HEPA stands for High Efficiency Particulate Air, and a true HEPA filter is widely regarded as the ultimate filter.

In World War II the Atomic Energy Commission needed a filter to protect researches from radioactive dust particles that might present a health hazard to them. The HEPA filter was born. It traps particles as tiny as .3 microns with an efficiency rating of 99.97%.

To give you an idea of the size of a micron, it takes 25,400 microns to equal 1 inch (2.54 cm). Conversely, 4/100,000ths of an inch equals one micron. In metric terms, a single micron is 1 millionth of a meter.

A particle of 10 microns is invisible to the naked eye. Pollen ranges between 5-100 microns. Human hair between 70-100 microns.

The rating for a HEPA filter is based on capturing nearly all microns .3 in size, verses .1 or even .001 because .3 microns are the hardest size to trap and the optimal size for passing into the human respiratory system. Therefore the .3 micron efficiency rating sets the highest standard.

A HEPA filter is so efficient that for every 10,000 particles that enter the filter within its filtering range, only 3 particles will get through. Surprisingly, HEPA filters become even more efficient the longer they are in use.

For this reason HEPA filters are used in environments that rely on high clean-air standards. Surgical facilities, tuberculosis wards, NASA clean rooms, and laboratories are a few examples of environments that utilize HEPA filters. HEPA filters are particularly useful for allergy sufferers.

Many imitations of HEPA filters pervade the marketplace, usually called HEPA -type filters. They are less expensive than true HEPA filters, rated lower, and do an inferior job of filtering the air. If your health requires the specific benefits and efficiency of a HEPA filter, check the packaging to make sure the filter is rated at an efficiency of 99.97% for microns .3 in size. If it's a true HEPA filter, it will have this rating clearly marked.