Reply to the letters to the editor submitted by T. L. Ogden and K.T. Du Clos, and by R. Foster regarding the paper ‘SWeRF—a method for estimating the relevant fine particle fraction in bulk materials for classification and labelling purposes’.

The authors respond to the points raised in the Letters to the Editor raised by Ogden and Du Clos and by Foster. Ad 1: The debate of the classification of respirable cyrstalline silica is outside the scope of the technical paper. Ad 2: A standard for the determination of SWeRF is under development, in which indeed the provision is made that for a correct determination all quartz within the fine fraction needs to be liberated.

SWeRF--A method for estimating the relevant fine particle fraction in bulk materials for classification and labelling purposes.

In accordance with the European regulation for classification, labelling and packaging of substances and mixtures (CLP) as well as the criteria as set out in the Globally Harmonized System (GHS), fine fraction of crystalline silica (CS) has been classified as a specific target organ toxicity, the specific organ in this case being the lung. Generic cut-off values for products containing a fine fraction of CS trigger the need for a method for the quantification of the fine fraction of CS in bulk materials. This article describes the so-called SWeRF method, the size-weighted relevant fine fraction.

Is the transformation/dissolution protocol suitable for ecotoxicity assessments of inorganic substances such as silica fume?

Performing ecotoxicity tests on poorly water soluble substances and in particular metals, metalloids, and metal oxides such as silica fume, can be problematic. Such substances may not be directly toxic to aquatic organisms but often have high concentrations of impurities present, due to production processes, which may result in ecotoxicological effects. This combined with possibly testing above the limit of solubility further exacerbates the interpretation of ecotoxicity test results.

Comparison of non-crystalline silica nanoparticles in IL-1β release from macrophages.

Background: Respirable crystalline silica (silicon dioxide; SiO₂, quartz) particles are known to induce chronic inflammation and lung disease upon long-term inhalation, whereas non-crystalline (amorphous) SiO₂ particles in the submicrometre range are regarded as less harmful. Several reports have demonstrated that crystalline, but also non-crystalline silica particles induce IL-1β release from macrophages via the NALP3-inflammasome complex (caspase-1, ASC and NALP3) in the presence of lipopolysaccharide (LPS) from bacteria. Our aim was to study the potential of different non-crystalline SiO₂ particles from the nano- to submicro-sized range to activate IL-1β responses in LPS-primed RAW264.

An International comparison of the crystallinity of calibration materials for the analysis of respirable alpha-quartz using X-ray diffraction and a comparison with results from the infrared KBr disc method.

It is important that analytical results, produced to demonstrate compliance with exposure limits are comparable, to ensure controls are monitored to similar standards. Correcting a measurement result of respirable alpha-quartz for the percentage of crystalline material in the calibration dust is good analytical practice and significant changes in the values assigned to calibration materials will affect the interpretation of results by an analyst or occupational hygiene professional. The reissue of the certification for the quartz reference material NIST 1878a in 2005 and differences in comparative values obtained by other work created uncertainty about the values of crystallinity assigned to national calibration dusts for alpha-quartz.