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EPA-Expo-Box (A Toolbox for Exposure Assessors)

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Contamination of ambient or indoor air can occur from anthropogenic sources or natural sources.

  • Anthropogenic sources include point or stationary sources, area sources, mobile sources, and various indoor sources.
  • Natural sources of air contamination include the earth’s crust, wildfires, volcanic eruptions, and biota. Some natural sources (e.g., radon gas from the earth’s crust) may become indoor air contaminants as a result of vapor intrusion through building foundations or basements.
Tools for assessing exposure to contaminants from various outdoor and indoor sources are provided in this module.

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Outdoor Air

Outdoor Sources of Contaminants Diagram

Outdoor Sources of Contaminants
Source: U.S. EPA

There are many sources of outdoor air pollution as described below.

  • Point sources are a type of stationary source that include factories and other manufacturing plants, incinerators, and furnaces.
  • Area sources are stationary sources and include small businesses such as dry cleaners and auto body shops, home heating units such as woodstoves, and outdoor burning. Emissions generated by agricultural and forestry practices such as windblown dust from tillage, ammonia from animal wastes, and prescribed burning of crop residue are also classified as area sources and can be important contributors to the total inventory of releases.
  • Mobile sources of air contamination include on-road vehicles and engines (e.g., cars, trucks, buses, motorcycles) and nonroad vehicles and engines (e.g., locomotives, watercraft, aircraft, lawn and garden equipment). EPA's Office of Transportation and Air Quality (OTAQ) provides information on transportation-related sources.
  • Examples of natural sources of air pollution include gases emitted by the earth’s crust (e.g., radon); smoke and carbon dioxide (CO2) released during forest fires; particulate matter and gases from volcanic eruptions; volatile organic compounds (VOCs) released by some vegetation; and methane from digestive processes of some animals.
Under the Clean Air Act, EPA has set national air quality standards for six common air pollutants (see http://www.epa.gov/air/emissions/ for more information):

  • Carbon monoxide (CO)
  • Ozone (O3)
  • Lead
  • Particulate matter (PM)
  • Nitrogen dioxide (NO2)
  • Sulfur dioxide (SO2)
Data banks such as Toxics Release Inventory (TRI) provide information on onsite releases and disposal of toxic chemicals to air that are reported annually by industries. These types of data sets may be used to identify the sources and levels of specific air toxics of concern and to evaluate potential exposures and risks.

Emission factors are fundamental tools used in developing inventories of air pollutant sources. They can be used in estimating releases to the environment, making air quality management decisions, and developing emission control strategies.

  • EPA’s AP-42 provides emission factors for various source categories (i.e., industry sectors or groups of similarly-emitting sources). These emission factors have been developed from source test data, material balance studies, and engineering estimates.
  • The Clearinghouse for Inventories & Emissions Factors (CHIEF) provides information on emissions inventories and emissions factors.
Resources that describe sources and releases to outdoor air are provided below.

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Indoor Air

Indoor Sources of Contaminants Diagram

Indoor Sources of Contaminants
Source: U.S. EPA

There are many sources of indoor air contaminants. These contaminants can enter buildings from the outside or are generated inside (see figure)—and people spend a lot of time indoors. According to Chapter 16 of EPA’s Exposure Factors Handbook: 2011 Edition, an adult aged 18–64 years spends on average 1,159 minutes (i.e., more than 19 hours) indoors per day (U.S. EPA, 2011). An average child aged <18 years spends even more time indoors per day (U.S. EPA, 2011). Of course, these estimates might vary from region to region and by season.

Indoor-generated air pollution can result from the following:

  • Combustion of oil, gas, kerosene, coal, wood, and tobacco products
  • Building materials and furnishings
  • Consumer products (e.g., products used for household cleaning and maintenance, personal care, or hobbies)
  • Central heating and cooling systems and humidification devices
Outdoor pollution can also penetrate into a building. Vapor intrusion generally occurs when there is a migration of volatile chemicals from contaminated groundwater or subsurface soil into an overlying building. Volatile chemicals can enter buildings through cracks in the foundation and openings for utility lines. Radon and ozone are examples of two pollutants formed outside the home that can travel to indoor living spaces and become entrapped in the indoor environment. Radon is generated by certain types of rock and can accumulate in (or under) a home following emission from the ground. (See the Inorganics and Fibers Module in the Chemical Classes Tool Set for resources used to assess exposure to radon.)

Some of these sources of indoor air pollutants like building materials, furnishings, and household products like air fresheners release pollutants more or less continuously; others—such as smoking, use of household cleaning products, and use of space heaters—release pollutants intermittently. Indoor contaminants are primarily advected and dispersed on a relatively large scale by heating and air conditioning systems or natural ventilation. (See the Chemical Classes Tool Set of EPA-Expo-Box for additional information and resources for specific contaminants, including asbestos, radon, lead, pesticides, and flame retardants.)

EPA’s Indoor Air Quality (IAQ) website contains information on various indoor air contaminants, including:

  • Asbestos
  • Pesticides
  • Biological pollutants (e.g., mold)
  • Radon
  • Environmental tobacco smoke
  • Respirable particles
  • Lead
The IAQ website also provides links to several resources, including Tools for Schools and the Indoor Building Education and Assessment (I-BEAM) tool. Descriptions for these resources and others that describe sources and releases to indoor air and general information on indoor air quality are included in the table below.

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