As part of the hazard assessment of a given substance hazard identification, a comprehensive hazard characterisation, mode of action analysis and consistency check have to be performed in order to describe qualitatively and if possible quantitatively the nature of an observed effect. The criteria and weight of evidence for this evaluation have to be set in advance and comprise strength of evidence of all information available and then a hazard characterisation making use of a description of adversity, reversibility, severity of effects, potency for a given target as well as consistency of the observations. A hazard characterisation should be part of the science-based evaluation of endocrine disrupting effects as it is for any other toxicological effect. This approach was supported also by EFSA (2013). A proposal for a matrix based integrated decision strategy is illustrated in Fig. 1.
In brief, the proposed decision matrix could aid regulators to group chemicals into the three categories suggested in the roadmap by taking into account a number of criteria used in regulatory hazard identification and characterisation. These criteria include severity of effects, reversibility of effects, consistency, strength of evidence and potency. For each of these criteria a short definition is given and subgroups are suggested to facilitate matrix-based categorisation.
Strength of evidence
According to Hennes et al. (2014), strength of evidence defines the degree of association between chemical exposure and the toxicological effect, such as the integrity of the test system, dose–response relationship, strength of correlation based on comparison with concurrent, or in lack or complementation thereof historic control values and clarity of dose response. Insufficient or limited strength of evidence would support categorisation into category 3 while sufficient strength of evidence would support categorisation into category 2 or 1.
Severity of effects
For severity the following working definition has proven helpful: “A severe effect is an effect which is either described as severe in nature or irreversible. Typical severe effects comprise (but are not limited to) tumours, irreversible forms of organ damage, malformations or infertility” (Marx-Stoelting et al. 2014). Effects described as severe would thus support categorisation into a higher category (cat 1 or 2) in the decision matrix, while effects described as mild or slight would support categorisation into a lower category (cat 2 or 3) in the decision matrix.
Reversibility of effects
Irreversible effects (tumours, malformations, organ loss) would generally support a category 1 decision, while reversible effects would support categorisation into category 2.
Consistency refers to a situation, where several tests (e.g. for several durations, by several routes of administration and/or in several species) have been made and should only be applied in such a situation. The level of consistency increases, when an effect or a related effect is observed in several studies and/or in several species (if applicable) and/or confirmed by results in several tissues and/or can be substantiated by a conclusive mode of action. The mode of action is actually a very important underlying biological reason that defines a potential endocrine disruptor. It is thus one of the most convincing arguments when seen in conjunction with actual effects. The level of consistency is higher, when an effect is observed in two or more studies, in rats and mice or in a group of structurally related compounds. For many endocrine disruptors, several target tissues exist. Here, consistency is regarded higher if more than one tissue is affected. On the other hand, if only one target tissue in one species would be affected, than a lower category would be more appropriate. In brief: a high level of consistency would support categorisation into group 1, while a lower level of consistency would support categorisation into group 2 or 3.
Potency consideration with regard to endocrine effects can be understood as a dose required to induce a specific effect on targets of the endocrine system (e.g. an unwanted, adverse effect). If the dose required to induce this effect is low, the potency is considered high. Hence, a higher category in the decision matrix would be applied. Guidance values for potency exist in the EU Regulation on Classification, Labelling and Packaging of substances (CLP regulation, EC 2008). They have been proposed for hazard characterisation of substances producing specific target organ toxicity.