HEPA (High Efficiency Particulate Air) filters are the best type of air filter for removing almost any pollutants from the air. They're completely safe and effective, they and don't have any negative health effects like ozone or ionizers. In the US, an air filter must remove more than 99.97% of particles with a 0.3 µm (micron) diameter.
Simply put, HEPA filters work like a sieve. Air is moved across the filter material and it filters out the tiny solid and liquid particulates that are in the air. These filters are made of fibers of fiberglass and/or polypropylene. But the spaces between the fibers are much greater than the particle sizes that the filters remove. How is this possible?
If we zoom in to the microscopic level, there are actually 4 mechanisms that allow HEPA filters to remove particulates so efficiently. These different mechanisms have varying degrees of effectiveness for different sized particles. Keep in mind the air can flow through the filter fibers, but it has to weave in different directions to get around them.
Inertial impaction occurs when a heavier particle in the air can't follow the zig zag path that the lighter air molecules can. The particle's inertia carries it in a straighter line on a collision course with one of the filter fibers. When it hits, the particle becomes embedded in the filter fiber. This mechanism is dominant with particles larger than 1.0 µm.
When mid size particles in the air stream come within one radius of a filter fiber, they tend to adhere to it in a process called interception.
When tiny particles in the air stream collide with air molecules weaving around the filter fibers, those particles pushed in all different directions (Brownian motion). This increases the probability that impaction or interception will occur. This diffusion mechanism is dominant with particles smaller than 0.1 µm.
The smallest particles in the air tend to have an electrostatic charge. The filter fibers tend to have an electrostatic charge as well. When the particles and fibers of opposite charge come in close proximity, they are attracted and the fiber holds onto the particle.
These 4 mechanisms predominantly work for different particle sizes. Where they overlap there is a drop in fractional efficiency for the filter at those particle sizes. The particle diameter at the trough of this graph is called the "Most Penetrating Particle Size," or MPPS. This is going to be the filter's worst efficiency.
This fact is at first counterintuitive, since the efficiency of a filter doesn't keep going down as the particle sizes get smaller. Some companies mislead consumers by only publishing their filter's efficiency at a very large or very small particle size. They may say "removes 99.5% of particles 0.001 microns." While it's possible that may be true, that same filter can fail to meet HEPA standards at the 0.3 or 0.1 µm size where the efficiency may only be 98%. Companies may further deceive consumers by using marketing words such "True HEPA," "HEPA-like," "HEPA-style," or "99% HEPA." None of these terms have any meaning.
Aspen's Microparticle HEPA Filter has been tested for efficiency at a variety of particle sizes. It's lowest efficiency occurs at 0.1 µm, where it is still more than 99.97% efficient.