Fresh air? More like stinky, germ-filled and particle-laden air. The air that you breathe is filled with all manner of things, from visible dust to invisible (but stinky) chemicals. Most of these things are harmless, but some can be harmful to your health, especially for those with conditions such as asthma. For these people, relief can come in the form of a HEPA (short for "High Efficiency Particulate Arrestance" or "Air," depending on who you ask) filter that removes the ick, pumping out clean, stink and dust-free air.
HEPA is a US government standard, first set in 1983. The standard says that to earn the name, a filter must capture at least 99.97 percent of particles larger than 0.3 µm (micrometers, or microns) in size, while only impeding the flow of air by a relatively small amount. The progression of modern of air filters design goes back much further, though: the technology behind them was developed to capture tiny radioactive particles released in the making of atomic bombs for the Manhattan Project in World War II. Post-war, it was used in industry, and eventually found a home in consumer devices like your air filter, vacuum cleaner and car.
These days, we see HEPA filters in products that clear the air, ranging from thefor your home to the that cleans the air in your car. Many vacuum cleaners like the also use HEPA filters, as they stir up dust and dirt, and without HEPA filters, would simply eject this dust back into the air to irritate the user.
These HEPA filters work by pulling or pushing the air through a filter screen that catches these contaminants. Think of them as being like the sieve you use to separate vegetables from the water you cook them in: the veggies gets caught in the sieve, the water (or air) passes right through. While the peas boiling on your stove are easy for a sieve to catch, the things in the air are a bit harder; HEPA provides a way to set a standard for how many airborne "peas" get caught, and how quickly the air can pass through the sieve.
HEPA filters are made from a fabric of tangled, warped fibers. These fibers are in a random pattern, with strands bending and curving in all directions to form a confusing, twisty maze that the air must go through. As the particles pass through the filter, they are caught in this maze in four ways: impaction, sieving, interception and diffusion.
Some bump into threads and stick to them (a phenomenon called impaction). Like our water metaphor, some get stuck in gaps between two threads that are too small for them to pass through (called sieving). As some are pushed through by the air flow, some pass so close to a fiber that they can't follow the flow of air around it and move out of the way, so they hit the fiber and stick (interception). And, finally, some smaller particles are knocked about randomly by molecules in the air, eventually bumping into and sticking to a fiber (diffusion).
Near HEPA does not equal HEPA
HEPA definitely works, and is something to look for when buying an air filter. This phrase is often abused, though. Companies try and sneak the HEPA name in by using phrases such as "HEPA-type" or "near HEPA." These are meaningless terms and should be avoided. A good manufacturer will go the extra mile and have its products tested by an independent lab to show that they comply with the HEPA standard.
What HEPA doesn't catch
No filter is perfect, though -- some particles will always get through. A good air filter won't catch everything the first time the air passes through it, so a good air purifier should be large enough and powerful enough that it can pass the air in a room through it several times. John Holecek did an excellent test of multiple air purifiers for The Sweethome, and found that all of the products he tested (which had HEPA filters) "were very effective at removing even the smallest detectable particulates from the air -- it doesn't matter if you're paying $200 or $1,000, they all work about the same." The differences that Holecek found were mainly in the performance of products at filtering other contaminants, such as volatile organic compounds (VOCs) and other odors.
A HEPA filter doesn't capture everything: a small number of particles larger than 0.3 µm will still get through. Smaller particles will also still get through, including small bacteria (some as small as 0.1 µm), viruses measuring between 0.005 and 0.2 µm, some smoke particles, and gases. In particular, many of the particles in urban smog are too small to be mostly captured by a HEPA filter, such as the PM2.5 particles that are generally considered to be most harmful because they are small enough to get deep into the lungs and get lodged in there.
If you are concerned about smog in your area there is an interactive map of PM2.5 and other air quality sensors here. It is eye-opening to see the difference between a city like San Francisco (with a PM2.5 count of 25 parts per billion, which is classified as good) and Riverside, near Los Angeles, with a PM2.5 count of 155, classified as unhealthy at the time this was written. That's on a quiet Sunday afternoon: the counts in both cities would be much higher during a busy commute.
A HEPA filter won't catch PM2.5 particles, so many air purifiers will include additional filters that are designed to be more effective. However, these aren't covered by the HEPA standard, so don't assume that they are as effective as the HEPA filter. There is another standard (called Ultra-Low Particulate Air, or ULPA) that drops the size limit for the particles captured to 0.12 µm, but this is mainly used in industrial filters. An air purifier designed to reach this standard is more difficult to build and usually costs hundreds of dollars more than a HEPA model.