Characterization of Airborne Dust Generated from the Grinding of Natural and Engineered Stone Products

Details

• Personal Author:
  Qi, Chaolong ; Thompson, Drew
• Corporate Authors:
  National Institute for Occupational Safety and Health. Division of Applied Research and Technology
• Description:
  Background: Workplace exposure to respirable crystalline silica (RCS) can cause silicosis, a progressive lung disease marked by scarring and thickening of the lung tissue. Crystalline silica is found in several materials, such as brick, block, mortar, and concrete. Construction and manufacturing tasks that cut, break, grind, abrade, or drill those materials have been associated with overexposure to dust containing RCS. Stone countertop products can contain various levels of crystalline silica (can be >90 wt%) and working with this material during stone countertop fabrication has been shown to cause excessive RCS exposures. NIOSH scientists are conducting a study to develop a control strategy for workers' RCS exposure during stone countertop fabrication. The laboratory research described in this report is part of that study. Assessment: NIOSH scientists systematically characterized the airborne dust generated from grinding engineered and natural stone products using a laboratory testing system designed and operated to collect representative respirable dust samples. The laboratory experiments in this study determined dust and crystalline silica generation rates, dust size distributions, and crystalline silica content during the dry grinding of nine stone countertop products including seven engineered stones containing crystalline silica in a polymer resin matrix from five major manufacturers (labeled Stone A, B, and 1 through 5 throughout), one engineered stone containing recycled glass in a Portland cement matrix (Stone C), and one natural stone, granite. Results: For each stone product, the corresponding crystalline silica content in bulk dust samples and respirable dust samples were found to be similar. The crystalline silica content in the respirable dust was within or below the ranges reported in the manufacturers' safety data sheets for each of the respective stone products. It is worth noting that no crystalline silica was detected in Stone C. In addition, Stone B, whose new formulation was advertised by the manufacturer as having a reduced crystalline silica content (≤ 50 wt%) with respect to their previous formulation, had a crystalline silica content comparable to that of the granite evaluated in this study (about 20-30 wt%). Since sample thickness may influence the rate at which material is removed from the stone product sample during grinding, the normalized generation rate results from differing sample thickness are presented separately. Among the four stone products with samples about 30 mm thick, the mean normalized generation rates of respirable dust ranged from 24 to 43 mg cm-3 with granite being the highest and Stones A, B, and C being comparable. The mean normalized generation rates of RCS for these samples ranged from 0.0 to 16 mg cm-3 with Stone A being the highest followed by granite, Stone B, and finally Stone C which generated no detectable crystalline silica. Among the five engineered stone products with a resin matrix and sample thicknesses of around 20 mm, mean normalized generation rates of respirable dust and RCS both remain in relatively narrow ranges. The mass-based distributions showed the most prominent modes at 5.1 - 8.0 µm for all the stone products evaluated in this study. This suggests that the mechanical process of the fabrication task, in this case a pneumatic angle grinder equipped with a coarse diamond grinding cup wheel, rather than the type of stone product predominantly determines the shape of the dust size distribution. The results of particle size distribution and RCS generation rate both contributed to the observation published previously that the highest normalized generation rate of RCS consistently occurred at 3.2 - 5.6 µm for all the stones containing crystalline silica. Conclusions and Recommendations: Workers are likely to be exposed to lower concentrations of RCS when working with engineered stones containing no crystalline silica (e.g., Stone C), followed by engineered stones specifically designed with lower silica content (e.g., Stone B), then granite similar to the one in this study, and finally engineered stones that contain high silica content (up to about 90 wt% in a resin matrix). Working with the engineered stones from different manufacturers that have similar thicknesses and colors and contain similarly high levels of silica content is likely to lead to similar levels of RCS exposure for workers. Thus, following the hierarchy of controls, a layered, overall control strategy can incorporate elimination (e.g., products similar to Stone C), substitution (e.g., products similar to Stone B), and engineering controls at the top to minimize workers' RCS exposure during stone countertop fabrication. For developing engineering controls, prioritizing the removal of particles in the range of 3.2 - 5.6 µm near the generation sources should help maximize RCS reduction, since the highest normalized generation rate of RCS consistently occurred in this size range for all the stones containing crystalline silica in this study.
• Subjects:
  Aerosols Air Pollutants, Occupational Dust Engineering Occupational Health Particulate Matter Safety Silicon Dioxide
• Keywords:
  Aerosol Sampling Airborne Dusts Control Methods Crystalline Silica Engineering Controls Grinding Equipment Particle Size Respirable Dust Stone Products X-ray Diffraction
• Series:
  Engineering and Physical Hazards Survey Reports
• DOI:
  https://doi.org/10.26616/NIOSHEPHB2023DFSE1489
• Publisher:
  National Institute for Occupational Safety and Health
• Document Type:
  Text
• Genre:
  Field Study
• Place as Subject:
  Ohio ; OSHA Region 5
• CIO:
  National Institute for Occupational Safety and Health (NIOSH)
• Division:
  DFSE - Division of Field Studies and Engineering ; EPHB - Engineering and Physical Hazards Branch
• Topic:
  Workplace Safety & Health
• Location:
  North America
• Pages in Document:
  1-50
• NIOSHTIC Number:
  nn:20068745
• Citation:
  Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, EPHB Report No. 2023-DFSE-1489, 2023 Oct ; :1-50
• CAS Registry Number:
  Quartz (SiO2) (CAS RN 14808-60-7)
• Federal Fiscal Year:
  2024
• NORA Priority Area:
  Construction ; Manufacturing
• Peer Reviewed:
  False
• NAICS and SIC Codes:
  327991 - Cut Stone and Stone Product Manufacturing - (NAICS)
• Collection(s):
  National Institute for Occupational Safety and Health
• Main Document Checksum:
  urn:sha-512:ad1e8d0d0269c7637a94cd3e335e438566bec06381ea8e4bbdf2e69d4cecd5aa6e6317b67255adae4c5215d925a2db5e967b997fde028bd6efb8a4ed25b36e03
• Download URL:
  https://stacks.cdc.gov/view/cdc/174852/cdc_174852_DS1.pdf
• File Type:
  [PDF - 1.70 MB ]