Many compounds used by the building construction industry are bulk materials, including, but not limited to, gypsum, calcined gypsum, mica, cement, calcium carbonate and sand. These dry compounds comprise a population of particles of different sizes.
Particles which are smaller than 20 microns are referred to respirable particles because these particles can be dispersed into the air. These particles can be then inhaled by workers, which should be avoided. Thus, there is a need to analyze a powder sample and provide an accurate estimate whether the sample comprises respirable particles and the percentage of such particles in the sample.
The standard procedure for respirable silica requires the sample in analysis to be treated with acid to eliminate most of the sample matrix. The remaining material is examined for crystalline silica. The weight of crystalline silica is determined using X-ray Diffraction or FTIR. This number is reported as Total Crystalline Silica. The second step requires the sample to be dispersed in alcohol and transferred to a silver membrane and the mass of crystalline silica determined for the <10μ fraction. A determination is also provided for the <5μ fraction.
The criteria in the Globally Harmonized System (GHS) have the fine fraction of silica classified as specific target organ toxicity in this case, the lung. Generic cut-off values for products containing a fine fraction of crystalline silica trigger a need for a method for the quantification of the fine fraction of crystalline silica in bulk materials.
The European Union has promoted an analysis for silica in bulk materials which culminated in the publication of a new standard for measuring the amounts of respirable particles in bulk materials. The standard has two parts:
1. Determination of Size-Weighted Potential Respirable Fraction (SWeRF); and
2. Size-Weighted Potential Respirable Fraction of Crystalline Silica (SWeRFCS)
The method is to be used for comparing the potential health risks of bulk materials. The method does not predict how a material will disperse in air, but quantifies the respirable fraction. The particles that present a larger health risk are weighed more in the calculation. The advantage of the method is it provides an unambiguous characterization of the bulk material. The term “potential” is used to indicate that the standard does not analyze airborne particles.
The standard describes a method using sedimentation and a calculation method based on particle size distribution (PSD). The calculation can only be used after the results are validated using the sedimentation data. The calculation method requires that the particles have the same density or in the case of mixture with different materials that they have the same PSD. A plot is made to compare the sedimentation PSD with the Stokes' Law and the convention described in CSN EN 481 (European Standards EN 481 “Workplace atmospheres—size fraction definitions for measurement of airborne particles”). However, in this calculation method, the dynamic form factor is neglected where in the sedimentation method, the dynamic form factor is assumed to be equal in air and liquid. Thus, there remains a need for accurate and rapid detection of respirable particles in a bulk material.