To establish the time that must be allowed to elapse before workers can safely re-enter a crop that has been sprayed with a pesticide, one must be able to estimate the hazard associated with working in the crop at any particular time after application of the pesticide. The acute hazard for such re-entry can be estimated by monitoring workers to determine the potential dermal and respiratory exposures they receive. Then, using toxicity data from laboratory animal studies, one can arrive at an estimate for the percent of an acute lethal dose of the pesticide that a worker can be expected to receive during a normal day's work (DURHAM & WOLFE 1962, DAVIS et al. 1982a). A recently postulated system also allows one to be determined whether or not it is safe for a worker to receive the observed level of exposure for an entire working season.