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Report on the Environment

Indoor Air

Other Air Topics

What are the trends in indoor air quality and their effects on human health?

"Indoor air quality" refers to the quality of the air in a home, school, office, or other building environment. Most pollutants affecting indoor air quality come from sources inside buildings, although some originate outdoors. Typical pollutants of concern include combustion products such as carbon monoxide, particulate matter, and environmental tobacco smoke; substances of natural origin such as radon; biological agents such as molds; pesticides; lead; asbestos; ozone (from some air cleaners); and various volatile organic compounds from a variety of products and materials. Indoor concentrations of some pollutants have increased in recent decades due to such factors as energy-efficient building construction and increased use of synthetic building materials, furnishings, personal care products, pesticides, and household cleaners.

The potential impact of indoor air quality on human health nationally is considerable, for several reasons. Americans, on average, spend approximately 90 percent of their time indoors,29 where the concentrations of some pollutants are often 2 to 5 times higher than typical outdoor concentrations.30 Moreover, people who are often most susceptible to the adverse effects of pollution (e.g., the very young, older adults, people with cardiovascular or respiratory disease) tend to spend even more time indoors.31 Health effects that have been associated with indoor air pollutants include irritation of the eyes, nose, and throat; headaches, dizziness, and fatigue; respiratory diseases; heart disease; and cancer.

Indoor air pollutants originate from many sources. These sources can be classified into two general categories:

  • Indoor sources (sources within buildings themselves). Combustion sources in indoor settings, including tobacco, heating and cooking appliances, and fireplaces, can release harmful combustion byproducts such as carbon monoxide and particulate matter directly into the indoor environment. Cleaning supplies, paints, insecticides, and other commonly used products introduce many different chemicals, including volatile organic compounds, directly into the indoor air. Building materials are also potential sources, whether through degrading materials (e.g., asbestos fibers released from building insulation) or from new materials (e.g., chemical off-gassing from pressed wood products). Other substances in indoor air are of natural origin, such as mold and pet dander.

  • Outdoor sources. Outdoor air pollutants can enter buildings through open doors, open windows, ventilation systems, and cracks in structures. Some pollutants come indoors through building foundations. For instance, radon forms in the ground as naturally occurring uranium in rocks and soils decays. The radon can then enter buildings through cracks or gaps in structures. In areas with contaminated ground water or soils, volatile chemicals can enter buildings through this same process. Volatile chemicals in water supplies can also enter indoor air when building occupants use the water (e.g., during showering, cooking). Finally, when people enter buildings, they can inadvertently bring in soils and dusts on their shoes and clothing from the outdoors, along with pollutants that adhere to those particles.

In addition to pollutant sources, the air exchange rate with the outdoors is an important factor in determining indoor air pollutant concentrations. The air exchange rate is affected by the design, construction, and operating parameters of buildings and is ultimately a function of infiltration (air that flows into structures through openings, joints, and cracks in walls, floors, and ceilings and around windows and doors), natural ventilation (air that flows through opened windows and doors), and mechanical ventilation (air that is forced indoors or vented outdoors by ventilation devices, such as fans or air handling systems). Outdoor climate and weather conditions combined with occupant behavior can also affect indoor air quality. Weather conditions influence whether building occupants keep windows open or closed and whether they operate air conditioners, humidifiers, or heaters, all of which can impact indoor air quality. Weather also has a large effect on infiltration. Certain climatic conditions can increase the potential for indoor moisture and mold growth if not controlled by adequate ventilation or air conditioning.

The link between some common indoor air pollutants and health effects is very well established. Radon is a known human carcinogen and is the second leading cause of lung cancer.32,33 Carbon monoxide is toxic, and short-term exposure to elevated carbon monoxide levels in indoor settings can be lethal.34 Episodes of Legionnaires’ disease, a form of pneumonia caused by exposure to the Legionella bacterium, have been associated with buildings with poorly maintained air conditioning or heating systems.35,36 In addition, numerous indoor air pollutants—dust mites, mold, pet dander, environmental tobacco smoke, cockroach allergens, and others—are “asthma triggers,” meaning that some asthmatics might experience asthma attacks following exposure.37

While these and other adverse health effects have been attributed to specific pollutants, the scientific understanding of some indoor air quality issues continues to evolve. One example is “sick building syndrome,” which occurs when building occupants experience similar symptoms after entering a particular building, with symptoms diminishing or disappearing after they leave the building; these symptoms are increasingly being attributed to a variety of building indoor air attributes.

Researchers also have been investigating the relationship between indoor air quality and important issues not necessarily related to health, such as student performance in the classroom and productivity in occupational settings.38 Another evolving area is research in “green building” design, construction, operation, and maintenance that achieves energy efficiency and enhances indoor air quality.

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