Air Cleaning Devices

February 2000 "The overall effectiveness of an air cleaning device depends on the efficiency of the unit and on the amount of air drawn through it.

WHAT ARE AIR CLEANERS?

Air cleaning devices are either central filtration (often called "in-duct") systems put into the heating, ventilation, and air conditioning (HVAC) system of a home or portable units with an attached fan to assist in reducing indoor air pollutants.

WHAT TYPES ARE AVAILABLE?

MECHANICAL FILTERS draw air through a flat, pleated or high efficiency particulate air (HEPA) surface to trap particles.

HEPA FILTERS are the most efficient mechanical filters for removing small particles which can be breathed deep into the lungs. The highest efficiency air cleaners, widely available today, remove 0.3 micron (1 micron = 1 millionth of a meter) sized particles at a minimum of 99.97% efficiency.

ELECTRONIC AIR CLEANERS, in their simplest form, include negative ion generators that charge airborne particles so they become attracted to and settle on room surfaces. Activity within a room, however, may stir up and release the settled particles back into the air. More effective units called electrostatic precipitators electrically charge airborne particles and then trap them on oppositely charged metal plates. These plates must be cleaned regularly for proper performance.

HYBRID FILTERS are made up of two or more of the above filters. An example is the "electret" filter which uses electrically charged flat or pleated fabric to attract airborne particles. These are efficient when clean, but as they fill with particles the efficiency decreases and they must be replaced.

GAS PHASE FILTERS are often attached to air cleaning devices to filter specific gases, vapors and odors.

HOW DO I CHOOSE?

CENTRAL FILTRATION SYSTEM VS. PORTABLE DEVICE

Generally, portable units are used when air cleaning is desired in a room. Central filtration systems are used when whole-house air cleaning is needed. However, these systems require that the central heating or air conditioning fan be "on" in order for air cleaning to occur.

EFFICIENCY

The efficiency of an air cleaner is measured in terms of the particle size that the device can capture. In most cases, very small particles are the ones that penetrate deep into the lungs, causing health problems. Look for mechanical filters such as HEPA filters and electronic air cleaners that can effectively trap large and small particles.

CLEAN AIR DELIVERY RATE

The overall effectiveness of an air cleaning device depends on the efficiency of the unit and on the amount of air drawn through it. Although no industry-wide set of performance standards exists to help consumers compare units, the Association of Home Appliance Manufacturers (AHAM) has developed a standard for portable air cleaners called the Clean Air Delivery Rate (CADR). The CADR expresses the number of cubic feet of clean air a unit delivers each minute.

OZONE PRODUCTION

Some air cleaners (called ozone generators) use an electrical charge to generate ozone. Although ozone (also referred to as trivalent oxygen or saturated oxygen) is a necessary part of the upper atmosphere (10-30 miles above us), in the part of the atmosphere we breathe, ozone is a potent lung irritant. It can have damaging health effects, especially for persons with asthma and other lung diseases, children and the elderly. It is produced directly by ozone generators and indirectly by ion generators and some other electronic air cleaners. The FDA has set a limit of 0.05 parts per million of ozone in indoor air. Ask whether any electronic air cleaner you are considering buying has been tested for ozone production. The American Lung Association suggests that ozone generators not be used.