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.
There are many sources of outdoor air pollution as described below.
Outdoor Sources of Contaminants
Source: U.S. EPA's Air Pollution Training Institute (APTI) Learning Management System
- 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.
- Carbon monoxide (CO)
- Ozone (O3)
- Particulate matter (PM)
- Nitrogen dioxide (NO2)
- Sulfur dioxide (SO2)
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.
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
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:
- Biological pollutants (e.g., mold)
- Environmental tobacco smoke
- Respirable particles