Towards a surface metric to measure the dustiness of nanomaterial powders.

The relevance of dustiness methods is increasingly recognized in the preliminary exposure evaluation of workers handling nanomaterials in powder form, and should also be transposed to the assessment of environmental risk in the future. The methods currently recommended in the European standards are mainly based on determining a mass-based dustiness index [mg kg-1], whereas surface area is regularly put forward as a more appropriate determinant to assess the pulmonary toxicity of nanoparticles. In this study, we describe an operational methodology leading us to propose a surface metric to determine the dustiness index [m2 kg-1] of nanoparticulate matter. To this end, we demonstrate the equivalence between the external specific surface area of a nanopowder and that of its aerosol with five nanomaterials produced and used on an industrial scale, and covering a range of external specific surface areas from 35 to 230 m2 g-1. Compared to the conventional mass-based dustiness index, the surface-based dustiness index (1) is more discriminating, covering an additional order of magnitude, and (2) has an impact on the powder ranking with potential consequences on the preventive measures to be implemented. Finally, our proposal has the potential to be included in future revisions of European standards for workplace exposure and dustiness measurement, provided that further experimental results on surface-based dustiness indices support these preliminary data.

Identification of nanomaterials by the volume specific surface area (VSSA) criterion: application to powder mixes.

We demonstrate the relevance of the Volume Specific Surface Area (VSSA) parameter to identify the nanoparticulate character of powder mixes based on either spherical constituent particles with bimodal size distributions (TiO2), or fiber-like constituent particles with unimodal size distributions (sepiolite and sepiolite-based pigments). These new results indicate that VSSA could reasonably be proposed as an optional criterion in the future for the definition of nanomaterials based on the European Commission recommendation, provided certain requirements are fulfilled.

Toward an operational methodology to identify industrial-scaled nanomaterial powders with the volume specific surface area criterion.

Nanoparticulate powders are increasingly found in the workplace. Inhalation exposure to airborne nanoparticles (NPs) is possible throughout the life-cycle of the powders. As the toxicity of NPs has never been demonstrated, it remains essential to evaluate the risks associated with NPs in order to propose preventative measures. The first step of a risk assessment strategy consists in the identification of the 'nano' nature of a material, which suffers from a lack of an operational methodology. Here, we present a simplified and operational strategy relying on the volume specific surface area (VSSA) for nanomaterial identification, based on the recommendation stemming from the European Commission and previous work on this topic from the European Project Nanodefine. The proposed strategy was tested on a set of 15 representative industrial powders (TiO2, SiO2, CuO, and ZnO), covering a wide range of properties, and previous published data. The VSSA classification was validated via a comparison with the particle size obtained by transmission electron microscopy (TEM). It was evidenced that the VSSA is in accordance with particle size for nanomaterial powder classification. The proposed methodology involves relatively accessible methods such as thermogravimetric analysis, nitrogen adsorption and helium pycnometry and limits the detection of false negatives. Moreover, it does not imply systematic confirmation of the results with the reference particle size criterion. Our results suggest that the VSSA is a promising parameter to be used for risk assessment and should be further investigated on powder mixings to confirm its relevancy to define nanomaterial powders.