A Systematic Review of the Routes and Forms of Exposure to Engineered Nanomaterials.

Background: Establishing the routes of exposure is a fundamental component of the risk assessment process for every dangerous substance. The present study systematically reviews the available literature to assess the relevance of the different routes and forms of exposure that are of concern for the protection of workers during the manufacture, handling, or end-use of engineered nanomaterials (ENMs).
Methods: A systematic review of the peer-reviewed literature published between 2000 and 2015 was completed. Only studies including measurements of inhalation or dermal exposure were selected and used to identify the exposure situations for which the measurements were collected. The identified exposure situations were grouped based on the type of ENM (i.e. carbon nanotubes and fibres, silicon-based, titanium dioxide, other metal oxides, pure elemental metals, and other ENMs) and activity involved. The grouped exposure situations were assessed to provide a conclusion regarding the likelihood, form, and route of exposure. Assessment of the likelihood of exposure was based on well-defined criteria using a previously established decision logic for inhalation exposure and the outputs from measurements and/or conceptual models for dermal/ingestion exposure. For each combination of nano-activity and type of ENM, the aggregated likelihood across all relevant individual assessments was used to draw conclusions about the relevance of both the inhalation and dermal/ingestion routes. Based on the quality of the data, the strength of the evidence was also evaluated.
Results: One hundred and seven studies were identified during the review process, reporting 424 individual exposure assessments. Measurement data were limited for dermal/ingestion exposure and for inhalation exposure for downstream use and end-of-life. However, the data provided high-quality evidence that in occupational settings all three routes can be of relevance for exposure to ENMs. In general, whenever inhalation exposure occurs then dermal and inadvertent ingestion exposure may occur due to surface deposition and transfer due to the ENMs release. However, for some forms of exposure (e.g. suspension/liquids), dermal exposure can occur even when inhalation exposure is unlikely. An increased likelihood of exposure was observed for manual activities such as cleaning and maintenance, collection/harvesting, spraying, and finishing as well as those involving feeding into a process and handling of powders outside enclosures. The likelihood of exposure was affected by the presence of risk management measures and the scale of the production involved.
Conclusion: This literature review provides evidence that for ENMs, as found for other materials, the likelihood of the exposure depends largely on the physical form of the substance as well as the applied process and operational conditions. These results can be used to provide first indications of the likelihood of exposure and guidance for exposure controls in workplaces. However, there is a clear lack of high-quality exposure data, in particular for downstream use and end-of-life scenarios and in low- and medium-income countries.