Most public controversies about the effects of chemicals on human health revolve around the risk of cancer--hardly surprising, considering that it is the second leading cause of death in the United States. People are concerned about the dangers of carcinogens in air, water, and food, and they expect their representatives in government to protect them from such hazards. On the other hand, the economic costs of eliminating every suspected carcinogen from the environment would cause tremendous economic hardship. How should policymakers use science to help strike a balance between the benefits...