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

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Inhalation

Calculations

Using EPA’s current methodology, it is unnecessary to calculate an inhaled dose when using dose-response factors from IRIS in a risk assessment. However, inhalation risk assessments may require that an adjusted air concentration be used to represent continuous exposure. The adjusted air concentration (Cair-adj) may be estimated as shown below. As described under the Methods tab, for noncarcinogens, the air concentration is adjusted based on the time over which exposure occurs (i.e., the exposure duration). For carcinogens, the concentration is averaged over the lifetime of the exposed individual (often assumed to be 70 years).

Cair-adj=Cair x ET x 1day/24 hours x EF x ED/AT
Where:
Cair = Concentration of contaminant in air (mg/m3)
ET = Exposure time (hours/day)
EF = Exposure frequency (days/year)
ED = Exposure duration (years)
AT = Averaging time (days)

  • The concentration in air (Cair) is either a measured or modeled value. Air concentrations may be measured in the breathing zone of individuals using personal monitoring equipment or in indoor or outdoor air using stationary or portable monitoring devices. Air measurements may represent gas phase or particulate-phase contaminants, or both.
  • Temporal parameters in the equation include the following:
    • Exposure time (ET) and exposure frequency (EF) refer to the frequency with which the exposure occurs and might be provided in hours per day and days per year, respectively.
    • Exposure duration (ED) is the amount of time that an individual or population is exposed to the contaminant being evaluated and is typically given in years.
    • Averaging time (AT) is the amount of time over which exposure is averaged and is equal to ED for assessing non-cancer risks. For chronic assessments (e.g., cancer), potential lifetime average daily dose (LADD) is calculated in which lifetime (LT, in days) is substituted for AT.

In some cases, it may be necessary to calculate an inhalation dose using the equation below. This algorithm can be used to calculate the average daily potential dose from inhalation of a contaminant in air. The potential dose of a contaminant is the product of the contaminant concentration, inhalation rate, exposure time, exposure frequency, and exposure duration divided by the product of averaging time and body weight. The equation parameters below must be defined for each inhalation exposure scenario, and all parameters must be expressed in consistent units; in some cases, unit conversion factors may be necessary. Average Daily Dose (ADD) is generally expressed as mass of contaminant per unit body weight over time (e.g., mg/kg-day).

ADD = Cair x InhR x ET x EF x ED/BW x AT
Where:
ADD = Average daily dose (mg/kg-day)
Cair = Concentration of contaminant in air (mg/m3)
InhR = Inhalation rate (m3/hour)
ET = Exposure time (hours/day)
EF = Exposure frequency (days/year)
ED = Exposure duration (years)
BW = Body weight (kg)
AT = Averaging time (days)

See above for descriptions for many of the relevant equation parameters. The additional parameters are described below.

  • Inhalation rate (InhR) represents the volume of air inhaled over a specified timeframe. Long-term inhalation rates are typically expressed in units of m3/day. Short-term inhalation rates are typically indexed to activity levels and are expressed in units of m3/hour or m3/minute. Assessors should choose inhalation rate data that best represent the population for which exposures are being assessed. For example, some assessments might focus on certain subsets of the general population (e.g., older adults) whose inhalation rates might vary from those of the general population. Chapter 6 of the Exposure Factors Handbook: 2011 Edition (U.S. EPA, 2011) provides inhalation rate data for various age groups (see the Factors tab in this module).
  • Body weight (BW) of an individual, typically expressed in kilograms (kg), is also included so that the dose is normalized to that value. Sometimes the inhalation rate is already normalized to body weight (e.g., in units of m3/kg-day). In this case, a separate term for body weight would not be necessary.

Additional information on exposure scenarios involving the inhalation route can be found in the Indirect Estimation Module in the Approaches Tool Set of EPA-Expo-Box. The Exposure Calculation Spreadsheet (XLSX) (360 K) estimates the inhalation exposure concentration or inhalation dose when user-defined values are entered for the various exposure parameters that are indicated in BOLD.

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