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

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To estimate human exposure to contaminants in ambient or indoor air, information is needed about the exposed population(s), exposure pathways, and the concentrations of contaminants in air. When determining inhalation risk, inhalation rate and body weight might not be needed. The Superfund Program’s updated approach for determining inhalation risk eliminates use of inhalation rates when evaluating exposure to air contaminants (U.S. EPA, 2009). The updated methodology recommends using contaminant concentrations in air as the exposure metric (e.g., in mg/m3) instead of the intake of a contaminant in air based on inhalation rate and body weight (e.g., mg/kg-day). Inhalation dose-response factors like the IRIS and Superfund reference concentrations (RfCs) and inhalation unit risks (IURs) incorporate inhalation rate into their values.

Microenvironments are the various changing spaces in which people spend their time that will influence how they will be exposed to contaminants. A microenvironment can be defined as a space with a contaminant concentration that is assumed to be relatively well-mixed, homogeneous, and temporally constant while an individual is located within the microenvironment. Examples of microenvironments included in EPA assessments are automobiles, schools, work places, and other buildings. The Exposure Factors Handbook: 2011 Edition presents data on the amount of time that an individual is expected to spend in various microenvironments. For buildings, important factors might include size, volume, number of windows, and air exchange rates, and the Handbook provides information about these factors for both residential and nonresidential buildings (U.S. EPA, 2011). 

When inhalation rates are needed, data are available in Chapter 6 of EPA’s Exposure Factors Handbook: 2011 Edition (U.S. EPA, 2011). Inhalation rate is a volume of air inhaled per unit time. Long-term inhalation rates are reported as daily rates in units of m3/day or m3/kg-day (adjusted for body weight). Short-term rates are described for various activities (e.g., walking, running, playing, housework, yard work) or activity levels (e.g., light, moderate, heavy) in units of m3/minute.

Characteristics of the exposed individual and population are important to define for inhalation scenarios because they determine inhalation rate. For example, inhalation rates will vary between children and adults due to differences in size, physiology, behavior, and activity levels. Individuals in an occupational setting are often assumed to be working harder than residents and so might be assumed to have a higher inhalation rate. Personal characteristics also dictate activity patterns, which determine where a person is located (i.e., his/her microenvironments; see text box) and the intensity of the activities in which the individual is engaged.

The Exposure Factors Tab of the Indirect Estimation Module provides links to data on inhalation rates.

Activity-specific factors that may be relevant for assessing inhalation exposures associated with specific activity levels are available in Chapter 16 of the Handbook. These might include amount of time spent in various microenvironments (e.g., indoors at home, indoors at school, outdoors, in vehicle) or time spent doing specific activities (e.g., household work, physical leisure activities, passive leisure activities).

Other exposure factors that might be needed for assessing inhalation exposures include:

  • Body weight (Chapter 8)
  • Consumer product use (Chapter 17)
  • Life expectancy values, specifically when evaluating cancer risk (Chapter 18)
  • Building characteristics when assessing indoor air exposures (Chapter 19)

Inhalation exposure factors should be selected to represent the age, gender (if appropriate), timeframe/activity level, and population group (e.g., resident, occupational worker) for the exposure scenario of interest. Exposure factors related to inhalation exposures are also described in the Soil and Dust, Water and Sediment, and Consumer Products Modules in the Media Tool Set of EPA-Expo-Box.

The Exposure Factors Tab of the Indirect Estimation Module provides links to data on other factors.

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