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POSSM, the PCB On‐Site Spill Model, is a contaminant transport model developed to predict environmental concentrations associated with a chemical spill. The model predicts daily changes in chemical concentrations on a spill site (e.g., in soil and on vegetation) and losses of chemical due to volatilization, surface runoff/soil erosion, and leaching to groundwater. Spill areas consisting of soil/vegetation and/or an impervious surface (e.g., asphalt and concrete) can be analyzed, as can different spill cleanup practices. POSSM is used to analyze exposure levels associated with a hypothetical capacitor spill. While the model was developed for PCB spills, it is generally applicable to a number of organic compounds.

Measurements of intermediate end points in the carcinogenic process may reduce uncertainty in human risk assessment from bioassay data, by identifying sources of interspecies variation and dose nonlinearity. This paper describes desirable properties of such markers: persistence, predictive power, temporal relevance, and consistency across dose rate and species. We illustrate these properties by evaluating markers for squamous cell nasal carcinoma in rodents exposed to formaldehyde. We also discuss design issues for bioassays that evaluate markers and tumors simultaneously at necropsy.

Children are becoming an increasingly important focus for exposure and risk assessments because they are more sensitive than adults to environmental contaminants. A necessary step in measuring the extent of children's exposure and in calculating risk assessments is to document how and where children spend their time. This 1990‐1991 survey of 1000 households was designed for this purpose, targeting children between 5 and 12 years of age, in six states in varied geographic regions. The behavior of children was sampled on both weekdays and weekends over all four seasons of the year using a retrospective time diary to allocate time to activities during the previous 24 h. Information was obtained on the kinds and locations of activities, the nature of the microenvironments of the locations, and the time spent in the different environments. Measures of variability in addition to mean hours per day are reported. Results of this study closely match those of earlier research on California children's activities done by the California Air Resources Board. One important finding of the survey was that 5‐ to 12‐year‐old children in all geographic regions spend most of their time indoors at home, indicating that risk assessments should focus on indoor, on‐site hazards.

This paper presents benchmark (BMD) calculations and additional regression analyses of data from a study in which scores from 26 scholastic and psychological tests administered to 237 6‐ and 7‐year‐old New Zealand children were correlated with the mercury concentration in their mothers' hair during pregnancy. The original analyses of five test scores found an association between high prenatal mercury exposure and decreased test performance, using category variables for mercury exposure. Our regression analyses, which utilized the actual hair mercury level, did not find significant associations between mercury and children's test scores. However, this finding was highly influenced by a single child whose mother's mercury hair level (86 mgkg) was more than four times that of any other mother. When that child was omitted, results were more indicative of a mercury effect and scores on six tests were significantly associated with the mothers' hair mercury level. BMDs calculated from five tests ranged from 32 to 73 mgkg hair mercury, and corresponding BMDLs (95% lower limits on BMDs) ranged from 17 to 24 mgkg. When the child with the highest mercury level was omitted, BMDs ranged from 13 to 21 mgkg, and corresponding BMDLs ranged from 7.4 to 10 mgkg.

Applying a hockey stick parametric dose‐response model to data on late or retarded development in Iraqi children exposed in utero to methylmercury, with mercury (Hg) exposure characterized by the peak Hg concentration in mothers'hair during pregnancy, Cox et al. calculated the "best statistical estimate" of the threshold for health effects as 10 ppm Hg in hair with a 95% range of uncertainty of between 0 and 13.6 ppm.(1)A new application of the hockey stick model to the Iraqi data shows, however, that the statistical upper limit of the threshold based on the hockey stick model could be as high as 255 ppm. Furthermore, the maximum likelihood estimate of the threshold using a different parametric model is virtually zero. These and other analyses demonstrate that threshold estimates based on parametric models exhibit high statistical variability and model dependency, and are highly sensitive to the precise definition of an abnormal response. Consequently, they are not a reliable basis for setting a reference dose (RfD) for methylmercury. Benchmark analyses and statistical analyses useful for deriving NOAELs are also presented. We believe these latter analyses—particularly the benchmark analyses—generally form a sounder basis for determining RfDs than the type of hockey stick analysis presented by Cox et al. However, the acute nature of the exposures, as well as other limitations in the Iraqi data suggest that other data may be more appropriate for determining acceptable human exposures to methylmercury.