Weight-of-Evidence Modification to the Hazard Index

The HI method does not incorporate information on interactions among components of the mixture (ATSDR 2004). Mumtaz and Durkin (1992) proposed a weight-of-evidence (WOE) method to systematically address this need. The method was designed to modify the HI to account for interactions, using the weight of evidence for interactions among pairs of mixture components. Thus, the basic assumption is that pairwise interactions will dominate in the mixture and adequately represent all the interactions. For example, if chemicals A and B interact in a certain way, the presence of chemical C will not cause the interaction to be substantially different. 

It should be noted that experience with the method has revealed that it is mainly useful for a qualitative prediction as to whether the hazard may be greater or less than indicated by the HI (ATSDR 2004). 

The method evaluates the data relevant to joint actions for each possible pair of chemicals in the mixture in order to make qualitative binary weight-of-evidence (BINWOE) determinations for the effect of each chemical on the toxicity of every other chemical. Two BINWOEs are needed for each pair one for the effect of chemical A on the toxicity of chemical B, and another for the effect of chemical B on the toxicity of chemical A. 

The BINWOE determination indicates the expected direction of the interaction, such as greater than additive, less than additive, additive, or intermediate. It scores the data qualitatively by using an alphanumeric scheme that takes into account mechanistic understanding, toxicological significance, and relevance of the exposure duration, sequence, bioassay, and route of exposure. The alphanumeric 

Toxicological Risk Assessments of Chemicals A Practical Guide 

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