Let me start by defining HEPA Filter. It stands for High Efficiency Particulate air filter. From what I can tell, the term "true HEPA" is used to refer to where the filter is placed in the unit in conjunction with the motor. This is more of a consideration for HEPA vaccuums (which have exhaust) as opposed to the stationary room filters. When something says it is a true HEPA, it is basically saying that its efficiency in removing certain contaminants from the air is 99.97% effective at removing particles that are .3 microns and larger in diameter.

What exactly is a micron? A micron is one millionth of a meter, but that probably isn't very helpful to you, so I'll put it another way. A grain of salt is approximately 60 microns in size. To give you an even better idea of how small this is, here is a list of airborne pathogens and their sizes. The items listed in blue are generally visible to the naked eye. Those in red are very difficult to see as individual particles. The smaller the diameter, the easier it is for the pathogen to get into our small airways and cause irritation.

* Pollen 10-60 microns
* Mold spores 1-60 microns
* Lint 10-60 microns
* Dust 1-60 microns

* Bacteria .25-20 microns
* Smog .05-20 microns
* Tobacco Smoke .05-5 microns
* Viruses .01-.1 microns

As you can probably infer by looking at the list above, HEPA filters-which can remove particles .3 microns in diameter and larger- are most effective at removing almost all of the things in that list. With the exception of viruses, airborne pathogens of the greatest concern to CFers are easily filtered from the air with a HEPA filter.

What about ionizing filters?

Ionic air cleaners work by breaking up the contaminant and pushing it around the room in a different form. They split the offending molecule into separate positive and negative charges (ions). Ionizing units are great for removing odor, and many people like them because all you have to do is wipe of the metal plates inside the unit as opposed to changing out filters. However, when the airborne pathogen is merely ionized, the separated atoms, particularly oxygen atoms, have the potential to combine with one another and form what is called "ground level ozone."

Ground level ozone varies slightly from the stratospheric ozone that makes up the ozone layer surrounding the earth. Stratospheric ozone is "good" ozone; ground level ozone is "bad" ozone. "Bad" ozone (i.e. ground level

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