What Does an Air Purifier Do?
An air purifier is a device for removing pollutants from indoor air. According to the U.S. Environmental Protection Agency, air purifiers fall into two general types --- devices installed in the ductwork of a home's central heating, ventilating and air conditioning system to clean the air in the entire house, and portable units for cleaning the air in a single room or other specific area in the home.
Indoor Air Pollutants
Indoor pollutants fall into two categories. Particulate pollutants include dust, insect and animal dander, pollen, smoke, mold spores, bacteria and viruses. Gaseous pollutants come from sources such as cooking stove combustion byproducts, gases emitted by building materials, carpeting and upholstery, and solvents used in paints, cleaning compounds, pesticides and adhesives.
Purifying with Filters
Most portable and whole-house air purifiers rely on mechanical filters to trap particles borne in the air passing through the filter. Filters for whole-house systems fall into low, medium and high efficiency ranges, as measured by their Minimum Efficiency Reporting Value, or MERV. Common flat panel filters with a MERV of 1 to 4 trap large particles. Medium efficiency filters have a pleated surface, a MERV of 5 to 13, and can trap all large and most small particles. High efficiency pleated filters with a MERV of 14 or more will trap almost all airborne particles, but these require a powerful blower to overcome the airflow resistance of high efficiency filters. There are no set efficiency standards for portable air-filtering units.
Portable and whole-house electronic air cleaners employ electrostatic precipitation to remove airborne particles. These devices draw air through an ionization chamber where particulate pollutants pick up a static electrical charge. The charged particles are attracted to collector plates with an opposite static charge and stick there. Another type of electronic air cleaner, called an ionizer, disperses negatively charged ions into the air, where they attach to airborne particles and transfer the negative charge to them. The negatively charged particles are drawn to floor, walls and furniture that normally carry a positive static charge and stick there. The trapped particles are removed by cleaning the collector surfaces.
Devices for purifying gaseous pollutants typically use a "sorbent" such as activated charcoal to attract and hold the gases through a principle called "adsorption," where molecules of the pollutant gases are electrochemically attracted to the surface of the sorbent's particles and stick there. Eventually the adsorbent can't hold any more gas molecules and must be replaced. A more active approach called photocatalytic oxidation destroys pollutant gases by chemically converting them into harmless substances. These units work by shining strong ultraviolet light on a catalyst material as air passes through the PCO device. Unfortunately, these devices aren't effective against carbon monoxide gas, a deadly byproduct of incomplete combustion, says the EPA.