Conducting a Risk Assessment of Mixtures of Disinfection By-Products (DBPS) for Drinking Water Treatment Systems
The risk assessment of disinfection by-product (DBP) mixtures in drinking water is an important issue in environmental health. To generate improved assessments of DBP mixture health risk, EPA scientists have explored a number of novel approaches to generating realistic, central tendency estimates of potential health risks, despite data limitations and uncertainties. A response addition approach to DBP mixtures risk estimation is presented, the state of DBP toxicity and exposure data are discussed, alternative methods for mixtures risk characterization are explored, and recommendations for future applications and methodological developments are provided. This effort has resulted in the production of three interrelated research reports and an external review report, all contained within this document as follows:
Report 1, September, 2000: EPA's conclusions, recommendations, conceptual approach and future directions regarding the conduct of a DBP mixture risk assessment.
Report 2, January, 2000: A summary of presentations and discussions at an April 1999 workshop where scientists examined an illustrative example of a DBP mixtures risk assessment, presented ideas to advance the approach, and recommended research and development directions.
Appendix I, April, 1999: An illustrative example of a DBP mixtures risk assessment (using a response addition approach), including data, assumptions and statistical methods used. Shows resulting risk distributions and uncertainty analysis.
Appendix II, July, 2000: External scientific review comments and recommendations for Report 1.
In this research effort, EPA scientists have produced the following:
1) An initial assessment of human health risks for developmental and reproductive effects and cancer from exposure to DBP mixtures, using a response addition approach that incorporates data on the unidentified fraction of the DBPs and uses a probabilistic approach.
2) A workshop report containing a wealth of information on the exposure, dose-response and risk characterization issues relative to DBP mixtures health risks that can be used by risk assessors interested in this area.
3) A new conceptual approach, the Cumulative Relative Potency Factors (CRPF) method, to assessing DBP mixtures health risks that improves on the response addition method by integrating data on mode of action and multiple routes of exposure over physiologically-relevant time frames.
4) A list of research recommendations most critical to improving DBP mixtures health risk assessment. The most important scientific directions included: integration of both human and animal toxicity data into the assessment; development of exposure models that incorporate dermal, oral and inhalation routes, human activity patterns, and measures of internal dose; collection of concentration data that are representative of real world drinking water samples, including additional information on the unidentified fraction of the DBPs; application of risk characterization methods that incorporate data on the toxic mode of action for the physiologically-relevant exposure time frame; consideration of sensitive subgroups in the population; and analysis of variability in the data and uncertainty of the final risk estimates.