In May 2019, the Nordic Council of Ministers published a working paper entitled The applicability of the GHS classification criteria to nanomaterials.  The goal of the project was to review the applicability of the Globally Harmonized System of Classification and Labeling of Chemicals (GHS) to manufactured nanomaterials, taking into account the progress of international scientific work.  The report notes that the Organization for Economic Cooperation and Development (OECD) Working Party on Manufactured Nanomaterials (WPMN) has generated and compiled much data on nanomaterials under the Testing Program of Manufactured Nanomaterials.  These data were further assessed for some pre-selected nanomaterials:  single-walled carbon nanotubes (SWCNT), nano silicon dioxide, nano silver, and nano zinc oxide.  Additionally, the appropriateness of the GHS classification criteria for the generated data were evaluated for five health hazard classes for which an initial screening had shown a need for classification.  Finally, if applicable, relevant classifications of the nanomaterials were assessed.  For each of the relevant hazard classes, the available test data of the nanomaterials were summarized and evaluated with respect to:

• Applicability of the test methods;

• Applicability of the GHS criteria and proposed classification;

• Identified data gaps and uncertainties; and

• Need for revision of GHS criteria or further guidance.

The report lists the following overall observations and reflections:

• In general, the current GHS classification criteria for the five evaluated hazard classes were found to be applicable to the generated data on SWCNT, nano silicon dioxide, nano silver, and nano zinc oxide.

• Differences in toxicity exist between the various types/qualities (g., related to production methods (e.g., silicon dioxide) or impurity profile (e.g., SWCNT)) of the same nanomaterials that may result in different classifications of the various types/qualities.

• Specific target organ toxicity – repeat exposure (STOT RE) is considered a highly relevant hazard class to examine for all the nanomaterials especially considering the lung as the target organ.

• For voluminous nanomaterials (e., nanomaterials with a relatively high specific surface area and low density), testing at high dose levels may not be technically achievable. Hence, testing in accordance with OECD Test Guideline (TG) methods covering all relevant dose levels for acute toxicity classification and STOT RE classification according to the GHS criteria values may not be possible. This is especially relevant for testing via inhalation route.

• For acute toxicity and STOT RE, the GHS criteria based on a mass-based dose metric can be applied for voluminous nanomaterials, however, the dose levels corresponding to the less severe hazard categories cannot be technically achieved. It may be examined whether another dose metric (g., specific surface area or particle number concentrations) would be a better metric for enabling differentiation in toxicity and the classification of nanomaterials.

• It is noted that most testing regarding repeated inhalation exposure has focused on identification of no observed adverse effect concentration (NOAEC)/lowest observed adverse effect concentration (LOAEC) levels and the examination of early signs of toxicity (g., various inflammatory markers) rather than establishing data for STOT RE classification. So mostly very low exposure levels compared to the STOT RE criteria have been used.  Thus, there are data gaps for assessing the proper STOT RE classification of nanomaterials.

• As support for a STOT RE classification, it should be considered how to use an adverse outcome pathway (AOP) or mode of action (MOA) approach using inflammatory signs/markers or mild/moderate histopathological effects induced in target organs at very low exposure levels for classification purpose.

• Also, it may be examined how and under which circumstances data from intratracheal instillation or pharyngeal aspiration may be used as support for STOT RE classification if data from inhalation testing are limited or do not cover the relevant dose ranges for classification.