A commentary on the NRC report "Science and judgment in risk assessment".

This article provides a brief overview of the report "Science and Judgment in Risk Assessment," prepared by a Committee of the National Research Council/National Academy of Science in response to a U.S. Environmental Protection Agency request mandated by the Clean Air Act Amendments of 1990 (CAAA-1990). The report critiques EPA's current approaches for characterizing human cancer risks from exposure to chemicals and offers recommendations for the conduct of future cancer risk assessments, especially those required in implementing the CAAA-1990 provisions which are concerned with hazardous air pollutants. The report offers specific recommendations that address the role of default options, data needs, methods and models, uncertainty, variability, and the aggregation of data. A cross-cutting theme of the report is the use of an iterative approach in which screening assessments with limited data and, of necessity, default options used in the absence of specific scientific data may be performed initially followed by subsequent assessments, as needed, in which increasing amounts of data are developed and incorporated. In some instances, the specific data on a given chemical or pollutant source will replace conservative default options used in earlier assignments. The report includes two authored appendices that address issues related to the use of default options and their replacement by specific scientific information. One appendix by Finkel advocates a principle of "plausible conservatism" for choosing and altering default options and in making cancer risk estimates. A second appendix by McClellan and North advocates the full use of scientific information in the risk assessment process. This article gives major attention to the key aspects of the NRC/NAS report, especially those dealing with the use and replacement of default options. The default options and the extent to which the options are replaced with specific science have major impact on the final quantitation of cancer risk for exposure to chemicals.