From correlation to causation: Integrating cohorts with experimental studies in mixture toxicology

Harmful chemical mixtures are pervasive in the environment, yet traditional epidemiological designs face major challenges in establishing causal links between individual chemicals or mixture exposures and health outcomes. These challenges arise from the high dimensionality and inter-correlation of exposures, their mediation through complex molecular pathways, and the practical absence of truly unexposed control groups, due to the ubiquity of synthetic chemicals. However, environmental health research is entering a new era defined by integration of epidemiological and experimental studies as well as recent advances in molecular technologies and computational modelling. Here, we introduce four approaches designed to advance our understanding of chemical mixtures and move beyond correlation to causation and intervention: 1) 'hMIX' which integrates human relevant reference mixtures with experimental evidence of adverse effects; 2) the Similar Mixture Approach (SMACH) that translates hazards of chemical mixtures to risks across populations; 3) hybrid epidemiology that bridges experimental and population-based mechanistic insights; and 4) counterfactual theoretical interventions tailored to examine the health benefits of reducing exposure to specific harmful chemicals or mixtures. We propose an integrative framework combining these four approaches to move the chemical mixture field towards causality - a critical step toward predicting and preventing chemical mixture related health effects.