From biomonitoring we know that people are exposed to several different exogenous chemicals at the same time. For example, food packaging can be a source of chemical exposure, and in general several different substances will migrate from the packaging simultaneously (but at varying concentrations). Regulatory toxicology requires toxicity testing for compounds that are used to make food packaging, but they are tested individually and not in the final combinations that consumers are exposed to. In addition, people are exposed to many other compounds via food, personal care products, air, and all these exposures add to chemical body burden and can cause cumulative effects.
In recent years it has clearly been established by scientific studies that mixtures of chemicals can act together to cause effects, even if the individual compounds are below their regulatory relevant effect concentration. These phenomena are observed in vitro (cell-based toxicity testing) (Rajapakse et al. 2002) and in whole organisms (Hass et al. 2007; Christiansen et al. 2009; Christiansen et al. 2012).
Mixture effects can be predicted with existing models, and cumulative risk assessment of similarly acting chemicals has been suggested (Kortenkamp and Faust 2010). However, exact data on exposure and toxicological properties are often missing and this reduces model reliability.
Mixture phenomena are undisputed in the scientific community, but their translation into risk assessment is proving to be challenging. An ongoing EU research project addresses how risk assessment for mixtures can be managed.