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Air-Cleaning Devices
In general, controlling or eliminating pollutant sources and ensuring effective ventilation should be the primary methods used for reducing indoor air contaminants. Advanced air-cleaning devices are also available to filter particles, absorb gases or destroy pollutants. These air cleaners should always be used in addition to, not in lieu of, controlling or eliminating sources of pollutants and using effective ventilation (including effective particle filtration in HVAC systems).
Effective air filtration in heating ventilation and air-conditioning systems (HVAC) can reduce levels of particulates in buildings. Air filters are rated for particle removal by several methods. Minimum Efficiency Value Rating (MERV) is a common rating. Installing MERV 8 to 12 disposable pleated filters is the most effective and least costly method of removing common particles found in buildings. Air filters should be securely installed to prevent air from bypassing the filter, be replaced regularly, and should not add additional resistance that would affect the performance of the HVAC system. In some circumstances, addition of portable room air cleaners equipped with High Efficiency Particulate Air (HEPA) filters may reduce indoor levels of airborne particles that can trigger allergic reactions or asthma episodes. HEPA filters are rated by the manufacturer to capture particulates of 0.3 microns in size with 99.97 percent efficiency. An effective air cleaner must also be designed so that all the air drawn into the machine is expelled through the HEPA filter.
Electronic air cleaners such as electrostatic precipitators use a process called electrostatic attraction to trap charged particles. Air is drawn through an ionization section, where particles obtain an electrical charge. The charged particles then accumulate on a series of flat plates, called a collector, that is oppositely charged. Ion generators, or ionizers, disperse charged ions into the air, similar to the electronic air cleaners but without a collector. These ions attach to airborne particles, giving them a charge so that they attach to nearby surfaces such as walls or furniture, or attach to one another and settle faster. Any equipment that produces strong electric charges or ions can, in some circumstances, produce ozone, an indoor air pollutant that can cause health problems.
Gas-phase air cleaners use sorbent materials, such as activated carbon, for reduction of gases. Sorbents are materials used to absorb or adsorb liquids or gases, and are typically specific to a specific type or chemical class of gaseous airborne pollutants. These devices will not reduce concentrations of pollutants for which they were not designed, and they will not remove carbon monoxide.
Pollutant destruction technologies include ultraviolet (UV) radiation from UV lamps. If used, they should be applied with, but not as a replacement for, filtration systems. UV air cleaners mav be used for surface disinfection to inactivate or inhibit biological pollutants such as microbial growth on moist HVAC surfaces (e.g., found on cooling coils, drain pans or ductwork). UV air cleaners for air stream disinfection may reduce viability of bacteria and mold spores but provide limited disinfection of viruses. Air stream disinfection is less effective than surface disinfection, because airborne microorganisms are only exposed to ultraviolet radiation for a short time-period. The effectiveness of UV air cleaning systems is dependent on regular maintenance, such as cleaning dust from lamps and replacing old lamps. In many cases, maintenance must follow manufacturer's recommendations and must be performed by technicians representing the manufacturer, due to safety concerns from high-voltage electricity and potential exposure to ultraviolet light. UV air cleaners may have limited effectiveness in reducing allergy symptoms, because non-viable mold spores and fragments contain the irritants and allergens and are not susceptible to the light.
Photo catalytic oxidation (PCO) air cleaners use a UV lamp along with a substance called a catalyst, which reacts with the light. They are intended to destroy gaseous pollutants by converting them into harmless products, but are not designed to remove particulate pollutants. To achieve effective conversion, the reaction rate of the PCO cleaner must match the rates of contaminant generation. Some other issues with PCO have not been fully resolved, including the relatively large power consumption of PCO units, the complex chemistry of the PCO process, and the ability of PCO to remove multiple compounds from the air stream. Some PCO cleaners fail to destroy pollutants completely and instead produce new indoor pollutants that may cause irritation of the eyes, throat and nose. At this time, information on the performance of PCO cleaners is limited and inconclusive.
Ozone generators use UV light or an electrical discharge to intentionally produce ozone. Ozone is well known to be an irritant gas that reacts with lung tissue and can cause asthma attacks, coughing, chest discomfort, irritation of the nose and throat, and other adverse health effects. At concentrations that are lower than public health exposure limit standards, ozone has little effect in removing most indoor gaseous air contaminants and is ineffective at removing particle air pollutants. In addition, even low levels of ozone can react with existing chemicals in the air to create other, more-hazardous pollutants, including formaldehyde and ultrafine particles. Since ozone generators are not always safe and effective in controlling indoor air pollutants, the best advice is never to use ozone-generating air cleaners in occupied spaces.
See NC DPH advice about ozone generators and protecting yourself from ground-level ozone.
When selecting an air cleaning system, the first step should always be to eliminate and control pollutant sources, reduce pollutant concentrations, and increase ventilation using methods proven to be both safe and effective. Before purchasing an air cleaner, see the guidance in the EPA document, Guide to Air Cleaners in the Home 
, and consider initial costs, ease of maintenance, and replacement filter costs.
