HEPA stands for High Efficiency Particulate Air. It is a standard defined and developed by the U.S. Department of Energy during the 1940s as part of their efforts to contain the spread of particles and contamination resulting from nuclear testing. This standard has since moved into the consumer market and has become commonplace for air purifiers.
Credit: A Fresher Home
To meet the HEPA standard, the filter must remove 99.97% or more of all particles which are 0.3 microns (micrometers) in diameter. In other words, for every 10,000 particles that are 0.3 microns in diameter, only three of them pass through.
The HEPA standard traps very small particles, many of which are invisible to the human eye and harmful to your health. To provide perspective for size, human hair is approximately 80-100 microns in diameter.