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11. Multiple effects on the same health endpoint

Some assessments may involve multiple nutrients and/or contaminants that affect the same endpoint.

Where the combined effect of different substances can be modelled using toxic equivalency factors, e.g. for dioxin-like substances, this can be incorporated by converting the intakes to toxic equivalents outside the Qalibra framework and then using the combined intake as the “dose” for a single effect in the Qalibra model.

In other cases, combining intakes using toxic equivalency factors may be inappropriate, or the nature of the interaction between different effects on the same endpoint may be unknown. For example, oily fish contain nutrients that may increase IQ of the next generation but also methylmercury, which may decrease next generation IQ. There is no special provision for such effects in the Qalibra framework. A practical approach is to include the different nutrients and/or contaminants separately, and then take special care in interpreting the results. If the health endpoint is continuous (e.g. IQ) and if its relationship to the severity weight (DALY or QALY weight) is linear, then the additive aggregation of DALYs/QALYs in the Qalibra framework should give an appropriate result. Where the endpoint is quantal, or if the relationship to severity weights is non-linear, additive aggregation may give misleading results. To guard against this, the user should inspect very carefully the contributions of the individual substances: if either the negative or positive effects are very dominant, the result may be reasonable, whereas if both negative and positive effects are non-negligible then additive aggregation may either over- or under-estimate the net impact; in the latter case, it may be necessary to develop a bespoke model (outside Qalibra) to model the interaction of effects more appropriately.