Release of Crystalline Silica Nanoparticles During Engineered Stone Fabrication

Inhalation exposure to respirable crystalline silica (RCS) during fabrication of engineered stone-based kitchen countertops has been on the rise in recent years and has become a significant occupational health problem in the United States and globally. Little is known about the presence of nano-crystalline silica (NCS), i.e., particles below 100 nm. We present a methodology to quantify the crystalline silica content in the sub-100 nm size fraction of the aerosol released during engineered stone fabrication using X-ray diffraction (XRD) and Fourier transform Infrared (FT-IR) spectroscopy. Aerosol was generated in test chamber designed per EN 1093-3 and sampled using cascade impactors. XRD and FT-IR analysis showed the presence of both α-quartz (15 – 60 %) and cristobalite (10 – 50 %) polymorphs in all size fractions. With increasing particle size, the cristobalite content increased. 70 % of the total aerosol mass in the sub-100 nm fraction was found to be crystalline silica, qualitatively confirmed by electron diffraction and electron energy loss spectroscopy. Presence of other minerals was detected in all size fractions; no polymeric resin binder was detected in the sub-100 nm fraction. Although, the sub-100 nm fraction was about 1 % of the aerosol mass, it accounted for 4 – 24 % of the aerosol surface area based on total lung deposition. If the surface area is a more relevant exposure metric, the assessment of efficacy of current engineering control systems using mass as an exposure metric may not provide adequate protection.