Presentations on the Relative Source Contribution Project
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Title: Exposure Assessments for Contaminants of Emerging Concern
Authors: Chris Greene, Pam Shubat
Presented at: Society for Risk Analysis, 2014 Annual Meeting
Abstract: Development of drinking water guidance for chemicals of emerging concern (CECs) requires a change in perspective from the methods used for other water contaminants. For many CECs, even those which may be present in drinking water sources, drinking water is not the primary source of exposure. Some CECs (phthalates, BPA, flame retardants) exhibit near ubiquitous exposure in the U.S. population from the use of consumer products; others (fragrances, pharmaceuticals, DEET) are deliberately placed on or in the body. A few CECs (triclosan, microcystin, acrylamide) may approach or exceed toxic thresholds from consumer product or dietary exposures. The Minnesota Department of Health (MDH) addresses these exposure assessment issues in a rigorous review process when it develops health-based guidance values for CECs in drinking water. The review process includes an exposure assessment involving multiple exposure routes and the development of chemical-specific Relative Source Contribution (RSC) factors to account for potential non-drinking water exposures. RSCs and guidance values are developed for multiple exposure durations. Exposure and RSC assessments for CECs include analysis of published data on occurrence, exposure estimation, and biomonitoring, as well as the use of appropriate computer models to estimate exposures from multiple media and routes. MDH’s practical application of the RSC involves the consideration of measured and modeled data and the judicious apportionment of exposure and risk across both water and non-water routes. Results, discussion, and policy implications of exposure and RSC assessments for chlorpyrifos, isobutanol, acrylamide, nonylphenol, and other chemicals will be presented.
Title: Computer-Based Exposure Modeling to Support Drinking Water Guidance
Authors: Christopher Greene, Charlie Wilkes, Michael Koontz, Pam Shubat
Presented at: Society for Risk Analysis, 2013 Annual Meeting
Abstract: The Minnesota Department of Health (MDH) develops health-based guidance values for contaminants of emerging concern (CECs) in drinking water. To account for non-drinking water exposures, MDH uses a chemical-specific Relative Source Contribution (RSC) factor to allocate only a fraction of the toxicological reference dose to drinking water exposure. Pharmaceuticals, personal care products, pesticides, and other CECs found in drinking water sources have complicated exposure profiles that may include ubiquitous exposure at low levels, deliberate self-exposure, high exposures to infants and children, and exposures approaching the reference dose. CEC-specific data to quantify these exposures and accurately understand cumulative and relative risks are not readily available and, until now, MDH has relied on default assumptions based on U.S. EPA guidance. Working with a contractor, MDH explored the potential for using computer models to improve upon the default approach by estimating multipathway, multiroute exposures. We have identified the key media and exposure routes of concern, evaluated numerous models that cover these key media and routes, and developed a set of preferred models based on the model’s fitness to meet our exposure evaluation needs, its strength in representing actual physical/chemical processes, its input demands, and its user friendliness. The preferred models include EPA’s Exposure and Fate Assessment Screening Tool (E-FAST) and Multimedia, Multipathway, and Multireceptor Risk Assessment (3MRA) model, the California Population Indoor Exposure Model (CPIEM), and the Total Exposure Model (TEM). We developed a set of procedures to apply these models to the problem of estimating RSC values for CECs. The procedures were evaluated using a test group of six CECs that cover a range of exposure pathways and routes. Beyond estimation of the RSC, the modeling process may also be of use to risk managers seeking to target resources towards reducing total exposures.