Toxicological benchmark concentration

An approach to estimating thresholds of effect for multiple endpoints within a species has been proposed in which distributions of effect measures from different assay endpoints in a species can be used to extrapolate to a no-effect measure for all possible endpoints (Hanson and Solomon 2002). In practice, a distribution of effect measures is constructed and extrapolated to a low probability (Figure 1.5). This value is used as an estimate of the toxicological benchmark concentration (TBC), below which no effect... 

FIGURE 1.5 Illustration of the method for determining a toxicological benchmark concentration (TBC). Note A distribution of endpoints for a species is used to extrapolate to a TBC, below which the likelihood of unmeasured responses being observed is very small. 

Occupational and toxicological studies have demonstrated adverse health effects from exposure to toxic contaminants. Emissions data from stationary and mobile sources are used in an atmospheric dispersion model to estimate outdoor concentrations of 148 toxic contaminants for each of the 60,803 census tracts in the contiguous United States for 1990. Approximately 10% of all census tracts had estimated concentrations of one or more carcinogenic HAPs at a greater than l-in-10,000 risk level. Twenty-two pollutants with chronic toxicity benchmark concentrations had modeled concentrations in excess of these benchmarks, and approximately 200 census tracts had a modeled concentration 100 times the benchmark for at least one of these pollutants. This comprehensive assessment of air toxics concentrations across the United States indicates hazardous air pollutants may pose a potential public health problem (Woodruff et al., 1998). 

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