Statistical properties of the NOAEL.

The use of the no observed adverse effect level (NOAEL) in setting allowable exposure levels for noncancer endpoints is a source of controversy. Based on computer simulations and empirical studies, several authors have criticized the use of the NOAEL in terms of its sensitivity to sample size and its high sampling variability from experiment to experiment. The purpose of this paper is to derive the statistical distribution of the NOAEL. Using Weibull models, we investigate the impact of the shape of the underlying dose-response curve on the distribution of the NOAEL. The results confirm previous criticisms of the NOAEL and show that average risk levels associated with the NOAEL may be substantial. These results provide additional motivation for developing alternative approaches to risk assessment.