HEPA is a type of pleated mechanical air filter known as a "high-efficiency particulate air filter.” HEPA filters are typically constructed using borosilicate glass fibers, plastic fibers (also known as polypropylene), or fiberglass bound together with up to 5% acrylic binder.
HEPA filters are made by randomly arranging the polypropylene or fiberglass fibers (between 0.5 and 2.0 µm). While sometimes the fibers are arranged in a tangle, the fibers can pass air through and trap some of the particles. The important part is that the fibers create narrow pathways for the air to pass through. Larger particles are trapped because they can’t keep up with the constant movement and maneuver through the fibers. Critical factors of HEPA filtration are the fiber diameter, filter thickness, and face velocity. The filters capture particles through diffusion, interception, and impaction.
The standards that HEPA falls under require that any filter approved to be HEPA must remove 99.95% to 99.97% of particles in the air that passes through it. This refers to a specific diameter of particles, specifically (for HEPA) ones equal to 0.3 µm (a micrometer or micron, one-millionth of a meter).
As the most microscopic particles around, viruses exist in all organisms. Particles of the SARS-CoV-2 (COVID-19) virus measure approximately 0.125 microns on their own.
Bacteria can also cause a range of illnesses. There are many different varieties, which means the size can vary widely. Some bacteria are as small as 0.3 microns to 5 microns.
Visible and often odorous, these spores range in size from 5 microns to 10 microns. HEPA air purifier systems work to reduce the number of particles floating through the air by attacking the spores.
Particles of smoke from cigarettes and other types of tobacco are dangerous despite their small size. At 0.3 microns to 1 microns, particles from smoke are respirable, meaning you can breathe them in. An air purifier functions to trap particles in the HEPA filter.
Household dust particles range in size from 1 microns to 10 microns, and they can cause respiratory illnesses and other issues when inhaled.
With dander ranging in size from 1 microns to 5 microns, furry friends tend to trigger allergy and asthma symptoms. Without an air purifier, dander remains airborne for hours.