Mechanical — uses fans to draw air through a series of filters.
HEPA filter — HEPA, or High Efficiency Particulate Air filter, was created by the government during World War II in order to reduce radioactive dusts in nuclear power plants. By definition, 99.97% of particles in the air 0.3 microns or bigger are removed by a HEPA filter. A HEPA filter is made up of one continuous sheet of paper-like glass-fiber pleated into a "V" configuration. The pleats then have separators of aluminum in between. These filters remove allergens such as dust, dust mites, pollens, and mold spores.
HEPA filters cannot remove particles finer than 0.3 microns or kill viruses and germs. Yet, they can remove many bacteria in the air and these solid particles that are captured in the air are not released back into the air again. Because these particles are so efficiently trapped, some can breed or reproduce in the filter, causing increased microorganism populations. Other drawbacks include a difficulty removing particles further distances from the filter and an inability to remove chemical fumes, gases, cigarette smoke and other odors.
ULPA filter — ULPA, or Ultra Low Penetrating Air filters, were designed originally for use in hospitals and are now available in units for the home. They have a filter made of a micro-fibered web that is able to lock in 99.99% of particles 0.1 micron and larger. While this filter can remove smaller particles than the HEPA, it still shares in that filters' other drawbacks.
Carbon air filter — These filters contain highly absorbent molecular-sized pores. These pores form a tough bond to liquids and gases of an odorous nature. This is the most absorbent filter available that can capture gases, cigarette smoke, odors, and chemical fumes. The carbon air filter does not re-release them back into the air. This filter has many of the same drawbacks as the other filters listed above, while, in addition, it does not remove particles in the air such as dust, microorganisms and other allergens.