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Part ~ Uncertainty In the opening paper of this section, Alvin M. Weinberg describes the "regulator's dilemma": agencies are required by law to regulate products and processes that may harm human health and the environ- ment, but science is often unable to provide a factual basis for the regulations. This dilemma has become increasingly important as pub- lic concern focuses on hazards arising from chronic, low-level expo- sures and from high-consequence hazards that are estimated to have a low probability of occurrence. Each of the four papers in this section characterizes components of the regulator's dilemma, and several authors recommend ways in which the dilemma can be circumvented. Weinberg argues that questions about Tow-leve} and low- probability risks lie in a region of trans-science and are therefore beyond the power of science to answer. He suggests that probabilistic risk assessment be used to estimate the likelihood of harm from rare events, and cites its successful application in the nuclear industry. But in regulating products and processes that pose chronic, low-level risks, Weinberg tells us that the regulator needs to finesse the trans- scientific limits to science; one suggestion is that the regulator invoke the principle of de minimis. This principle argues that for naturally occurring insults to the human environment, regulators need concern themselves with man-made exposure only when it exceeds the natu- ral, background exposure.

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8 INTRODUCTION TO PART I Chris G. Whipple discusses advantages and disadvantages ofthe de minimis principle in this section. His particular concern is that adher- ence to the principle may reinforce the long-standing tradition of regulatory approaches that use conservative, protective assumptions in assessing risks. Whipple argues that the use of such assumptions is unlikely to protect either human health or the environment. Also in this section, Victor P. Bond illustrates the difficulty of linking a hazard outcome cancer to a particular cause low-level exposure to radiation and he proposes a method for calculating the "probability of causation" of an outcome from a given exposure. He suggests that this method would provide the courts with a reasonable threshold for determining when the victim of a particular exposure can collect damages from the responsible party. In the last paper in this section, Ronald Bayer describes the evolu- tion of the occupational standard for airborne lead. In doing so, he provides an example of differences among scientists over apparently value-centered interpretations of incomplete data. Bayer argues that whenever regulatory standards are based on inconclusive data, the outcome will depend on political resolution of conflicting claims of equity and distributive justice, not on scientific fact.