Indoor air pollution is among the top five environmental health risks. Usually the best way to address this risk is to control or eliminate the sources of pollutants and ventilate a home with clean outdoor air… But opportunities for ventilation may be limited by weather conditions or by contaminants in the outdoor air.
If the usual methods of addressing indoor air pollution are insufficient, air-cleaning devices may be useful. Air filters and other air-cleaning devices are designed to remove pollutants from indoor air. Some are installed in the ductwork of a home’s central heating, ventilating, and air-conditioning (HVAC) system to clean the air in the entire house. Portable room air cleaners can be used to clean the air in a single room or in specific areas, but they are not intended to filter the air in the whole house. Air-cleaning devices are categorized by the type of pollutants – particulate and gaseous – that the device is designed to remove or destroy.
Filters & Cleaners
Two types of air-cleaning devices can remove particles from the air: mechanical air filters and electronic air cleaners.
Mechanical air filters, such as high efficiency particulate air (HEPA) filters, remove particles by capturing them on filter materials. Most mechanical air filters are good at capturing larger airborne particles – such as dust, pollen, some mould spores, and animal dander – and particles that contain dust mite and cockroach allergens. But because these particles settle rather quickly, mechanical air filters are not very good at completely removing them from indoor areas.
Electronic air cleaners, such as electrostatic precipitators, use a process called electrostatic attraction to trap particles. Ion generators, or ionizers, disperse charged ions into the air. These ions attach to airborne particles, giving them a charge so they can attach to nearby surfaces such as walls or furniture, or to one another, and settle faster. However, some electronic air cleaners can produce ozone, a lung irritant.
Gas-phase air filters remove gaseous pollutants by using a material called a sorbent, such as activated carbon, to adsorb pollutants. Because these filters are targeted at one or a limited number of gaseous pollutants, they will not reduce concentrations of pollutants for which they were not designed. None are expected to remove all of the gaseous pollutants in the air of a typical home. Gas-phase filters are much less common in homes than are particle air filters. One reason may be the filter can become overloaded quickly and may need to be replaced often.
Three types of air cleaners on the market are designed to deactivate or destroy indoor air pollutants: ultraviolet germicidal irradiation (UVGI) cleaners, photocatalytic oxidation (PCO) cleaners, and ozone generators sold as air cleaners.
UVGI cleaners use ultraviolet radiation from UV lamps that may destroy biological pollutants such as viruses, bacteria, and moulds that are airborne or growing on HVAC surfaces (e.g., cooling coils, drain pans, or ductwork). UVGI cleaners should be used with, but not as a replacement for, filtration systems. Typical UVGI cleaners used in homes have limited effectiveness in killing bacteria and moulds. Effective destruction of some viruses and most mould and bacterial spores usually requires much higher UV exposures than a typical home unit provides.
PCO cleaners use UV lamps along with a substance, called a catalyst that reacts with the light. These cleaners are designed to destroy gaseous pollutants by changing them into harmless products, but they are not designed to remove particulates. The usefulness of PCO cleaners in homes is limited because currently available catalysts are ineffective in destroying gaseous pollutants in indoor air.
Ozone generators use UV lamps or electrical discharges to produce ozone that reacts with chemical and biological pollutants and transforms them into harmless substances. Ozone is a potent lung irritant, which in concentrations that do not exceed public health standards, has little potential to remove indoor air contaminants. Thus ozone generators are not always safe and effective in controlling indoor air pollutants.
Portable air cleaners generally contain a fan to circulate the air and use one or more of the air-cleaning technologies discussed above. They may be an option if a home is not equipped with a furnace or a central air-conditioning system. Many portable air cleaners have moderate to large air delivery rates for small particles. However, most of the portable air cleaners on the market do not have high enough air delivery rates to remove large particles such as pollen and particles that contain dust mite and cockroach allergens from typical-size rooms.
Several other factors should be considered when making decisions about using air-cleaning devices.
• Installation: In-duct air-cleaning devices have certain installation requirements that must be met, including sufficient access for inspection during use, repairs, and maintenance.
• Major costs: These costs include the initial purchase price and the cost of maintenance (such as cleaning or replacing filters and parts) and operation (electricity).
• Odours: Air-cleaning devices designed to remove particles cannot control gases and some odours. The odour and many of the carcinogenic gas-phase pollutants from tobacco smoke, for example, will remain.
• Soiling of walls and other surfaces: Typical ion generators are not designed to remove from the air the charged particles that they generate. These charged particles may settle on, and soil, walls and other room surfaces.
• Noise: Noise may be a problem with portable air cleaners that contain fans. Portable air cleaners that do not have fans tend to be much less effective than units that have them.
The ability to remove some airborne pollutants, including microorganisms, is not, in itself, an indication of an air-cleaning device’s ability to reduce adverse health effects from indoor pollutants. Although air-cleaning devices may help reduce levels of smaller airborne particles including those associated with allergens, they may not reduce adverse health effects, especially in sensitive populations such as children, people who have asthma and allergies, and the elderly. For example, the evidence is weak that air-cleaning devices are effective in reducing asthma symptoms associated with small airborne particles such as those that contain cat and dust mite allergens. There are no studies linking the use of gas-phase filtration, UVGI systems, or PCO systems in homes to reduced health symptoms in sensitive populations.
Three Strategies
Three basic strategies to reduce pollutant concentrations in indoor air are source control, ventilation, and air cleaning.
The use of air cleaners alone cannot ensure adequate air quality.
Ventilation is also a strategy for decreasing indoor air pollutant concentrations. It exchanges air between the inside and outside of a building. The introduction of outdoor air is important for good air quality. In a process known as infiltration, outdoor air flows into the house through openings, joints, and cracks in walls, floors, and ceilings, and around windows and doors. Natural ventilation describes air movement through open windows and doors. Most residential forced air-heating systems and air-conditioning systems do not bring outdoor air into the house mechanically. Two primary ventilation methods can be used in most homes: general ventilation and local ventilation.
• General ventilation of the living space, by way of infiltration, natural ventilation, or mechanical ventilation, brings outdoor air indoors, circulates air throughout the home, and exhausts polluted air outdoors. Although limited by weather conditions, this method removes or dilutes indoor airborne pollutants, thereby reducing the level of contaminants and improving indoor air quality (IAQ). Special consideration should be given to the outdoor air used for ventilation. It should be of acceptable quality and should not contain pollutants in quantities that would be considered objectionable or harmful if introduced indoors. The use of ventilation to reduce indoor air pollutants should be evaluated carefully where there may be outdoor sources of pollutants.
• Localized ventilation by means of exhaust fans in bathrooms and kitchens, and in some cases by open windows and doors, removes excess moisture and strong, local pollutants and keeps them from spreading to other areas. Using exhaust fans increases the amount of outdoor air that enters a house.
Advanced designs for new homes are starting to add a mechanical feature that brings outdoor air into the home through the HVAC system. Some of these designs include energy efficient heat recovery ventilators to mitigate the cost of cooling and heating this air during the summer and winter.
Air cleaning may be useful when used along with source control and ventilation, but it is not a substitute for either method. The use of air cleaners alone cannot ensure adequate air quality, particularly where significant sources are present and ventilation is insufficient. While air cleaning may help control the levels of airborne particles including those associated with allergens and, in some cases, gaseous pollutants in a home, air cleaning may not decrease adverse health effects from indoor air pollutants.
Types of Air Cleaners
Various technologies can be used in air-cleaning devices. Filtration and electrostatic attraction are effective in removing airborne particles. Adsorption or chemisorption captures some gaseous and vaporous contaminants. Some air cleaners use ultraviolet light (UV) technology. Ultraviolet germicidal irradiation (UVGI) has been used to kill some microorganisms growing on surfaces. Photocatalytic oxidation (PCO), another UV light technology under development, has the potential to destroy gaseous contaminants. Ozone-generating devices sold as air cleaners use UV light or corona discharge and are meant to control indoor air pollutants.
Table 1 provides a brief summary of air-cleaning technologies and the pollutants they are designed to control. Some air-cleaning devices are designed to be installed in the ductwork of HVAC systems or to be used in portable, stand-alone units.
In-duct or whole-house air cleaning devices typically are installed in the return ducts of HVAC systems, as shown in Figure 1. The typical furnace air filter is a simple air cleaner that captures particles in the airstream to protect fan motors, heat exchangers, and ducts from soiling. Such filters are not designed to improve indoor air quality, but the HVAC system can be upgraded by using more efficient air filters to trap additional particles. Other air-cleaning devices such as electrostatic precipitators, UV lamps, and gas-phase filters use sorption and chemical reaction and are sometimes used in the ductwork of home HVAC systems.
The fans in residential HVAC systems may operate intermittently or continuously. Continuous operation improves air circulation and air cleaning, but this operation mode also increases electrical energy consumption and costs.
Portable air cleaners are available as small tabletop units and larger console units. They are used to clean the air in a single room, but not in an entire house. The units can be moved to wherever continuous and localized air cleaning is needed. Larger console units may be useful in houses that are not equipped with forced air-heating systems and air-conditioning systems. Portable air cleaners generally have a fan to circulate the air and a cleaning device such as a mechanical air filter, electrostatic precipitator, ion generator, or UV lamp.
Some units marketed as having the quietest operation may have no fan; however, units that do not have a fan typically are much less effective than units that have one. Air cleaners may also have a panel filter with bonded fine particles of activated carbon, or an activated carbon filter encased in a frame, to remove gases and odours. Some portable air cleaners referred to as hybrid air cleaners use a combination of two or more of the devices discussed above.
Courtesy: Environmental Protection Agency