Risks are quantified under the traditional ecological risk paradigm using the hazard quotient approach. A hazard quotient is defined as the ratio of an organism's estimated chemical exposure relative to a lowest-observed-effect level or a no-observed-effect level derived from laboratory toxicity testing or field data. The hazard quotient approach is rooted in human health risk assessment as a tool to measure the potential hazard (e.g., level of concern) posed by chemical exposure. As acknowledged by the US Environmental Protection Agency (EPA) in its Risk Assessment Guidance for Superfund (RAGS, Part A), a hazard quotient is in fact not a true measure of risk, but rather simply an indication of whether a threshold has been exceeded. This differs from EPA's approach toward carcinogenic risks to humans, which uses the linear low-dose cancer risk equation to estimate the probability of an individual's cancer risk over a lifetime. One proposed strength of the hazard quotient approach is its utility in ranking and prioritizing chemicals for more detailed risk characterization, but even this use is fraught with problems if implemented in a deterministic way. Nevertheless, this approach is included among other assessment methods for ranking of high-production-volume chemicals in the European Union (see regulation 1488/94 and directive 93/67) and the United States (as per the Toxic Substances Control Act).
For ecological assessments, hazard quotients are typically based on effect measures targeted at individual response levels (e.g., growth, survival) determined under laboratory conditions. This approach is limited in its ability to estimate risks to individual organisms, let alone estimate risks to groups of organisms. As described in numerous references within the open literature from the last decade, criticisms against reliance on the hazard quotients are vast. The criticisms are based on three chief arguments. First, hazard quotients do not denote risk per se, rather only potential hazard. Second, traditional hazard quotients provide limited insight, if any, to potential risks posed to populations and higher levels of organization. This is chiefly the result of a preponderance of laboratory toxicity tests based upon individual-level responses. Third, hazard quotients do not permit an explicit examination of potential impacts beyond standard laboratory exposure durations (generally from hours to days). Hazard quotients do not provide more meaningful expressions of impacts across multiple generations of organisms under field settings. While many researchers have tried to bridge the gap between hazard quotients to more meaningful expressions of ecological value described by assessment endpoints, few empirical data exist to conclusively support the continued sole reliance on a hazard quotient approach.
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