Changes in the distribution of short-term exposure concentration with different averaging times

For 10 workers (16 worker-chemical combinations) exposed to organic solvents, the distributions of 7.5-min time-weighted average (TWA) concentrations, 15-min TWAs, 30-min TWAs, and 60-min TWAs were examined by the Shapiro-Wilk W test and by plotting the TWA values and logarithmic values on probability paper. The hypothesis of normality of 7.5-min TWA and 15-min TWA was rejected for almost all combinations, while the hypothesis of lognormality was rejected only for a few combinations. For 30-min TWAs and 60-min TWAs, the hypothesis of normality was rejected for five and two combinations, respectively, while the hypothesis of lognormality could not be rejected for any of the combinations. The probability plot also showed that TWA values with averaging times of 7.5 to 60 min were approximately distributed in a lognormal manner. Consequently, there was no evidence against using the lognormal distribution to describe the short-term exposure distribution. Results suggest that statistical methods proposed for evaluating exposure conditions based on lognormal distribution can be useful regardless of averaging time. The appropriateness of estimating exposure distribution with different averaging times by Larsen's equations was also examined using 16 combinations. The geometric mean and geometric standard deviation estimated by Larsen's equations were nearly equal to those estimated by the traditional equations' average, but the difference of the two estimates in each case tended to be large when the ratio of the new averaging time to the original one was large.