There are three types of air cleaners: mechanical filters, electronic cleaners, and hybrids that are a combination of these two.

Mechanical filtration: These draw air through a flat, pleated, or high efficiency particulate air filter to trap particles.
Some of the purposes of air filters are:

• Protect the ventilation equipment from dust and dirt
• Protect furnishings and décor of the occupied space
• Reduce housekeeping and building maintenance
• Reduce furnace and heating equipment fire hazards
• Protect the general well being of people: in order to do this, a much higher performing filter must be used. It might not always be possible to put high efficiency filters on older ventilation systems so this should be investigated before doing so.
• The efficiency of a filter is the difference between an air cleaner and just an air filter. Air filters are less than 20 percent efficient at trapping or filtering out particles; air cleaners are more than 20 percent efficient. It is important to choose the right type of filter since not all air filters are equal. There are a wide range of filter sizes and thicknesses. The filter found in most home ventilation systems is a flat filter that is one-half inch to one inch thick. Though it is efficient in collecting large particles that might hurt equipment, it will not capture the particles that can be breathed deep into the lungs. Very small particles can be breathed deep into the lungs and cause health problems so high efficiency filters are needed to trap these small particles.
• One of the best ways that a filter's efficiency is increased is by pleating it. Medium-efficiency filters (20 percent to 50 percent efficiency) are usually pleated. This increases the total area available for filtration and extends the life of the filter. These use static electricity created by airflow, but their effectiveness may decline as the static charge decreases over time.
• The most efficient filters are High Efficiency Particulate Air (HEPA) filters. They can remove almost 100 percent (99.97percent) of all particles larger than 0.3 micrometers in diameter. Even though HEPA filters are expensive, they can be maintenance-free up to five years when used with a pre-filter. Most home ventilation systems do not have a powerful enough fan or horsepower to use this type of filter since these filters can be up to 36 inches thick (as compared to the one-half to one inch thickness of most furnace filters). A unit that is powerful enough to blow air through a HEPA filter also uses a lot of energy. Recently filters have appeared that call themselves "HEPA-type" filters that are less efficient (60 percent to 95 percent) but can run on a less powerful ventilation system.