ABSTRACT
Adverse health effects can be considered to be of two types (see Section 4.2): those considered to have a threshold, known as ‘‘threshold effects’’ (effects such as, e.g., organ-specific, neurological, immunological, non-genotoxic carcinogenicity, reproductive, developmental), and those for which there is considered to be some risk at any exposure level, known as ‘‘non-threshold effects’’ (effects such as, e.g., mutagenicity, genotoxicity, genotoxic carcinogenicity). Though it is not possible to demonstrate experimentally the presence or absence of a threshold, differences in the approach to the hazard assessment of threshold versus non-threshold effects have been adopted widely. The distinction in approaches is based primarily on the premise that simple events such as in vitro activation and covalent binding may be linear over many orders of magnitude, i.e., that these events occur even at very low exposure levels. However, a simple pragmatic distinction on this basis is increasingly problematic as it is likely that there is a threshold for a number of genotoxic effects; this is addressed in detail in Chapter 6.
