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Issues in Risk Assessment TABLE G-5 Vinyl Chloride* Dose Rate (mg/kg-d) Tumor Incidence Males Females Liver tumors (neoplastic nodules, hepatocellular carcinomas, angiosarcomas) 0 0/55 2/57 1.7 2/58 26/58 5.0 17/56 42/59 14.1 58/59 56/57 *Oral lifetime exposure. Surviving males and females were sacrificed at 135 and 144 weeks, respectively. Source: Feron et al., 1981. RESULTS 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