EPA-Expo-Box (A Toolbox for Exposure Assessors)
The following algorithm may be used for estimating the average daily potential dose from ingestion of food (including aquatic biota), water, or soil and dust. The potential dose of a contaminant is the product of the contaminant concentration, ingestion rate, exposure duration, and exposure frequency divided by the averaging time and body weight. The equation parameters below must be defined for each ingestion 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).
- Cmedium is the concentration in water, food, soil/dust, or aquatic biota; either a measured or modeled value.
- Ingestion rate (IngR) is the amount of food, water, or soil/dust containing the contaminant that an individual ingests during a specific period of time; expressed in units of mass or volume per unit time, such as g/day or L/day.
- IngR units should be consistent with the concentration term used in the exposure algorithm. For example, concentrations expressed in terms of whole, wet, or fresh weight (i.e., mg of contaminant per g of whole food) should be used with ingestion rates indexed to the whole weight of the food consumed (i.e., g of whole food consumed per day). Likewise, dry weight concentrations or lipid weight concentrations should be used with ingestion rates indexed to dry or lipid weights, respectively.
- 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 ingestion rate is already normalized to body weight (e.g., in units of mg/kg-day). In this case, a separate term for body weight would not be necessary.
- Temporal parameters in the dose equation include:
- Exposure frequency (EF) refers to the frequency with which the exposure occurs and might be provided in days per year or events per day.
- Exposure duration (ED) is the amount of time that an individual or population is exposed to the contaminant being evaluated and is typically provided 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.
Ingestion Rates Vary within Populations
Assessors should choose ingestion 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., farmers, recreational fishermen) whose ingestion rates might vary from those of the general population. Age also affects ingestion rate.
Exposure parameter inputs (e.g., ingestion rates, body weights, and relevant patterns of behavior such as timeframe of exposure) can be obtained from the Exposure Factors Handbook: 2011 Edition (U.S. EPA, 2011) or other sources of exposure factor data (see Factors tab in this module). Ingestion of contaminated media could occur over short durations (e.g., breastfeeding infant) or longer time periods (e.g., residence time for a receptor consuming homegrown food).
For ingestion of surface residues from hand-to-mouth or object–to-mouth contact, the following algorithm for average daily potential dose may be used.
For non-dietary ingestion exposures to chemical residues on surfaces or objects that are contacted via hand-to-mouth or object-to-mouth activity, additional parameters related to contact frequency and timeframe of exposure are incorporated into the ADD equation.
- Contact rate (CR) represents either the surface area of the hand or the surface area of the object being mouthed.
- Event frequency (EV) represents the number of times the hand or object is mouthed over a specified period of time (e.g., events /hour).
- Exposure time (ET) is added to represent the timeframe over which the transfer of residues from the surface or object to the receptor occurs.
Additional information on exposure scenarios involving the ingestion route can be found in the Indirect Estimation Module in the Approaches Tool Set of EPA-Expo-Box. The Exposure Calculation Spreadsheet (XLSX) (581 K) estimates ingestion dose when user-defined values are entered for the various exposure parameters that are highlighted in BOLD.