The Use of Exposure Source Allocation Factor in The Risk Assessment of Drinking-Water Contaminants

Abstract

In the risk assessment process, the reference dose, tolerable intake, or acceptable daily intake (RfD, TDI, ADI) is apportioned to specific exposure sources on the basis of a source allocation factor (AF) or relative source contribution (RSC). The U.S. Environmental Protection Agency (EPA) published an exposure decision tree framework in 2000 to guide the determination of AF (or RSC) of drinking-water contaminants (DWC). Besides that, there has not been any systematic analysis of the basis of the use of AF in DWC risk assessments. This article therefore critically reviews and integrates current knowledge and approaches for the development of AF, while focusing on its consistent use in DWC risk assessments based on consideration of (i) risk assessment endpoint, (ii) existing guidelines, (iii) exposure estimates, (iv) usage pattern and environmental fate information, (v) physicochemical properties, (vi) bounds of AF, (vii) multiroute exposures, and (viii) target population characteristics. Accordingly, for a DWC for which drinking water is not a major source of exposure and for which there is documented evidence of widespread presence in one or more of the other media (i.e., air, food, soil, or consumer products), the use of an AF value of 0.2 is suggested. For DWC for which drinking water represents nearly the single major source of exposure, a ceiling AF value of 0.8 is suggested. For other situations, chemical- and context-specific AF values can be developed based on exposure data or models, which should in turn be bounded by the floor and ceiling AF values as originally described by the U.S. EPA (i.e., 0.2-0.8). Future studies need to focus on improvements in methods for deriving AF, by basing it on the consideration of bioavailability, target tissue dose, and extent of route-specific absorption, as well as improvement in the modeling of dose received via direct/voluntary exposure through consumer products and at workplaces.

Deriving A Drinking Water Guideline for A Non-Carcinogenic Contaminant: The Case of Manganese

“…The relative source contribution (RSC) of drinking water corresponds to a fraction of the TRV to which the exposure resulting from the use of drinking water can contribute. Indeed, it is necessary from a public health perspective to ensure that this contribution does not result into exceeding the TRV if summed to the exposure resulting from the non-drinking water potential sources [ 1 , 41 , 42 , 43 , 44 ]. Given the default assumption that total exposure to an environmental contaminant may come from 5 medias (drinking water, air, soil, food and household products; [ 1 , 41 ]), it is considered that up to one fifth of this total exposure can be attributed to drinking water, thus triggering a default 20% RSC value for all age groups.…”

“…In certain circumstances, for which further details are provided in Supplemental materials , an RSC value that differ from this 20% default can be chosen. That is, a RSC greater than 20% is justifiable when drinking water is believed to be the main or the only source of exposure, while an RSC lower than 20% can be attributed when it is estimated that the other exposure sources than drinking water, notably food, may contribute in total, to more than 80% of the TRV [ 1 , 41 , 42 , 43 , 44 ]. Manganese being an essential element and given that its main exposure source is in fact food, its RSC was herein validated based on data of its concentrations found in cow or soya milk formula, which constitutes the main manganese dietary exposure source for non-breast-fed neonates.…”

A Method for Developing Rapid Screening Values for Active Pharmaceutical Ingredients (APIs) in Water and Results of Initial Application for 119 APIs

“…An RSC is used to account for exposure other than ingestion of water (e.g., inhalation of volatilized chemicals, dermal absorption) as well as exposure from other media (e.g., diet) to ensure that the cumulative exposure does not exceed the RfD, in this case the sRfD [ 25 ]. MDH used the EPA Exposure Decision Tree [ 25 , 28 ] to identify the appropriate RSC value [ 28 ]. Within the EPA Decision Tree framework, RSCs can range from 0.2 up to 0.8.…”

“…Within the EPA Decision Tree framework, RSCs can range from 0.2 up to 0.8. The EPA methodology uses a ceiling of 0.8 (80%) and minimum of 0.2 (20%) so that no more than 80% nor less than 20% of the RfD can be accounted for from ingestion of water at the developed guidance value [ 28 ]. WSVs were calculated using an RSC of 0.8 for the majority of APIs, based on the assumption that individuals not taking a prescription medication could receive the majority of their exposure through drinking water.…”

An analysis of the use of the relative source contribution term in derivation of drinking water standards using perfluorooctanoic acid as an example

“…The USEPA should consider how available chemical information and exposure data may be used to inform an assessment of the relative sources of exposure in a semiquantitative manner that might support an RSC higher than the default but that is still adequately protective of health. Stronger consideration of available data such as physicochemical properties could be an additional line of evidence in setting an RSC, as described in Krishnan and Carrier (2013). The USEPA seems to suggest this more holistic consideration at certain points along its Decision Tree: "Are there significant known or potential uses/sources other than the source of concern" (Box 6, Exposure Decision Tree), and "Is there some information available on each source to make a characterization of exposure" (Box 8A, Exposure Decision Tree).…”