A model-free approach to low-dose extrapolation

Benchmark Dose Analysis via Nonparametric Regression Modeling

Estimation of benchmark doses (BMDs) in quantitative risk assessment traditionally is based upon parametric dose-response modeling. It is a well-known concern, however, that if the chosen parametric model is uncertain and/or misspecified, inaccurate and possibly unsafe low-dose inferences can result. We describe a nonparametric approach for estimating BMDs with quantal-response data based on an isotonic regression method, and also study use of corresponding, nonparametric, bootstrap-based confidence limits for the BMD. We explore the confidence limits' small-sample properties via a simulation study, and illustrate the calculations with an example from cancer risk assessment. It is seen that this nonparametric approach can provide a useful alternative for BMD estimation when faced with the problem of parametric model uncertainty.

Nonparametric estimation of benchmark doses in environmental risk assessment

An important statistical objective in environmental risk analysis is estimation of minimum exposure levels, called benchmark doses (BMDs), that induce a pre-specified benchmark response in a dose-response experiment. In such settings, representations of the risk are traditionally based on a parametric dose-response model. It is a well-known concern, however, that if the chosen parametric form is misspecified, inaccurate and possibly unsafe low-dose inferences can result. We apply a nonparametric approach for calculating benchmark doses, based on an isotonic regression method for dose-response estimation with quantal-response data (Bhattacharya and Kong, 2007). We determine the large-sample properties of the estimator, develop bootstrap-based confidence limits on the BMDs, and explore the confidence limits' small-sample properties via a short simulation study. An example from cancer risk assessment illustrates the calculations.

Evaluation of health risks for contaminated aquifers

This review focuses on progress in the development of transport models for heterogeneous contaminated aquifers, the use of predicted contaminant concentrations in groundwater for risk assessment for heterogeneous human populations, and the evaluation of aquifer remediation technologies. Major limitations and areas for continuing research for all methods presented in this review are identified.

Lead, cadmium, arsenic and zinc in the ecosystem surrounding a lead smelter

A lead smelter has been operating at Belledune in the province of New Brunswick, in eastern Canada, since 1966. This paper presents data on the concentrations of the four primary metals emitted from the smelter - lead, cadmium, arsenic and zinc - which were measured in the terrestrial environment near the smelter and the concentrate transport route. Deposition of these metals to the snowpack and the uptake by grass forage are discussed in relation to non-regulatory guidelines, toxicity and atmospheric emissions. A 1992 snowpack transect survey extending 0.5-40 km northwest, southeast and south of the smelter revealed lead concentrations of 2-3193 ppb, cadmium <0.10-49.7 ppb, arsenic <3.0-72.0 ppb, and zinc 3-401 ppb. Deposition estimates within this zone for lead were between 0.046 and 20.1 kg/ha/yr, cadmium <0.007 and 313 g/ha/yr, arsenic <0.016 and 453 g/ha/yr and zinc 0.020 and 2.52 kg/ha/yr. Concentrations of these metals in the snowpack were highest within 3 km of the smelter and were detectable at greater distances SE of the smelter. Lead was dispersed greater distances from the smelter than cadmium or arsenic. Snowpack samples collected within 5-20 m of the railway contained 140-7270 ppb of lead, 0.4-36.9 ppb of cadmium, <3.0-72.0 ppb of arsenic and 41-13100 ppb of zinc. Grass forage sampled within 0.6-16 km of the smelter contained lead 5-152 ppm, cadmium 0.10-4.1 ppm, and zinc 22-154 ppm. Highest concentrations of lead, cadmium and zinc in grass forage were found were found within 2.2 km of the smelter. Grass forage collected within 10-70 m of the railway contained lead 13-288 ppm, cadmium 0.4-1.3 ppm and zinc 98-831 ppm.