A HEPA filter is composed of a high efficiency cloth type filter, which is essentially a dense mat of randomly arranged fibers. They were originally designed in the 1940s by the US military to prevent the spread of radioactive particles. A common misconception is that a HEPA filter behaves like a sieve and traps particles based on the smallest diameter opening in the mat (circa 0.3 microns). This is incorrect as the HEPA uses some clever technology to trap much smaller particles. This is best explained in a diagram.
How it works
Interception – Where particles following the air stream stick to a fibre due to attraction.
Impaction – Larger particles impact a fibre and embed in the fibre.
Diffusion – This happens to the smallest particles. It is the collision of particles with gas molecules which in turn leads to a higher chance of Interception or Impaction.
HEPA filters are very effective at clearing out contaminants such as allergens and microbes. They are not the most effective for chemical fumes or gases. They usually have a replacement filter that can last several years.