There are a few different ways to measure dose U. EPA, :. EPA, The following general equation may be used to estimate the average daily dose average daily dose The mean amount of an agent to which a person is exposed on a daily basis, often averaged over a long period of time. EPA is transitioning from average daily dose methodologies to more refined aggregate and cumulative approaches for estimating exposure across each lifestage.
ADD from intake of food, water, soil, dust, or other non-dietary exposure from hand- or object-to-mouth contact. 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 potential dose The amount of a chemical contained in material ingested, air breathed, or bulk material applied to the skin.
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.
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. Age also affects ingestion rate. Exposure parameter inputs e. EPA, or other sources of exposure factor data see Factors tab in this module. Ingestion of contaminated media could occur over short durations e. 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 there additional parameters related to contact frequency and timeframe of exposure are incorporated into the ADD equation.
Media-specific concentrations are needed for estimating exposure via the ingestion route. Information on sampling techniques and analytical methods is available to support the measurement of contaminants in potentially ingested media. EPA ExpoBox. Contact Us. Exposure Assessment Tools by Tiers and Types. Contact Us to ask a question, provide feedback, or report a problem.
Please click here to see any active alerts. Exposure assessment is one of the four major steps in risk assessment; others include hazard identification, dose-response assessment, and risk characterization. Exposure may be estimated for any of the various exposure media: air, water and sediment, soil and dust, food, aquatic biota, or consumer products.
Alternatively, exposure assessments may be categorized according to exposure assessment approaches, exposure route, assessment tiers and types, lifestages and populations, or chemical classes.
Some of the types of transformations that can occur in the environment are shown in the figure below. Once released into the environment, the form and distribution of stressors among various media or environmental compartments can be affected by the physical and chemical properties of the stressor e.
Characteristics of the environment can also impact fate and transport. Some categories of characteristics that can influence stressor fate and transport include:. In exposure assessments, fate and transport of stressors is typically evaluated via some level of modeling.
Modeling applies mathematical representations to the processes that distribute and transform stressors in the environment. Resources are available that provide information on physicochemical properties that affect fate and transport. Other resources provide tools for evaluation of fate and transport based on media type. EPA, , exposure is dependent upon the intensity, frequency, and duration of contact. Exposure magnitude is usually expressed as the concentration of contaminant per unit mass or volume e.
Characterizing contaminant concentrations for an exposure scenario is typically accomplished using one or more of the following approaches:. Stressor concentrations are typically measured or estimated in air, water, soil, food or food webs, microenvironments, surfaces, biota, or a combination of any of these.
Environmental concentrations or exposures can be measured directly through media sampling or monitoring and analysis, or indirectly estimated using models. A common approach for quantifying exposure for risk assessment is to combine the use of environmental monitoring data with model outputs. This approach integrates measured concentrations, and the effects of fate and transport processes. Depending on the assessment, the modeling approach can be characterized in multiple ways including:.
Mechanistic i. Deterministic i. Steady-state i. Further, the fate and transport processes might be modeled based on one or more of these approaches:. Many resources are available describing modeling techniques, sampling techniques, and analytical methods employed for estimating or measuring media concentrations in air, water and sediment, soil and dust, food, aquatic biota, and consumer products.
Exposure can vary across populations. Differences in age, gender, dietary preferences, occupation, cultural practices, geographical locations and settings may affect exposures. Certain behaviors, activities, or sociodemographic factors may also be associated with differences in contact with environmental agents. For example, infants might experience higher exposures to certain types of contaminants than adults because of mouthing behaviors that increase the likelihood of ingesting soil or dust.
Older adults may be more affected by exposures to other types of environmental agents because of physiological differences associated with age. Individuals living in buildings in disrepair e. EPA, suggest that it is often helpful for risk assessors to characterize and quantify the magnitude of risk for specific highly exposed, highly sensitive, or highly susceptible subgroups within the larger population. Considering vulnerability and susceptibility is critical to protect those populations at greatest risk when making risk management decisions.
EPA, b provides guidance to help risk and exposure assessors identify and enumerate populations that may potentially experience greater contact with environmental contaminants due to unique activity patterns, preferences, behaviors and various sociodemographic. The tables below provide information to help assessors enumerate populations, based on a variety of population characteristics.
The intent is to provide data for selected populations of concern in common potential exposure scenarios—not for every possible population group.
In some cases, these resources can be used directly to quantify a population of interest e. Census Bureau data on the number of individuals in a certain age group. In other cases, the resources can be used to help characterize potential exposure for a population in a certain category e.
Tools for estimating exposure among specific population groups e. About the Exposure Factors Handbook. Exposure factors are factors related to human behavior and characteristics that help determine an individual's exposure to an agent.
These factors include, rates of ingestion e. Information from the Handbook and data and resources used in developing the recommendations provided in the Handbook may be accessed using the the tables provided on the About the Exposure Factors Handbook page.
It is important to note that these recommendations are not legally binding on any U. EPA program and should be interpreted as suggestions that program offices or individual exposure assessors can consider and modify as needed. A selected number of these tables are available in spreadsheet format. These tables were selected because they contain distributional information that may be suitable for probabilistic analyses.
There are similar efforts in other countries that provide data on exposure factors. For example, the European Union developed the ExpoFacts database which contains data from 30 European countries.
Approaches for quantifying exposure vary depending on the level of refinement or complexity required. Directly related to the level of refinement incorporated into an assessment is whether the results of the assessment are a point estimate or a distribution of possible values.
The Tiers and Types Tool Set provides further discussion and links to resources related to these methods. Exposure descriptors are estimates for a specific point on the exposure distribution e. Exposures vary due to differences among individuals, populations, spatial and temporal scales, and other factors.
Exposure descriptors are useful when characterizing exposure and can help exposure assessors communicate with risk managers. Exposure Descriptors. Source: U. EPA, Exposure scenarios can be developed to derive a bounding estimate that captures the highest possible exposure, or theoretical upper bound, for a given exposure pathway.
Bounding estimates are often used to perform screening-level assessments. If the highest possible exposure is evaluated and found to be not of concern, other potential lower exposures will also not be of concern.
Upper percentile values are selected for the key input parameters to the exposure or dose equation. The combination of these assumptions results in a highly conservative exposure estimate. Exposure scenarios can be developed to derive high-end estimates of exposure. These are generally considered to be more realistic or more likely to occur compared with bounding estimates.
They are often calculated using a combination of high and central inputs for exposure parameters. High-end estimates of exposure are, by definition, intended to fall within the actual distribution, rather than above it.
Estimates above the distribution are bounding estimates U. The following descriptors all account for individuals at the high end of the exposure distribution at or above the 90th percentile :. These terms all refer to exposures that are within the population distribution and not outside the distribution.
The terms are expected to describe "an individual who exists, or is thought to exist, in the population.
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