Exposure to silica

Many composite raw materials and molded composite products contain crystalline silicaAccording to OSHA's special topics page, prolonged inhalation to crystalline silica at high concentrations may lead to the development of disabling and sometimes fatal lung diseases, including silicosis and lung cancer.

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Sand, quartz, calcium carbonate, gypsum, dolomite, mica and other materials used in the production of cast polymer, engineered stone, tub/showers and many other composite products contain crystalline silica. Glass fibers and fumed silica are forms of amorphous silica and are not considered toxic except possibly as a nuisance dust (classified as "particulates not otherwise regulated" in OSHA's Table Z-1 on air contaminants). The term "silica" in OSHA regulations and other references to workplace health and safety typically refers to crystalline silica.

OSHA's 2008 national emphasis program on crystalline silica identifies its use as a functional filler in the manufacture of plastics as a major source of exposure. The cutting of granite is also identified as a source of silica exposure.

OSHA's exposure limits for crystalline silica are provided in the agency's Table Z-3 on mineral dusts.

OSHA's proposed silica standard

In August of 2013, OSHA released a proposed new regulation (officially released in a Sept. 12, 2013 Federal Register notice) to control occupational exposure to respirable crystalline silica. The deadline for submission of written comments on the proposal will be 90 days following publication in the Federal Register. OSHA plans to hold a workshop to collect additional stakeholder input in March, 2014.

Under the OSHA proposal, employers would be required to conduct an initial assessment of occupational exposure to silica. If 8-hour weighted average exposures exceeded an action level of 25 µg/m3 (micrograms respirable crystalline silica per cubic meter of air), the employer would be required to implement a periodic monitoring program.

For workplaces where 8-hour weighted average exposures exceed a revised permissible exposure limit (PEL) of 50 µg/m3, employers would be required to install engineering controls and adopt workpractices to reduce exposures to the extent possible. Job rotation would not be permissible as a technique to reduce silica exposures.

If exposures still exceeded the PEL after installation of engineering controls and adoption of workpractices, personal protective equipment (PPE) could be used to meet the PEL. Employers must establish procedures to prevent unauthorized employees from entering areas where they might be exposed to silica in excess of the PEL. Regular medical monitoring of employees would be required. 

All exposure assessments would be conducted in the workers' breathing zones, but without PPE. Exposure testing would be conducted by specialized accredited laboratories. Exposure assessments would be measured and calculated as 8-hour time weighted averages.

Under the agency's hazard communication program, employers would be required to inform workers and provide training and equipment to control silica exposures. Warnings would also be provided to employees of customers who may be exposed to silica as a result of working with engineered stone or other composite products. OSHA's proposed rule identifies the manufacture and cutting of engineered stone as a source of silica exposures.

In the detailed but important technical and economic analysis supporting the rule, OSHA provides a list of likely compliance methods, as well as feasability and cost analyses. (A summary of sections that likely apply to cast polymer and post-mold operations for composite laminate is available here.) For small companies, OSHA estimates the average annualized cost of compliance with the proposed silica standard to be $4,284, amounting to 0.25% of revenue and 4.51% of profits.

Health impacts

OSHA estimates the lifetime risk of workers dying from cancer as a result of workplace exposure to respirable cystalline silica is reduced from a range of 13-60 cases per 1,000 workers under the current PEL, to a range of 6-26 cases per 1,000 workers under the proposed revised PEL. In the preamble to the proposed rule, OSHA also provides estimated reductions in deaths due to silicosis and renal disease, and in incidents of non-fatal silicosis.

The table below presents OSHA's estimates of health impact per 1,000 workers exposed over 45 years, for the current PEL, the proposed PEL and the proposed action level. The table also compares the worker health benefit OSHA expects under its silica rule to the benefits achieved under its asbestos and methylene chloride standards.

Silica - Summary of OSHA Risk Assessment