How do HEPA filters work?
HEPA stands for high-efficiency particulate air. They come from a need to filter high volumes of very clean air, to produce microprocessors and other sensitive equipment. Also, HEPA filtration is simple and cheap, which means it’s available to everyone today.
HEPA filtration is a physical process, but it’s not like what most people envision when they think of a filter. Instead, in HEPA purifiers a fan will bring particles that are suspended in the air through the dense HEPA filter that has an array of gaps to filter all sizes of particles. Air passes through almost uninterrupted, which makes them “high-efficiency” but the web of fibers traps almost all the particles, from “big” stuff like pollen (10 microns or so) down to ultrafine, 10-nanometer (0.01 micron) stuff. That’s smaller than an individual virus.
The fibers in a HEPA filter capture airborne particulates in three basic ways.
- Impaction – Particulates 0.5 micron and above, slam into fibers and stick to them.
- Interception – Particulates 0.5 micron and below, can flow around some fibers but will eventually stick to one.
- Diffusion – Particulates below 0.1 microns, randomly bounce around with atmospheric atoms and will eventually stick to the fibers.
The final result allows all particulates to be captures quickly while allowing full air flow.
Crucially, the hardest particles to capture are what you might call the Baby Bears: At 0.3 micron, they’re at the low limit of interception momentum and above the limit of diffusion—in other words, they’re “just right” to get through a HEPA filter. The solution is to make the filter dense enough that it has a sufficient amount of fibers to capture most of the 0.3-micron particles. And again, according to the US HEPA standard, “most of them” means 99.97% of them in a single pass.
Written By: AML Staff