.3
microns—often described as "true HEPA" or "absolute HEPA")
by checking the particulate size quoted by the manufacturer. Avoid
manufacturers who make vague, meaningless descriptions like "HEPA-like" and "HEPA-type" that
aren't quantified in any way. A true HEPA filter will always quote the
numbers. Some professional-grade vacuums will have extra mechanisms for dealing with tiny particles, such as activated carbon granules (similar to those in water filters). Finally, if you need industrial-strength air cleaning, there's a tougher standard called ULPA (Ultra-Low Penetration Air) that can catch 99.99 percent of particles 0.12 microns and above. However, the relationship between ULPA and HEPA can be confusing:
it's worth noting that "a simple HEPA-rated filter will perform as an
ULPA-rated or better filter" if the air flow rate is low enough. 
Knowing a bit about how filters work comes in handy when you're deciding what to buy.
A vacuum with proper HEPA filtration will channel virtually all (over 90 percent) of the dirty airstream through the filter; if it doesn't do this, it's simply rearranging the dirt. HEPA filters obstruct the airstream in vacuum cleaners: it takes quite a lot of suction to pull air past all those convoluted fibers. That's why vacuums with true HEPA filtration need more powerful motors and ones with low power may not clean effectively. Air purifiers that use HEPA filtration may have higher power consumption for the same reason (which means they're more expensive to run for long periods and noisier than rival technologies, such as
photocatalysis, that work in different ways). A HEPA filter will also need cleaning or replacing periodically as well (which will add to the running costs); you
might want to check how much replacement filters cost before you commit yourself.
Who invented HEPA filters?
Like many other inventions, HEPA filters developed over time: there doesn't appear to be a single inventor
who deserves sole credit.
Photo: Air filters in gas masks: HEPA filters were originally developed in the 1940s for military use—and they remain an important line of defense against protection against chemical, biological, and radiological warfare to this day. Photo by Lamel J. Hinton courtesy of US Navy and
The ancestors of modern HEPA filters were folded paper filters developed by
companies such as Kimberley Clark (makers of Kleenex tissues) to
protect soldiers from gas attacks during World War I.
In World War II, much more sophisticated fiber-based air filters were developed by military scientists on both sides.
German scientists seem to have invented the basic technology, which was captured and further developed by Allied rivals,
notably during the Manhattan Project (the scientific effort that produced the world's first atomic bomb) as a way of trapping extremely dangerous, tiny radioactive particles. These filters used different combinations of fibrous materials, including cellulose, asbestos, viscose, glass, and different types of paper, which were pleated, folded, and separated in various
The Manhattan connection explains why, for a long time, according to Melvin First of the Harvard School of Public Health: "Everything about these filters was classified 'secret' during WWII and for a number of years after the end of the war." The technology was eventually declassified in the early 1950s. Some of the wartime research (and the theory of how HEPA filters work) was carried out by Nobel-Prize-winning chemist
Irving Langmuir. 
Germany, which pioneered military HEPA technology in the war, continued to play a part in its peace-time evolution. In the post-war years, German brothers Klaus and Manfred Hammes began developing relatively inexpensive, home-air filtering systems during the early 1960s to reduce soot particles produced by coal-fired stoves. Over in the United States, the first inexpensive, pleated air filter was patented on April 19, 1966 by Kenneth W. De Baun and you can read all about it in his US Patent 3,246,457: Pleated Air Filter Cartridge.
Artwork: Kenneth De Baun's pleated HEPA filter from the 1960s.
Artwork from US Patent 3,246,457 courtesy of US Patent and Trademark Office.
HEPA filters by Melvin W. First, Harvard School of Public Health, Journal of the American Biological Safety Association, 3(1) pp. 33–42, 1998. This is a great technical introduction, with some useful history.
Report on Smokes and Filters by Irvin Langmuir, OSRD 865 (OSRD, Office of Technical Services, Washington, DC, 1942). One of Langmuir's original studies into HEPA filters. I've not managed to find the text online.
↑HEPA filters by Melvin W. First, Harvard School of Public Health, Journal of the American Biological Safety Association, 3(1) pp. 33–42, 1998.
↑Report on Smokes and Filters by Irvin Langmuir, OSRD 865 (OSRD, Office of Technical Services, Washington, DC, 1942).
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