Electronic air cleaners are effective in removing airborne particles, but not gases. One major drawback to many of them is that they can produce ozone that may irritate the lungs.

• Negative ion generators: These give a charge to airborne particles so that they become attracted to and settle on walls, floors, tabletops, draperies, people, etc. Activity within a room, however, may stir up and release these settled particles back into the air. Some devices have a collector to pull the charged particles back to the unit. If a unit does not have a collector, then this kind of a unit can result in the soiling of walls and other surfaces.

  While ion generators may remove small particles (such as those in tobacco smoke) from the indoor air, they do not remove gases or odors. The odor of tobacco smoke is mostly due to gases in the smoke rather than the particles. So even if the particles in tobacco smoke are removed, you may still smell the smoke; and with over 4000 chemicals in tobacco smoke, many pollutants are still probably present. These units may also not do a good job at removing large particles, such as pollen and house dust allergens, since these large particles tend to settle quickly.

• Electrostatic precipitators: These give a charge to airborne particles and trap them on metal plates that are oppositely charged. These plates must be cleaned regularly for these units to be effective since their efficiency declines rapidly as the collector plates become coated with particles. Cleaning the plates should be done every few months, at least. When these units are built into a home or building's ventilation system, there is the advantage of cleaner ventilation equipment resulting in lower energy costs and performance that is more reliable. However, since these units can produce some ozone, you need to make sure that the unit does not emit more than the FDA limit of 0.05 ppm.

  As long as the units are the correct size for the room and do not give off ozone, they can be effective. For example, in a study in two Swedish day care centers, the absenteeism rate of the children was recorded over three years. In the second year, electrostatic air cleaners were added. The amount of mold in the air decreased after four to five months and absenteeism rates fell by 55 percent.[3]

  Since many indoor air complaints are related to temperature complaints, one manufacturer came up with an air cleaner that also provides personal heating and cooling. This device uses an electrostatic filter to remove airborne particulates and includes a separate filter for volatile organic compounds (VOCs). The system has been designed to include a range of options such as controls for thermal comfort and lighting. The cooling mechanism operates without the use of CFCs and allows a user to lower the temperature at his or her work station by as much as 10°F or to raise it by as much as 25°F compared to the ambient temperature in the building.[4]

An advantage to these types of units is that there are no long-term filter replacement costs.

The drawbacks are:

• They become less efficient with use
• They require frequent cleaning
• They can produce ozone
• They can be expensive if installed into existing ventilation systems