Chapter 21Statistics in Conventional and Probabilistic Risk Assessment

21.1 Introduction and Overview

Chemical risk assessment arises out of the reality that exposure to chemicals is inescapable in modern society. The issue therefore is not whether there will be exposure, but rather, how to limit or minimize the potential adverse health or ecological effects associated with chemical exposure. For example, maximum contaminant levels or MCLs have been established for potable water, such as 5 μg/l for benzene or 10 μg/l for arsenic. An MCL is the concentration of a chemical or contaminant that the environmental protection authorities believe would not cause substantial adverse public health effects. To establish an MCL, assumptions are made using statistical analyses and other means regarding exposure characteristics such as the rate of water consumption, body weight of individuals, duration of residency at any one location, expected human life span, and other exposure factors. It is further assumed that a risk of 1 in 1 million of developing cancer and a noncancer risk or hazard quotient of 1.0 due to exposure to the chemical are acceptable (i.e., minimal). These input factors are then combined with the estimated toxicity or potency of the chemical, to back-calculate an allowable concentration or MCL for the chemical, which is believed would not cause substantial adverse health effects.

The MCLs constitute the target concentrations to which water supply companies are required ...

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