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EPA Published Articles

Indoor air pollution is among the top five environmental health risks. Usually the best way to eliminate this

U.S. Environmental Protection Agency

Office of Air and Radiation

Indoor Environments Division

1200 Pennsylvania Avenue, NW Mail code: 6609J

Washington, DC 20460



This document has been reviewed in accordance with U.S. Environmental Protection Agency policy and approved for publication. Mention of trade names, products, or services does not convey, and should not be interpreted as conveying official EPA approval, endorsement or recommendation.



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.


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 mold 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.


Another type of air-cleaning device is a gas-phase filter designed to remove gases and odors by either physical or chemical processes.


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 xpected 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 molds 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 molds. Effective destruction of some viruses and most mold 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).
  • *  Odors: Air-cleaning devices designed to remove particles cannot control gases and some odors. The odor 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.

5. 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.




The best way to address residential indoor air pollution usually is to control or eliminate the source of the pollutants and to ventilate the home with clean outdoor air. But ventilation may be limited by weather conditions or the levels of contaminants in the outdoor air.


If the usual methods of dealing with indoor air pollutants are insufficient, air-cleaning devices may be useful. Air filters and other air-cleaning devices are designed to remove pollutants from indoor air. They can be installed in the ductwork of most home heating, ventilating, and air-conditioning (HVAC) systems to clean the air in the entire house, or the same technology can be used in portable air cleaners that clean the air in single rooms or specific areas. Most air-cleaning devices are designed to remove particles or gases, but some destroy contaminants that pass through them.


This publication focuses on air cleaners for residential use; it does not address air cleaners used in large or commercial structures such as office buildings, schools, large apartment buildings, or public buildings. It should be particularly useful to residential housing design professionals, public health officials, and indoor air quality professionals. In addition to providing general

information about the types of pollutants affected by air cleaners, this document discusses:

  • *  The effectiveness of air cleaning compared to other strategies, such as source control and ventilation, for reducing indoor air pollutants.
  • *  The types of air-cleaning devices available.
  • *  Guidelines that can be used to compare air- cleaning devices.
  • *  The effectiveness of air-cleaning devices in removing indoor air pollutants.
  • *  General information on the health effects of indoor air pollutants.
  • *  Additional factors to consider when deciding whether to use an air-cleaning device.


Please Note: The U.S. Environmental Protection Agency (EPA) neither certifies nor recommends particular brands of home air-cleaning devices. While some home air-cleaning devices may be useful in some circumstances, EPA makes no broad endorsement of their use, nor specific endorsement of any brand or model. This document describes the performance characteristics of several types of air cleaners sold for in-home use.


Federal pesticide law requires manufacturers of ozone generators to list an EPA establishment number on the product’s packaging. This number merely identifies the facility that manufactured the product. Its presence does not imply that EPA endorses the product, nor does it imply that EPA has found the product to be safe or effective.


Some portable air cleaners sold in the consumer market are ENERGY STAR® qualified. Please note the following disclaimer on their packaging: “This product earned the ENERGY STAR by meeting strict energy efficiency guidelines set by EPA. EPA does not endorse any manufacturer claims of healthier indoor air from the use of this product.”




There are two categories of indoor air pollutants that can affect the quality of air in a home: particulate matter and gaseous pollutants.


Particulate matter (PM) is composed of microscopic solids, liquid droplets, or a mixture of solids and liquid droplets suspended in air. Also known as particle pollution, PM is made up of a number of components, including acids such as nitric and sulfuric acids, organic chemicals, metals, soil or dust particles, and biological contaminants. Among the particles that can be found in a home are:

  • *  Dust as solid PM or fumes and smoke, which are mixtures of solid and liquid particles.
  • *  Biological contaminants,  including viruses, bacteria, pollen, molds, dust mite and cockroach body parts and droppings, and animal dander.


Particles come in a wide range of sizes. Small particles can be fine or coarse. Of primary concern from a health standpoint are fine particles that have a diameter of 2.5 micrometers (μm) or less. These particles (described as “respirable”) can be inhaled; they penetrate deep into the lungs where they may cause acute or chronic health effects. Coarse particles, between 2.5 and 10 μm in diameter, usually do not penetrate as far into the lungs; they tend to settle in the upper respiratory tract. Large particles are greater than 10 μm in diameter, or roughly one-sixth the width of a human hair. They can be trapped in the nose and throat and expelled by coughing, sneezing, or swallowing.


Respirable particles are directly emitted into indoor air from a variety of sources including tobacco smoke, ozone reactions with emissions from indoor sources of organic compounds, chimneys and flues that are improperly installed or maintained, unvented combustion appliances such as gas stoves and kerosene or gas space heaters, woodstoves, and fireplaces. This category of particles also includes viruses and some bacteria.

Among the smaller biological particles found in a home are some bacteria, mold fragments and spores, a significant fraction of cat and dog dander, and a small portion of dust mite body parts and droppings. Larger particles include dust, pollen, some mold fragments and spores, a smaller fraction of cat and dog dander, a significant fraction of dust mite body parts and cockroach body parts and droppings, and skin flakes.

Gaseous pollutants include combustion gases and organic chemicals that are not attached to particles. Hundreds of gaseous pollutants have been detected in indoor air.

Sources of indoor combustion gases such as carbon monoxide and nitrogen dioxide include combustion appliances, tobacco smoke, and vehicles whose exhaust infiltrates from attached garages or the outdoors.


Sources of airborne gaseous organic compounds include tobacco smoke, building materials and furnishings, and products such as paints, adhesives, dyes, solvents, caulks, cleaners, deodorizers, cleaning chemicals, waxes, hobby and craft materials, and pesticides. Organic compounds may also come from cooking food; from human, plant, and animal metabolic processes; and from outdoor sources. Some electronic air cleaners and laser printers may generate the lung irritant ozone by design or as a by-product.


Radon is a colorless, odorless, radioactive gas that can be found in indoor air. It comes from uranium in natural sources such as rock, soil, ground water, natural gas, and mineral building materials.  As uranium breaks down, it releases radon, which in turn produces short-lived radioactive particles called “progeny,” some of which attach to dust particles. Radon progeny may deposit in the lungs and irradiate respiratory tissues. Radon typically moves through the ground and into a home through cracks and holes in the foundation. Radon may also be present in well water and can be released into the air when that water is used for showering and other household activities. In a small number of homes, building materials also can give off radon.




Three basic strategies to reduce pollutant concentrations in indoor air are source control, ventilation, and air cleaning.


Source control eliminates individual sources of pollutants or reduces their emission. It is usually the most effective strategy for reducing pollutants. There are many sources of pollutants in the home that can be controlled or removed.2 For example, solid wood or alternative materials can be used in place of pressed wood products that are likely to be significant sources of formaldehyde. Smokers can smoke outdoors. Combustion appliances can be adjusted to decrease their emissions.


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 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 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 


The use of air cleaners alone cannot ensure adequate air quality.


Air cleaning may be useful when used along with source controland 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, air outdoors. Although limited by weather conditions, this method removes or dilutes indoor airborne pollutants, thereby reducing in some cases, gaseous pollutants in a home, air cleaning may not decrease adverse health effects from indoor air pollutants.




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......

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