Air Filter Ratings 

There are many types of air filters, but any filter that is labeled "HEPA" has a very specific rating.  The HEPA standard states that a HEPA air filter must trap 99.97% of all airborne contaminants that are 0.3 microns.

This means that for every 10,000 air particles that are 0.3 microns, the HEPA filter will only allow 3 of them to pass through.

How Big is a Micron?

A micron is 1 millionth of a meter, or about 0.00003937 inches, which means there are 25,400 microns in an inch. This dot (.) on your computer screen is about 615 microns.

The human eye can see particles as small as 10 microns, and an optical microscope allows us to see things as small as 0.1 micron. Electron microscopes allow us to see things as small as 0.001 microns.

HEPA air filters are the most efficient type of air filter because they can trap most air particles including bacteria and viruses.

Other Types of Air Filters

HEPA filters are classed as a mechanical air filter. Other types of air filters include Electronic or Electrostatic Air Cleaners and Gas-phase Absorption Air Filters. The American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) has come up with a rating system for each type of air filter. This rating system is called Minimum Efficiency Reporting Values (MERV) and most air filter manufacturers now label their products with a MERV rating.

Typical disposable air filters are rated between MERV 1 and MERV 10, with the higher ratings indicating a more efficient air filter.

The HEPA standard exceeds the MERV specifications because they are the only mechanical air filter with an efficiency of 99.97% at 0.3 microns. This makes them at least 50% more effective than other types of mechanical air filters.