Like humidifiers, air cleaners also come in styles that are part of the central ventilation system, that are room console units, and that are tabletop units. Here are the advantages and disadvantages of each style:

• Central filtration: These are air cleaners that are built into the home's central heating or air-conditioning system or a building's ventilation system. These are used when the air needs cleaning in an entire house or building. It may be difficult to add these to existing systems since a more powerful fan may be required to move sufficient air through some types of air cleaners. Another drawback is that the ventilation system must be running in order for the air to be cleaned. If you are having one of these systems installed, request one that has low-leakage, is easy to maintain, and is energy efficient. Be sure you go with a company or contractor that is very experienced in designing and installing central filtration systems.
• Console-size units: These are often used when air cleaning is needed in a particular room. They have effective filters or collecting plates. When room units were tested, all of them moved more air then tabletop units. One study even found this type of unit with a HEPA filter effective in removing large pollen and spores.[5] They usually work better when all the door and windows are closed and are usually the unit of choice if pollutants are confined to particular rooms.
• Tabletop units: These units usually have limited airflow and inefficient filters. Most reviews have shown these units to be ineffective. In a 1200 cubic foot room, 12 units were tested for their effectiveness in removing cigarette smoke. Out of the 12 units tested, 9 of them reduced smoke levels by less than 24 percent compared with a 17 percent reduction when no unit was used at all!
• "Personal" air cleaners: These are units designed to be worn around the neck, on a lapel, or in a pocket. To begin with, they are very dangerous because they emit ozone right at the breathing zone; therefore, they can trigger health problems in people with asthma and others with lung problems. They also are ineffective. When Consumer Reports tested one in a test chamber in January 2000, they said that it "barely reduced" the particle concentration in the surrounding air.[1] In another study by the California Air Resources Board in 1999, they found that both personal air purifiers they studied exceeded the Federal Drug Administration recommended limit of ozone as well as the World Health Organization eight-hour air quality guideline of 60 ppb. They recommended that these devices not be used anywhere near the breathing zone, especially by children and persons with asthma, other respiratory diseases, or allergies.[6]
• Houseplants: Houseplants are generally not effective at removing indoor air pollutants.