The percentage reported is based on particle weight, not particle count. Particles larger than 0.5 microns (um) make up 97% of the weight of all particles in the air. But 98.5% of the particles by actual count in the air are SMALLER than 0.5 um -- which means most go zipping right through the best HEPA filter (like gasses such as cigarette, chemical, cooking, pet and mold odors and virus pathogens). Understand that 98.5% of the particles in the air account for only 3% of the total particle weight. What happens to all of these particles?

It is particles of these sizes (0.3 microns and smaller) that can become lodged in the lungs resulting in irritation and worse.

Filters depend on bringing the problem particles to the filter. Powerful fans (which means high electric bills and noise levels) are needed to move the air in the room to the filter.

Only about a maximum of 20% of the air in the room where filter sits (or 5% of the entire home if you have only one machine in the home) ever gets to the machine to be filtered. Natural air flows, furniture, drapes, dead air pockets, etc., prevent particles from moving to the filter. Your real world environment is not the clean, unencumbered laboratory where the HEPA ratings were calculated. In addition, extremely small particles (like cat protein which some people are allergic too) are often too light to ever be effectively moved by fans.

A filter machine may be rated to move a particular amount of air (say 1000 Cu. Ft. per hour). However, it is normal for that same air once filtered to be re-filtered over and over again being caught in a figure eight effect.

Because you breath in all rooms of your house, you would need a filter machine in every room of your house to remove the larger particles. Even then, 60-80% of the air in your home never gets filtered!

This is why the EPA says that FILTERS ARE THE LEAST EFFECTIVE of all means to clean the air inside a building.

Filters do nothing to stop mold, bacteria or odors from being produced. In fact, if not cleaned or replaced according to the manufactures suggested schedule, filter machines can add to the problem by providing a medium for breeding these problems.

The original purchase price of a HEPA filter machine is only the beginning of a very expensive venture. Analysis has shown, believe it or not, that the total cost to filter the air in a 2,500 Sq. Ft. home using one of the leading HEPA filter machines over a ten year period can exceed $10,000 (for smaller models) to over $25,000 for their models rated to cover 440 Sq. Ft.! Add another $5,000 for a 3,000 Sq. Ft. home.

Lastly, the reported HEPA filter efficiency studies are based on laboratory results, not real world situations. The moment the filter is turned on and begins to collect particulate, it becomes less efficient because particles immediately begin to clog the filter. The longer the filter goes between replacements the less efficient it becomes. At what point the filter becomes more a health hazard (because of collected bacteria, mold spores, etc., which begin to grow and are blown back out into your breathing environment) rather than a health facilitator is anybody's guess.