Quantitative risk assessment is an important tool for regulating potentially carcinogenic chemicals including some pesticides. Although requiring that pesticide residue tolerances not exceed a specific dietary risk level may appear to be an effective risk management approach, it ignores significant limitations of calculated risk numbers that arise from uncertainties inherent in the risk assessment process. Determining what level of risk is to be considered negligible is a matter of policy rather than science. Different federal agencies and programs consider various levels of risk acceptable, but there appears to be some consensus that risks below 10^-6 are negligible.