Evaluating the relevance of seasonal differentiation of human health intake fractions in life cycle assessment

Abstract

The intake fraction (iF) is the fraction of an emitted mass of chemical that is ultimately taken in by an entire population, and it is used as an indicator of human health potential impacts related to environmental chemical persistence and bioaccumulation in the food chain. In chemical screening applications, the iF can be predicted using multimedia and multipathway fate and exposure models. One of the sources of iF uncertainty is the natural seasonal variability of the input parameters used in the models, i.e., the physicochemical properties of the pollutant and the landscape and exposure parameters. The objective of this article is to determine the relevance of including seasonal differentiation when assessing iFs in life cycle assessment. This was done by calculating and comparing seasonal iFs with each other and with iFs at 25° C, for both Canadian and global contexts. Two Canadian seasonal models based on the IMPACT 2002 predictive tool, and 2 models for the global context based on the USEtox consensus model were developed to calculate summer and winter iFs. Emissions into air and water and a set of 35 representative organic chemicals were considered. Partition coefficients for seasonal conditions were calculated using an integration of the van't Hoff equation. First-order degradation rate constants were calculated assuming that the rate constant doubles with each 10° C increase in temperature. For Canadian air emissions, results indicated that iFs for winter emissions could be up to 1 to 2 orders of magnitude higher than summer iFs or iFs calculated at 25° C. For Canadian water emissions, results showed that iFs for both summer and winter conditions were, in general, closer to each other with outliers within 1 order of magnitude to iFs calculated at 25° C. Results also indicated that seasonal variability was of lesser importance when assessing iFs within a global context. Because the ranking between chemicals was maintained, it can be concluded that seasonal variability is not relevant within a comparative context. However, this difference might be significant when comparing the magnitude of human toxicity impacts versus other impact categories contributing to human health damages.