We’re all keenly aware of air pollution. If you ask someone about it, they might point to the smoggy haze that hangs over Los Angeles, fumes from a smokestack at an industrial site, or smoke from a burning pile of leaves. What many fail to consider is that, even in the cleanest home, air pollution exists indoors as well. Indoor pollutants can result in irritation to the lungs, and contribute to worsening symptoms of allergies and asthma. Standalone HEPA air filters are an increasingly popular way to combat indoor pollutants. 

While most people dust and vacuum on a regular basis, these efforts may just be moving the dirt around rather than removing it from your environment. To effectively remove air–polluting irritants from the home, many are turning to HEPA (High Efficiency Particulate Air) air purifiers. This guide will explain what standalone HEPA air filters are, how they work, and what kinds are on the market today.

Environmental Pollutants

Common dirt particulate, invisible to the naked eye, include dust mite debris, bacteria, plants spore, mold and smoke, typically measured in microns of .5 to 100 microns. A micron is equivalent to 1/25,000th of an inch. A human hair is typically 50–100 microns. HEPA air filters are effective in trapping dirt as small as 0.3 micron. HEPA filters are typically composed of a non–woven synthetic fiber. Some HEPA filters may be made of glass fiber. HEPA filters traps dirt in four ways:

Impaction: As air flows through the filter, particles larger than 0.5 microns are propelled forward by the force of the air where they impact and stick to the surface of the filter.

Diffusion: Tiny particles of less than 1 micron are so light that rather than flowing with the air, they diffuse through the air. Because these tiny particles don’t follow the air path around the filter, the particles penetrate the fiber and become trapped.

Interception: Mid–range particles are not heavy enough to be inert, but are not light enough to be defused through an air stream. These particles will follow the air stream as it circulates the filter, and they are trapped in the filter as they pass by it.

Sieving: Sieving occurs when filter fibers are closely spaced together and particles cannot fit through the air space between the filter fibers, so they become lodged. This occurs with particles typically 1 micron or larger.

These four filtration methods combine for exceptionally high filtration of pollutants and environmental irritants, with the weakest point of filtration at 0.3 microns. HEPA filters, at their weakest, trap 99.97% of all microparticles. HEPA filters were originally developed by the nuclear industry, which needed this technology to ensure a thorough cleanup of dangerous, radioactive particles. The technology was also championed by factories and workplaces where the environment produces heavy amounts of dust as part of the manufacturing process. HEPA filters are also routinely used in places that require the highest hygienic standards because of their ability to trap mold spores, bacteria and viruses.

HEPA Filter Classifications