Silicosis is a disease of the lung caused by the deposition of fine crystalline silica particles (10 μm or less in diameter) in the lungs. The deposition of silica particles in the lungs triggers a chronic inflammatory response resulting in normal lung tissue being replaced with scar tissue (fibrosis). Symptoms of silicosis may include cough, shortness of breath, chest pain, early fatigue with exertion, and wheezing. Silicosis usually occurs after years of exposure (chronic) but may appear in a shorter period of time (acute) if exposure concentrations are very high. Acute silicosis is typically associated with a history of high exposures from tasks that produce small particles of airborne dust with a high silica content.⁽¹⁶⁾ The International Agency for Research on Cancer (IARC) in 1996 concluded that there was “sufficient evidence in humans for the carcinogenicity of inhaled crystalline silica in the form of quartz or cristobalite from occupational sources.”⁽¹⁷⁾ Workers diagnosed with silicosis are also at an increased risk of developing tuberculosis due to silica particles disabling the macrophages.⁽¹⁸⁾ For a diagnosis of occupational silicosis, workers must have a history of exposure to respirable silica and a confirmatory test such as a chest X-ray, chest computed tomography (CT), or lung biopsy.

When proper work practices are not followed or controls are not employed, respirable crystalline silica exposures obtained during activities involving cutting, drilling, and crushing of materials that contain silica can exceed the NIOSH recommended exposure limit (REL), the ACGIH® threshold limit value (TLV® ), and the Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL).^(19–21) NIOSH recommends an exposure limit for quartz of 0.05 mg/m³ for up to 10 hr as a time-weighted average (TWA) to reduce the risk of silicosis, lung cancer, and other adverse health effects. The ACGIH TLV for respirable quartz is 0.025 mg/m³ as an 8-hr TWA to protect against pulmonary fibrosis and lung cancer.⁽²⁰⁾

The OSHA PEL for respirable dust containing 1% quartz or more in general industry (29 CFR 1910.1000) is expressed as an equation:⁽²¹⁾

The OSHA PEL for respirable dust containing 1% quartz or more in construction (29 CFR 1926.55) is expressed as an equation:⁽²²⁾

The current OSHA PEL for respirable dust containing crystalline silica (quartz) for the construction industry is measured by impinger sampling. The PEL is expressed in millions of particles per cubic foot (mppcf); however, since the PELs were adopted, the impinger sampling method has been rendered obsolete by gravimetric sampling.⁽²³⁾ OSHA’s compliance of-ficers apply a conversion factor of 0.1 mg/m³ per mppcf when converting between gravimetric sampling and particle counts when characterizing construction operation exposures.⁽²⁴⁾ In this study dust concentrations are presented in mg/m³.

Roofers in home construction employed by four companies (A, B, C, and D) were included in these evaluations. Spanish was the primary language for most employees. The number of employees in each company ranged from 60 to 400; the smallest company installed approximately 200 roofs per month, while the largest company installed approximately 800 roofs per month. All four companies were located in the greater Phoenix area.

The employees were organized in crews of three to five, typically consisting of a foreman, a “second man,” laborers, and drivers. The work shift was typically 6:00 a.m. to 3:00 p.m. for 5 or 6 days per week. The roof installation included three phases: (1) laying tar paper and nailing boards to hold the tiles on the roof; (2) setting the tiles by stacking them in various areas of the roof; and (3) placing the tiles individually on the roof and cutting and nailing them in place. The tiles come in various colors and can be molded to look like wood shingles. They also come in different shapes (e.g., flat, barrel-shaped, and S-shaped). At least one hand-held gas-powered cutting saw is used per crew. Generally, the foreman or the second man cuts the tiles while the laborers and drivers lay and nail the tiles in place. At some work sites, laborers and drivers were also cutting tiles if the foreman and second man were not available. Dust is generated during the cutting of tiles to fit for size at the channels and valleys on the roof, at cupolas or turrets, and at the ends of the roof. At the completion of the roof installation, a gas-powered leaf blower is used to remove dust and debris from the tiles.

The sampling strategy consisted of selecting home sites each day where employees would be cutting and laying tiles throughout the day. Two or three days of sampling was conducted on workers on the roof (i.e., foreman, second man, laborers, and drivers) at each roofing operation evaluated. Bulk samples of tile dust were collected at each house to determine the silica content in the manufactured tile. The samples were analyzed for silica (quartz, cristobalite, and tridymite), using X-ray diffraction, per NIOSH Manual of Analytical Methods (NMAM) 7500.⁽²⁴⁾

Personal breathing zone (PBZ) air samples for respirable dust were collected and analyzed according to NIOSH Method 0600.⁽²⁴⁾ Samples were collected on tared 37-mm, 5-μm polyvinyl chloride (PVC) filters, at 1.7 L/min using a 10-mm Dorr-Oliver nylon cyclone pre-selector with a cut point of 3.5 μm for respirable dust. In addition, the respirable dust samples were analyzed for silica content by X-ray diffraction using NIOSH Method 7500.⁽²⁴⁾

For silica results between the limit of detection (LOD) and the limit of quantitation (LOQ), the minimum quantifiable concentration (MQC) was used. This was calculated from the laboratory LOQ and sample volume.

Real-time sampling for airborne particulates was conducted at the largest construction company (A) with an optical particle counter (GRIMM Aerosol, Ainring, Germany). The instrument operates at a flow rate of 2 L/min and can measure particle sizes ranging from 0.23 μm to greater than 20 μm. Data were integrated over 1-min intervals.

We collected one set of data each to monitor the particulates generated by three distinct events during roof tile cutting and clean-up on newly constructed houses. These events were cutting roof tiles in a roof valley, cutting roof tiles that would be used on a roof turret, and blowing tile dust off the roof with a leaf blower. Measurements were collected in the general vicinity of the worker’s breathing zone. Start and stop times for significant tasks were recorded during each sample collection period. The data collected revealed information on the mass distribution of particles, which is reported as a concentration in mg/m³. Estimates were made of the mass median aerodynamic diameter (MMAD) and the associated geometric standard deviation (GSD) based on the integrated particle size discrimination provided by the instrument. The density of the roofing tile particulate was assumed to be 1.0 (g/cm³).

During the initial evaluation at Company A, real-time particulate sampling was conducted during three separate tasks. Results are presented in Table III that summarize the total dust concentration, MMAD, GSD, and respirable mass fractions, by task. The MMAD value indicates the diameter at which half the total mass of particles is larger and half is smaller. Results indicate that approximately 11% to 17% of the particles generated during these tasks are in the respirable range. The respirable mass fractions reflect the percentage of total mass in the respirable range, less than 10 μm.