Evaluation of exposure concentrations used in assessing manufactured nanomaterial environmental hazards: are they relevant?

Manufactured nanomaterials (MNMs) are increasingly produced and used in consumer goods, yet our knowledge regarding their environmental risks is limited. Environmental risks are assessed by characterizing exposure levels and biological receptor effects. As MNMs have rarely been quantified in environmental samples, our understanding of exposure level is limited. Absent direct measurements, environmental MNM concentrations are estimated from exposure modeling. Hazard, the potential for effects on biological receptors, is measured in the laboratory using a range of administered MNM concentrations. Yet concerns have been raised regarding the "relevancy" of hazard assessments, particularly when the administered MNM concentrations exceed those predicted to occur in the environment. What MNM concentrations are administered in hazard assessments and which are "environmentally relevant"? This review regards MNM concentrations in hazard assessments, from over 600 peer-reviewed articles published between 2008 and 2013. Some administered MNM concentrations overlap with, but many diverge from, predicted environmental concentrations. Other uncertainties influence the environmental relevance of current hazard assessments and exposure models, including test conditions, bioavailable concentrations, mode of action, MNM production volumes, and model validation. Therefore, it may be premature for MNM risk research to sanction information on the basis of concentration "environmental relevance".