HEPA Air Filters

The term HEPA is an acronym for "High Efficiency Particulate Air." HEPA filters are a common tool used in many mechanical air cleaners to filter small, unwanted, and unhealthy particles out of the air. They consist of a web of fibers arranged in a random pattern.
The first HEPA filters were designed by United States Department of Energy in the 1940s, though it was not until the 1950s that they began to enjoy widespread commercial use. Since then, there have been advances which have improved upon the original HEPA filters, but their basic design and function has remained the same for over half a century.
 
According to the online encyclopedia Wikipedia, air purifiers with HEPA filters operate differently than a typical membrane filter. Non-HEPA filters generally have only very small pores, so that any bits of matter too large to fit through them are trapped. HEPA filters, however, operate based on a different principle. The space between HEPA fibers is actually often much larger than the particles a HEPA filter targets. Instead, HEPA filters catch particles through a combination of three different mechanisms:
HEPA Filter Particle Capture Technology
 
Interception - most commonly affects particles above .4 microns. The particle sticks to one of the filter's fibers after coming close enough to it. It is as if the fiber reaches out and catches the particle.
Impaction - most commonly affects particles above .4 microns. The particle is forced into contact with a fiber by the trajectory of the airstream in which it is traveling, and is embedded into it. It is as if the particle makes a head-on collision with the fiber and then cannot dislodge itself.
Diffusion - most commonly affects particles below .1 microns. The arrangement of the fibers makes the particle collide with air or other gas molecules, and it is thrown off its course through the filter. Basically this is a means of keeping the particle within the filter until one of the other two methods succeeds in ensnaring it. It is as if the particle is blown about by wind until it collides with a fiber.
 
A standard way to test the effectiveness of HEPA filters is known as the "DOP penetration test." This involves sending a cloud of particles .3 microns in size through filter and comparing how highly concentrated these particles are in the airstream before and after they pass through it.
Effectiveness of HEPA Filters in Air Purifiers
According to the United States Environmental Protection Agency, HEPA filters are the most effective filter commonly found in mechanical air cleaners. To quote the EPA's guide to residential air cleaners: "True HEPA filters with a MERV between 17 and 19 are defined ... as having a minimum efficiency between 99.97 percent and 99.999 percent in removing 0.3 [micron] particles." MERV stands for "Minimum Efficiency Reporting Value" and represents the purifier's ability to clean the air. Usually MERV numbers fall between 1 and 16, but ratings of 17-20 are sometimes used for HEPA filters.
However, the EPA also warns of diminishing returns, to the point that using a stronger and more effective filter is no longer cost-efficient. It claims that cleaners with a MERV rating from 7-13 "are likely to be almost as effective as true HEPA filters" for most common purposes, and they will also be quieter and less expensive. Installing HEPA filters in large air cleaners will often require professional installation, further adding to the cost.
Also, in small, portable air cleaners, HEPA filters "may not be preferable to medium-efficiency filters because of HEPA filters’ lower air delivery."
Therefore, there are limits to the effectiveness of HEPA filters. On the other hand, they may be preferable to the so-called "HEPA-type" filters that are commonly found in lower-end small air cleaners, simply because HEPA filters must meet certain standards to be called HEPA, while "HEPA-type" could mean nearly anything.
Limitations of HEPA Filters
 
 
 
 

Conclusion

 
Anyone who does much comparison between different air purifiers will soon come across HEPA filters. They are the most effective type of filter available for removing solid particles from the air. On the other hand, they are more expensive, and are not effective at removing gasses. It would be a good idea to consider a purifier with a HEPA filter, especially over an ill-defined "HEPA-type" alternative, but be aware that there are limitations, and the presence of a HEPA filter does not by itself indicate that the purifier is a good one.
HEPA filters only filter particles, not gasses
HEPA-type filters may be a viable alternative due to being able to move more air through the filter quicker than standard HEPA filters
HEPA filters tend to be more expensive - something to consider since air purifier filters require replacement over time