The case for integrating low dose, beneficial responses into US EPA risk assessments.

When conducting risk assessments, the US Environmental Protection Agency (EPA) does not currently consider the beneficial effects from exposure to concentrations of agents below the no observed adverse effect level (NOAEL). If such benefits were observed, and if the beneficial and toxicological mechanisms of action were identical, this would probably be represented as a 'j-shaped' hormetic dose-response curve. If such data are available, they should be considered when assigning uncertainty factors for safe exposure calculations. However, when such data are not readily available, as is likely the case when the mechanism of action of the benefit differs from that of toxicity, current US EPA methods appear adequate.