Air purifiers evolved in response to people's reactions to allergens like pollen, animal dander, dust, and mold spores. Reactions (sneezing, runny nose, scratchy eyes, and even more severe consequences such as asthma attacks) are the result of antigens found in the home. These antigens are major triggers of asthma, and there are more than 17 million asthmatics in the United States alone. Air purifiers remove a portion of these particles, thus reducing allergic-type responses.
Due to their extremely small size, allergens are able to pass through a standard vacuum cleaner bag and redistribute into the air where they stay for days. Even a single microgram of cat allergens is enough to invoke an allergic response in most of the six to 10 million Americans who are allergic to cats. Other airborne particles—such as bacteria and viruses—can cause illnesses, some of which are fatal. There are many reasons—allergies, asthma, fatal illnesses—that millions of air purifiers are sold in the United States every year.
There are two common types of air purifiers that can remove some or all of the disease and allergy-causing particles in the air: mechanical filters—the most effective are classified as High Efficiency Particulate Air filters (HEPA filters)—and electrostatic precipitators.
HEPA filters are made out of very fine glass threads with a diameter of less than 1 micron (a micron is 0.00004 in, 0.001 mm). By comparison, a human hair has a diameter of about 75 microns (0.003 in, 0.07 mm). The fine glass threads are tangled together and compressed to form a filter mat. Because the individual threads are so microscopic, most of the mat consists of air. The openings in the mat are very small, generally less than 0.5 micron (0.00002 in, 0.0005 mm). HEPA filters will collect particles down to 0.3 microns (0.00001 in, 0.0003 mm) in diameter. Even though the filter may only be 0.10 in (2.5 mm) wide, it would consist of 2,500 layers of glass threads.
Electrostatic precipitators rely on electrostatic forces to remove particles from the air. They work by creating a cloud of free electrons through which dust particles are forced to pass. As the dust particles pass through the plasma, they become charged, making them easy to collect. Electrostatic precipitators can collect particles down to a diameter of 0.01 microns (0.00001 mm).
Neither HEPA filters nor electrostatic precipitators can remove volatile organic compounds from the air, therefore do nothing to reduce odors. For this reason, most air purifiers are equipped with a pre- or post-filter composed of activated carbon. Activated carbon is produced by heating a carbon source (coconut shells, old tires, bones, etc.) at very high temperatures in the absence of oxygen, a process also known as pyrolysis or destructive distillation. Pyrolysis separates the pure carbon from the other materials contained in the raw material. The pure carbon is then exposed to steam at 1,500°F(800°C). The high temperature steam activates the carbon. The activation process forms millions of cracks in the carbon grains. These cracks have diameters of about 0.002 microns (0.000002 mm). Because there are so many cracks, the activation process provides the carbon with an enormous surface area per weight—about 6.5 acres/oz (1,000 m 2 /g). The millions of cracks provide locations where organic compounds can be adsorbed. In addition, the surface of the carbon carries a residual electrical charge that attracts non-polar chemicals (chemicals that do not have separated positive and negative charges) to it. Activated carbon is very effective at adsorbing odor producing compounds.