This page discusses the air filters that are used in the mechanical air purifiers available on the market.  There are many electronic air cleaners (i.e. ionic) that do not necessarily need to incorporate all of these filters to be effective.

With that said, there are basically three main types of air filters that most air purifiers incorporate.  These are the prefilter, the HEPA, and the activated carbon.

Prefilter

The prefilter is an air filter that initially cleans the air as it is draw into an air purifier.  It is the least expensive and least effective in the unit-- but it is also one of the most important.  The prefilter absorbs particles that are 1 to 10 microns in size.  These are the larger pollutants such as animal hair and very large dust.

The reason a prefilter is important is that it gets rid of the pollutants that you do not want to "clog" your more expensive HEPA filter.  HEPA filters can clear particulates down to 0.3 microns in size.  These are the ones that have the ability to get into your lungs and cause problems.

HEPA air filters

HEPA filters are discussed in detail on our page.  These are the most effective component of most air purifiers.

Activated Carbon air filters

Because HEPA filters cannot remove gases and odors that are typically 0.3 microns and smaller, activated carbon air filters are used in many air purifiers.  Activated carbon is charcoal that has been treated with oxygen to open millions of tiny pores between the carbon atoms.

Activated carbon filters then physically absorb gas molecules into these pores.  Odors and gasses tend to be too small to be captured by anything but activated carbon.  Keep in mind that as these pores get filled, eventually the filter will need to be changed as it drops in efficiency.