Exposure to respirable airborne particulate matter containing crystalline silica, particle size distribution and morphology.

Exposure to respirable airborne particulate matter containing crystalline silica, particle size distribution and morphology.

Hedges, Kevin
Occupational Health Clinics for Ontario Workers, Ottawa Clinic, Canada
Keywords: Respirable dust, silica, equivalent aerodynamic diameter, particle size.

Topic(s): Aerosol Physics, Aerosol Chemistry, Aerosol Toxicity

This study by Hedges K, (2016), evaluated the health risk from exposure to RCS for 47 quarry and dimension stone mine workers throughout Queensland, Australia. Personal exposures to RCS were measured.

Electron microscopy (EM) was used to describe the physical characteristics of respirable silica and dust particles in respirable dust samples. Data revealed that silica particles were generally less than 5 μm in physical diameter and many particles were elongated. These smaller particles are known to be most hazardous to lung health.

Findings demonstrated that larger length elongated particles were collected by the cyclone sampler, which influenced the particle size distribution curve. There was a good fit between the physical cumulative silica particle size distribution, representing particle counts for selected workplace samples, when plotted against the theoretical (density dependent) equivalent aerodynamic diameter (EAD) sampling efficiency curve. This means that density is not the only factor for particle capture in the cyclone sampler. These silica particles do not behave as perfect spheres, which is the premise underpinning the International Organization for Standardization (ISO) 7708-1995, sampling efficiency curve. The science for these standards is based on information that is at least twenty years old, and results from the current study confirmed that particle shape must be considered in the sampling efficiency curve and lung health risk assessment Analysis by electron microscopy of respirable dust and respirable crystalline silica provides an insight into the particle size distribution and physical form of the different particle sizes. The curve is skewed towards the smaller size fraction of the curve as % of RCS in respirable dust increases. This suggests that the risk from exposure is not linear.

The empirical nature of EAD is questioned, and the context in which it is used is over-simplified, especially as airborne particles do not exist as perfect spheres. The use of EAD that does not match workplace aerosols, nor consider shape, overly simplifies the empirical EAD model.

References
Hedges, K. J. G. (2016). Assessment and control of respirable crystalline silica in quarries and dimension stone mines (Doctoral thesis, Western Sydney University). Western Sydney University Research Repository. https://researchers.westernsydney.edu.au/en/studentTheses/assessment-and-control-of-respirable-crystalline-silica-in-quarri/

International Organization for Standardization. (1995). ISO 7708:1995: Air quality — Particle size fraction definitions for health-related sampling. ISO.