A HEPA filter is a High Efficiency Particulate Air Filter. The first HEPA filter was developed by the U.S. military in the 1940s to prevent the spread of radioactive particles. Today, while HEPA filters are still used in the nuclear fuel industry, they’ve also become popular in homes, hospitals, and various industries that require contaminant-free air.
A HEPA filter consists of a maze of randomly arranged fibers. As small particles pass through this maze, they inevitably collide with one of the fibers and stick to it. In the case of HEPA air purifiers, a large fan forces a room’s air through the filter repeatedly, thus ensuring a constant supply of fresh air.
By definition, according to NIOSH (the National Institute of Occupational Safety and Health), a HEPA filter must capture 99.97% of all particles 0.3 microns or larger. One micron is one millionth of a meter, or 0.000039 of an inch. To give you an idea of its size, a postage stamp measures about 25,000 microns across, and the period at the end of this sentence is about 400 microns. Dust mites, invisible to the human eye, can be up to 300 microns, and their allergenic fecal pellets are about 10 microns. Most household allergens measure between one and ten microns. Thus, a HEPA filter captures virtually all household allergens.
HEPA filters also capture viruses and bacteria; however, they do not filter chemicals, odors, or gases. For this reason, many HEPA air purifiers also feature activated carbon filters to remove chemicals and gaseous pollutants.
HEPA filters are also common in vacuum cleaners; HEPA vacuums are recommended for allergy and asthma sufferers. A HEPA vacuum will trap and hold onto the microscopic household allergens that trigger allergies and asthma (dust, mold spores, pollen grains, and pet dander). A vacuum cleaner that doesn’t have a HEPA filter will simply rearrange the dirt without picking up the most harmful particles. If you have allergies or asthma, don’t be fooled by marketing claims for “HEPA-like” vacuums; don’t settle for anything less than a true HEPA filter, and you’ll breathe easier knowing that you have the cleanest air possible.