HEPA filters

Last updated: December 1, 2009.

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 (high-efficiency particulate air) air purifier (or a vacuum with a HEPA filter) well worth the investment. Let's take a look at how HEPA filters work.

Photo: A typical HEPA filter from a household vacuum cleaner. You can see the HEPA 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 airflow.

How HEPA filters work to trap dust and dirt

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.

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 HEPA filter inside. However, the inner HEPA filter works in a completely different way.

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 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.

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 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).

According to NIOSH (the US National Institute for Occupational Safety and Health), a proper 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 metre). 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 effectively. 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. A genuine HEPA filter is much more hygienic than an ordinary one because it will stop mold spores and even some bacteria and viruses.

HEPA was originally developed by the nuclear industry 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 process.

Things to consider before buying a HEPA cleaner

If you're looking to buy a HEPA vacuum, take care that you're getting the real deal. There are several things worth noting:

• Make sure it 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 vague descriptions like "HEPA-like" and "HEPA-type" that aren't quantified in any way. A true HEPA filter will always quote the numbers.
• A vacuum with proper HEPA filtration will channel virtually all (over 90 percent) of the dirty airstream through the HEPA filter; if it doesn't do this, the vacuum is 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. Vacuums with true HEPA filtration need more powerful motors and ones with low power may not clean effectively.
• Some professional-grade vacuums will have extra mechanisms for dealing with particles even smaller than 0.3 microns, 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.

Photo right: 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.