Issues in Risk Assessment (1993)

For the 14 candidate agents on which comparable quantitative data are available, the right side of Table G-2 shows the observed TD₅₀ as calculated by Krewski's procedures with the one-stage model. The last three columns in Table G-2 show the log₁₀TD₅₀ predicted by Krewski's model, the deviation from the regression line (observed - predicted log₁₀TD₅₀), and the standardized deviation (observed deviation divided by r.m.s. error). The data are plotted in relation to Krewski's regression line in Figure 1. Most of the 14 candidate chemicals are within the 95% confidence limits (standardized deviation, 1.96); this is illustrated in Figure 2, which plots the calculated TD₅₀s for each of the 14 chemicals. For five of the 14 agents (the three benzidine dyes, carbon tetrachloride, and 1,3-butadiene), the calculated TD₅₀s are below the lower confidence limit on the regression line, i.e., inside Region C (Figure 2). Among the four agents on which comparable quantitative data are not available, only plutonium has a low ratio of TD₅₀ to HDT (1:13), but its HDT caused premature deaths and would not be accepted as an MTD in a conventional bioassay.

Neither of the TD₅₀s calculated for vinyl chloride fall in Region C. The TD₅₀ based on the data selected by the committee falls below the