Advancements in crystalline silica exposure assessment and control

Beauregard Middaugh, Purdue University

Abstract

Excessive exposure to crystalline silica remains prevalent in the U.S. construction industry. Chronic inhalation of respirable crystalline silica is associated with the development of progressive pulmonary fibrosis and has been implicated in the pathogenesis of lung cancer and various autoimmune diseases such as systemic sclerosis, chronic renal disease, lupus, and rheumatoid arthritis. Although the relationship between silica-containing dust and pulmonary disease has been documented for centuries, the pathways triggering a progressive and untreatable disease state remain largely unclear. Attenuation of silica-related disease is dependent upon improved methods for assessing and reducing dust exposure, but a latent exposure-response to this abundant mineral reinforces the need for better biological indicators of chronic exposure and improved knowledge of biological interactions from silica mixtures. In the most recent National Occupational Research Agenda (NORA) for Construction, the National Institute for Occupational Safety and Health (NIOSH) has requested research to advance current deficiencies in hazard communication, dust control, and exposure assessment for silica exposures and related illnesses. The central purpose of this dissertation was to characterize unevaluated silica-related dust exposures and exposure controls while advancing methods for outdoor exposure assessment and communication of risk for task-based operations in roadway construction. During an exposure assessment of concrete curb cutting with a hand-held cut-off saw, wet suppression and local exhaust ventilation (LEV) control were found to significantly (p<0.001) reduce personal respirable dust exposure by +78.0 percent and +73.2 percent respectively when compared to no dust control, an exposure scenario where crystalline silica exposure was estimated to exceed the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) by almost 40 times. Videos of these exposure control methods were presented side-by-side with real-time dust concentration to local construction supervisors followed by an anonymous survey. Supervisors answered "yes" on 85 percent of returned surveys when asked if they would consider changing current uncontrolled work practices as a result of the training. During an exposure assessment of asphalt grinding with a cold-planer equipment attachment, an 80 percent increase in respirable dust exposure was observed for wet suppression when compared to no dust control during matched wind conditions, identifying a unique flaw in wet control technology. A new systematic method of reporting wind condition also identified that exposure can differ significantly by more than 1000 percent across 30 degree by 400 feet per minute (fpm) wind condition categories (WCCs). Reporting outdoor exposure data using the methods developed in this assessment will allow future researchers to easily compare tool-specific exposure during specific wind conditions, ultimately improving outdoor exposure assessments and reducing selection error for dust controls. During an exposure assessment of soil stabilization with lime and cement mixtures, excessive overexposures to Portland cement and calcium oxide were identified for multiple work tasks, but simultaneous exposure to crystalline silica was either non-detectable or below the ACGIH TLV in all mixed dust samples. These same four workers evaluated in the soil stabilization assessment were individually matched with four non-exposed roadway workers by gender (male), race (Caucasian), age (± 2 years), height (± 3 in.), and pack-years (±2.5 pack years). The natural log of the difference in cumulative calcium exposure between each matched pair was a significant predictor (p<0.05, r2=0.98) of the natural log of the difference in sputum neutrophil count. In the context of prior research, this indicates that the neutrophils remain present during tissue repair and after inflammation subsides, providing further evidence of possible mechanisms of disease potentiation by metallic irritants co-generated with quartz. Collectively, this study advanced silica exposure assessment and control by quantifying exposure for multiple dust-generating tasks, determining the effectiveness of unevaluated tool-specific dust controls, surveying the perception of tool-specific video exposure training among construction supervisors, developing a new methodology for reporting and adjusting exposure by wind condition, and detecting a possible field-friendly biological marker of cumulative exposure for a substance that commonly accompanies silica in construction dust.^

Degree

Ph.D.

Advisors

Neil Zimmerman, Purdue University.

Subject Area

Health Sciences, Occupational Health and Safety

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