Unravelling the chemical exposome in cohort studies: routes explored and steps to become comprehensive

Abstract

Environmental factors contribute to the risk for adverse health outcomes against a background of genetic predisposition. Among these factors, chemical exposures may substantially contribute to disease risk and adverse outcomes. In fact, epidemiological cohort studies have established associations between exposure against individual chemicals and adverse health effects. Yet, in daily life individuals are exposed to complex mixtures in varying compositions. To capture the totality of environmental exposures the concept of the exposome has been developed. Here, we undertake an overview of major exposome projects, which pioneered the field of exposomics and explored the links between chemical exposure and health outcomes using cohort studies. We seek to reflect their achievements with regard to (i) capturing a comprehensive picture of the environmental chemical exposome, (ii) aggregating internal exposures using chemical and bioanalytical means of detection, and (iii) identifying associations that provide novel options for risk assessment and intervention. Various complementary approaches can be distinguished in addressing relevant exposure routes and it emerges that individual exposure histories may not easily be grouped. The number of chemicals for which human exposure can be detected is substantial and highlights the reality of mixture exposures. Yet, to a large extent it depends on targeted chemical analysis with the specific challenges to capture all relevant exposure routes and assess the chemical concentrations occurring in humans. The currently used approaches imply prior knowledge or hypotheses about relevant exposures. Typically, the number of chemicals considered in exposome projects is counted in dozens-in contrast to the several thousands of chemicals for which occurrence have been reported in human serum and urine. Furthermore, health outcomes are often still compared to single chemicals only. Moreover, explicit consideration of mixture effects and the interrelations between different outcomes to support causal relationships and identify risk drivers in complex mixtures remain underdeveloped and call for specifically designed exposome-cohort studies.

Keywords: Biomarkers; Biomonitoring; Cohort studies; Environmental exposure; Environmental health; Exposome; Mixture toxicity.

Fig. 1

Conceptual framework for exposome studies (top panel), and elements for estimation of multiple environmental exposures against chemicals (bottom panel). Estimation of multiple external human exposures approached from analysis of exposure sources, social environment, exposure pathways, behavioral patterns and immission samplers. Indoor and outdoor environments are often separated in these efforts. Internal body burdens are analyzed in individual or population-based biosamples, such as blood. Typical analytical techniques are indicated in the white egg shapes. All approaches cover different aspects of a comprehensive exposome with regard to aggregation level, spatio-temporal characterization or indication of biological impact

Fig. 2

Substances in the chemical universe, the exposome, and human biomonitoring studies