Determining the probabilistic limits for the uncertainty factors used in the derivation of the Reference Dose (RfD) is an important step toward the goal of characterizing the risk of noncarcinogenic effects from exposure to environmental pollutants. If uncertainty factors are seen, individually, as "upper bounds" on the dose-scaling factor for sources of uncertainty, then determining comparable upper bounds for combinations of uncertainty factors can be accomplished by treating uncertainty factors as distributions, which can be combined by probabilistic techniques. This paper presents a conceptual approach to probabilistic uncertainty factors based on the definition and use of RfDs by the U.S. EPA. The approach does not attempt to distinguish one uncertainty factor from another based on empirical data or biological mechanisms but rather uses a simple displaced lognormal distribution as a generic representation of all uncertainty factors. Monte Carlo analyses show that the upper bounds for combinations of this distribution can vary by factors of two to four when compared to the fixed-value uncertainty factor approach. The probabilistic approach is demonstrated in the comparison of Hazard Quotients based on RfDs with differing number of uncertainty factors.

At the time of this writing, more than 18 million people worldwide are known to have contracted COVID-19, the medical condition caused by the novel SARS-CoV-2 coronavirus (Johns Hopkins 2020). The World Health Organization confirmed the COVID-19 outbreak as a global pandemic in March 2020, as the virus spread rapidly worldwide. The end of this global health emergency is not yet in sight. Responses to the crisis from politicians and public health officials in different countries have adapted, with a few notable exceptions, to rapidly evolving medical and epidemiological information. The public's response to recommendations from public health officials, however, has varied widely. In some communities, variable public interpretation of public health recommendations has inhibited efforts to contain the outbreak.The COVID-19 pandemic is a stark reminder of the need to elevate the role of science in public and political decision making. The health sciences community must seize this opportunity as a call to action to more forcefully and effectively convey science and health risk information to the world, especially during this and future times of crisis. Now is the time for health and science professionals to sharpen communication strategies to guide decision makers to protect the health of individuals, families, communities, and nations. We believe the global pandemic offers important lessons and opportunities for improvements critical to effective risk communication. BARRIERS AND SOLUTIONS FOR EFFECTIVE RISK COMMUNICATIONThe COVID-19 pandemic has revealed several barriers to effective health and risk communication. To achieve the best

This paper presents an approach for characterizing the probability of adverse effects occurring in a population exposed to dose rates in excess of the Reference Dose (RfD). The approach uses a linear threshold (hockey stick) model of response and is based on the current system of uncertainty factors used in setting RfDs. The approach requires generally available toxicological estimates such as No-Observed-Adverse-Effect Levels (NOAELs) or Benchmark Doses and doses at which adverse effects are observed in 50% of the test animals (ED50s). In this approach, Monte Carlo analysis is used to characterize the uncertainty in the dose response slope based on the range and magnitude of the key sources of uncertainty in setting protective doses. The method does not require information on the shape of the dose response curve for specific chemicals, but is amenable to the inclusion of such data. The approach is applied to four compounds to produce estimates of response rates for dose rates greater than the RfD.

In deriving water quality standards and appropriate restoration levels for contaminated surface waters, the potential for human exposure is often the most important factor to be considered. For certain persistent compounds, like 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TODD) or mixtures of polychlorinated biphenyls, a primary pathway of human exposure is through ingestion offish obtained from affected waters. Pending water quality regulation for TCDD in Maine required that estimates be made of the rate of consumption of freshwater fish obtained from rivers that receive TCDD discharges. Because commercial freshwater fishers do not exist on Maine rivers, any freshwater fish that are eaten have been caught by anglers. A statewide mail survey of Maine's licensed anglers was undertaken to characterize rates offish consumption from rivers and streams in Maine. The survey was mailed to 2,500 licensed resident anglers who were randomly selected from state license files. The response rate of 70% (based on deliverable surveys) resulted in a usable sample of 1,612 anglers. Results ofthis study indicated that, if fish are shared with other fish eaters in the household, the annual average consumption of freshwater river fish per consuming angler in Maine is 3.7 g/d. Comparisons of findings of this study and of studies in other regions of the United States show considerable variations in fish consumption rates, supporting the use of state‐ or region‐specific estimates of fish consumption in establishing water quality regulations for persistent, biologically accumulative compounds.

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Indirect exposures to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other toxic materials released in incinerator emissions have been identified as a significant concern for human health. As a result, regulatory agencies and researchers have developed specific approaches for evaluating exposures from indirect pathways. This paper presents a quantitative assessment of the effect of uncertainty and variation in exposure parameters on the resulting estimates of TCDD dose rates received by individuals indirectly exposed to incinerator emissions through the consumption of home-grown beef. The assessment uses a nested Monte Carlo model that separately characterizes uncertainty and variation in dose rate estimates. Uncertainty resulting from limited data on the fate and transport of TCDD are evaluated, and variations in estimated dose rates in the exposed population that result from location-specific parameters and individuals' behaviors are characterized. The analysis indicates that lifetime average daily dose rates for individuals living within 10 km of a hypothetical incinerator range over three orders of magnitude. In contrast, the uncertainty in the dose rate distribution appears to vary by less than one order of magnitude, based on the sources of uncertainty included in this analysis. Current guidance for predicting exposures from indirect exposure pathways was found to overestimate the intakes for typical and high-end individuals.

Bioconcentration factors (BCF) or bioaccumulation factors (BAF) reported for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin) in aquatic environments encompass a wide range of values, from less than 1000 to 189,000 l/kg. These values are based on concentrations of TCDD in various environmental media including water, sediment, or food. Under the Federal Water Pollution Control Act and its enabling regulations (40 CFR 100-140, 400-470), point source discharge limits are established so that the nominal receiving water concentration will not exceed the water quality criterion. To be consistent with this regulatory process, the water quality criterion should also be calculated using an accumulation factor that is based on a nominal water concentration. The regulatory process for developing a water quality criterion for TCDD requires the selection of a BAF that describes the relationship between the source to be regulated and the fish tissue concentration.