Adequacy Conditions for Reporting Uncertainty in Chemical Risk Assessments

ABSTRACT

Uncertainty may influence decision-making. A prerequisite for a decision to be well founded is thus that scientific experts inform decision-makers about all decision relevant uncertainty. A set of conditions is provided for adequate characterization of scientific uncertainty for the purposes of regulatory decision-making. These conditions require specification of (1) the character and degree of uncertainty about the assessment variables, (2) the possibility of reducing the uncertainty, and (3) the degree of agreement among experts. Furthermore, it is required that (4) the information covered by the previous conditions is presented in a clear and comprehensible way. The point of departure is that characterizing scientific uncertainty conceptually means specifying all potentially important possibilities that are consistent with the state of scientific knowledge. The conditions are intended to be applied to human health risk assessment of chemicals. However, the basic approach, to consider potentially important possibilities, should be useful also to environmental, and site-specific risk assessment.

Keywords:

• uncertainty
• risk characterization
• characterization of uncertainty
• risk assessment
• risk management

ACKNOWLEDGEMENTS

The work has been funded by MISTRA, The Swedish Foundation for Strategic Environmental Research. I thank John Cantwell, Jonas Clausen, Stellan Fischer, Sven Ove Hansson, Christina Rudén, and Nils-Eric Sahlin for valuable comments and suggestions on drafts of the manuscript. Errors remain my own.

Notes

¹That practical difficulties may still remain is a well-known fact among historians and prosecutors as well as geologists and astronomers. As it is unclear how to analyze “knowable in principle,” using this notion as a criterion rather than determinateness, would not be suitable.

²The paradox of ideal evidence originates from Peirce (1932), Vol. 2, p. 421.

³This case has also been labeled decision-making under ignorance, which, strictly speaking, would be a better name (Resnik 1987, p. 14).

⁴What is of interest here is the internal dose at the site of each effect, and more specifically, the relation between this dose and the benchmark dose. The default assumption of 100% bioavailability of the chemical, in the face of ignorance, may result in overestimating exposure levels of concern.

⁵See USEPA (1992, pp. 26f) for some critical remarks on the use of “loosely defined” terms such as “reasonable worst case scenario” and “maximum exposed individual.” “High-end exposure” is said to differ from “reasonable worst case exposure” in that it always is intended to represent a level within the actual distribution of exposures. In the EU chemicals policy, “reasonable worst case scenario” in connection with work place exposure, is said to be “the level of exposure which is exceeded in a small percentage of cases over the whole spectrum of likely circumstances of use for that particular scenario,” excluding “extreme” uses and misuses. It “can include the upper end of normal use,” such as cleaning and maintenance (EU Commission 2003, pp. 59f). This may be called a loose “definition.” When actual versus predicted exposures are concerned, one should note that actual exposure levels are often difficult to obtain, especially so in the case of chemicals that are new to the market.

⁶This level should “approximate the 90th percentile values” (European Commission 2003, p. 60).

⁷The need of standardization of “subjective probability weights in particular inference contexts” has been pointed out by NRC (1994, p. 170).

⁸An illustrative discussion of this issue is found in NRC (1994), pp. 171–175.

⁹“[It is] not appropriate to aggregate such ‘schools of thought’ into a single distribution, but rather to show the two ‘paradigms’ and mention the amount of support expressed for each distribution,” p. 42.