Statistical Support for 4-Lab Project: Design and Analysis of Toxicology Studies on Complex Disinfection By-Products (DBPs) Mixtures

Objective/Intended Use

The 4-Lab Project will: reduce uncertainty in the evaluation of those developmental and reproductive endpoints identified as of concern in epidemiologic studies; provide information that bridges the gap between experimental animal single-chemical experiments and human exposures to multiple DBPs; improve estimates of risk from exposure to complex mixtures of DBPs at low exposure levels, including the unidentified fraction; and, provide data and risk assessment methods to the Office of Water to aid in assessing health risk from exposure to DBP mixtures. Another goal of this project is to develop generalizable complex mixtures methods using the 4-Lab exposure and toxicity data in the development and refinement of emerging mixture risk assessment methodologies (e.g., for evaluation of the accuracy of component-based approaches when applied to complex mixtures; to estimate health risks when part of the complex mixture is an unidentified fraction; to develop toxicological, chemical and statistical criteria for establishing sufficient similarity, a concept in the Agency guidelines on chemical mixtures risk assessment; and, to develop exposure models for a variable complex mixture sampled over time).

A wealth of data was produced in FY06 following conduct of the 4-Lab full study. This will be a unique data base featuring analytical chemistry and toxicity data on several end points over time. It is proposed that these data will be useful not only in the short term for determination of whether the complex DBP mixture is of concern for human health, but also in the long term for development of exposure modeling based on the chemical characterization of the complex mixture and the potential development of risk assessment models for complex mixture data. OW can use the laboratory data and statistical analysis results of the 4-Lab study to evaluate the health effects from exposure to the complex DBP mixture, including information on whether the unidentified fraction contributes to health risk.

Results from this project will be significant in providing responses to the drinking water mixtures research requirements of 42 USC � 300 of the Safe Drinking Water Act Amendments (SDWAA) of 1996. In addition, this research is providing information in response to the ORD and OW jointly drafted Research Plan for Microbial Pathogens and DBPs in Drinking Water in 1997 that calls for the characterization of DBP mixtures risk. End users of this research include the risk assessment community and parties interested in chemical risk assessment of drinking water, such as the American Water Works Association and Stakeholder groups. OW will use the DBP approaches while it evaluates Stage 2 M/DBP rule during its 6-year review.

Abstract

At high laboratory-concentrations, a number of disinfection by-products (DBPs) have been shown to be either carcinogenic or to cause target-organ toxicity. However, based on animal study results, exposure to any single DBP at its typically low concentration in the drinking water is unlikely to result in significantly increased human health risk. In contrast, epidemiologic investigations suggest associations between human consumption of disinfected drinking water and various health effects, including bladder, colon and rectal cancer and reproductive and developmental effects. As most animal DBP toxicity data are from single chemical studies and the results of the human epidemiologic studies are from exposure to DBPs mixtures, there is a need to conduct scientifically sound and defensible animal experiments with complex DBP mixtures.

The four ORD laboratories are proposing to produce and evaluate complex mixtures of DBPs using a coordinated series of well designed studies that involve water treatment, analytic chemistry, toxicologic studies and statistical analysis. The basic design of this project is to: 1) produce concentrates of drinking water samples by selecting a high bromide source water and subjecting it to chlorination treatment; 2) perform analytic chemistry to determine the amounts and proportions of both known and unknown DBPs in the mixture and to test for the integrity of the mixture to be used in toxicologic testing; 3) perform a series of in vivo and in vitro toxicologic studies to assess toxicity for relevant endpoints; 4) use appropriate statistical design and analysis techniques to provide data that is of maximal use in risk assessment, including evaluation of the amount of toxicity attributable to the known and unknown fractions of the complex mixture. A trial run was performed in August 2000, range-finding and defined mixture studies were performed in 2005, and a full study was completed in 2006. The trial run determined the amount of concentrated water that can be made, the chemical stability of these waters, and initial palatability and volume requirements for the laboratory animals. This project will conduct research necessary to address two areas: Predictive Modeling and Risk Assessment; and Hypothesis Testing and Modeling of Laboratory Data. In the 4-Lab full study, by representing the unidentified fraction of the complex mixture in the model as if it were a single chemical, its contribution to toxicity or interaction effects can be tested and estimated. The uses of the full study data set extend beyond conclusions that may be drawn directly from that particular study. The purpose of this second research effort is to use the data produced by the 4-Lab effort to estimate human health risks for the treatment train and source water in the study, to determine when these estimates are generalizable to other sufficiently similar DBP mixtures, to contrast 4-Lab health risk estimates with those predicted in the literature using epidemiologic data, and to develop generalizable complex mixtures methods using the 4-Lab data.

Project Status

Ongoing

Project Start Date

08/01/2000

Project Completion Date (Actual/Projected)

09/30/2010