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Background on Silica

Silica is a compound (SiO 2 ) of the element silicon and oxygen. Silicon does not occur uncombined in nature but occurs chiefly as the oxide (silica) and as silicates. Over 95% of the earth’s crust is made of silica containing minerals and crystalline silica. Quartz is one form of crystalline silica commonly found in the environment. Approximately 12% of the earth’s crust by volume is quartz. Other less common forms of crystalline silica, including tridymite and cristobalite, are found in rocks and soils. Silica occurs both in crystalline minerals (quartz, cristobalite & tridymite) and amorphous or seemingly amorphous minerals. Both the crystalline and amorphous types are common substances and have been used in materials for centuries. Unfortunately, silica’s crystalline forms have hazardous properties that can cause serious health effects in some circumstances.

Respirable crystalline silica refers to particles 10 microns and smaller particles. The respirable size dust is at least 100 times smaller than ordinary sand you might find on beaches and playgrounds. Respirable silica can be created when cutting, sawing, grinding, drilling, and crushing stone, rock, concrete, brick, block, and mortar. Activities such as abrasive blasting with sand; sawing brick or concrete; sanding or drilling into concrete walls; grinding mortar; manufacturing brick, concrete blocks, stone countertops, or ceramic products; and cutting or crushing stone are examples of activities that can result in worker exposures to respirable crystalline silica dust. Industrial sand used in certain operations, such as foundry work and hydraulic fracturing (fracking), is also a source of respirable crystalline silica exposure.

Health Effects
Crystalline forms of silica (quartz, cristobalite and tridymite) that become airborne as dust with particle sizes of 10 microns and smaller are considered respirable. Only very small particles of silica, less than 5 microns, are more likely to be deposited in the lungs. Particles visible to the naked eye are typically 100 microns in diameter and larger. It has been estimated that over 2.3 million American workers are potentially exposed at work each year to the crystalline silica.

Diseases that are associated with inhalation of respirable crystalline silica include:

  •  Silicosis, an incurable lung disease that can lead to disability and death
  • Lung cancer
  • Chronic obstructive pulmonary disease (COPD)
  •  Kidney disease

The primary nonwork-related silica exposure route is thought to be inhalation of crystalline silica during the use of commercial products containing quartz.

According to ATSDR, the current number of silicosis cases in the United States is not known; however, it has been estimated that during the period of 1987–1997, approximately 3,600–7,300 new silicosis cases were diagnosed yearly in the United States. In the United States, 13,744 deaths were attributed to silicosis from 1968 to 1990 and 4,313 deaths were attributed to silicosis from 1979 to 1990. (1) Silicosis mortality trends have shown a marked decline over the past 50 years due to improved industrial hygiene practices and more stringent regulatory standards and guidelines. However, silicosis deaths in younger adults (ages 15– 44 years) have not  declined since 1995, which may reflect more recent, intense exposures, such as those associated with construction, drilling and abrasive blasting industries. Some adverse health effects can be acute from very high short-term exposures, but the predominant health effects are associated with long term work related cumulative exposures.

Risk of Silica Diseases
The risk factors for silica related diseases are:

  •  Exposure to respirable (microscopic) size airborne crystalline silica particles. Respirable size particles are less than 10 microns in diameter.
  • Exposure to airborne respirable crystalline silica in concentrations of greater than 25 micrograms per cubic meter (25 μg/m 3 ) averaged over an 8-hour work day are of potential concern. The new OSHA silica standard sets the permissible exposure limit
    (PEL) at 50 μg/m 3 averaged over an 8-hour day and establishes an action level of 25 μg/m 3 .
  • Exposures maintained below these levels are intended to be protective of human health for workers who work 8 hours per day, 40 hours per week over a working lifetime.

Managing the Risks
Employers that work with crystalline silica either as a raw material, a product or by the disturbance of rock or sand are at risk of exposing their employees to silica diseases. It is recommended that all employers determine whether any potential crystalline silica sources in their operations. If so, the first consideration should be to eliminate the crystalline silica material and substitute a safer substance, if feasible. If not feasible, then the employer may be subject to the 2016 OSHA silica regulations covering employers and employees in industrial, construction and maritime operations. 2 The construction regulation compliance date was September 23, 2017 while the general industry and maritime standards compliance date is June 23, 2018. Compliance with these regulations will eliminate the risk of regulatory penalties while
helping to reduce employee health risks. Affected employers should take the following actions:

  •  Measure the amount of respirable crystalline silica that workers are exposed to if the exposure may be at, or above, the action level of 25 μg/m 3 averaged over an 8-hour day. Personal air sampling devices are used by industrial hygienists to measure breathing zone concentrations of respirable crystalline silica over the course of the work day. Accredited industrial hygiene laboratories analyze these samples and provide the industrial hygienists with the analytical data used to calculate the time weighted average exposures.
  • Protect workers from respirable crystalline silica exposures above the PEL of 50 μg/m 3 , averaged over an 8-hour day by:
  • Limiting workers’ access to areas where they could be exposed above the PEL;
  • Using dust controls such as wetting and local exhaust ventilation to protect workers from silica exposures above the PEL;
  • Providing appropriate respirators to workers when dust controls cannot limit exposures to below the PEL. Industrial hygienists can assist in respirator selection and fit testing.
  • Restricting housekeeping practices that generate dust that could expose workers to silica where feasible alternatives are available. Examples are prohibiting use of compressed air, restricting dry sweeping and employing use of high efficiency vacuums or wet cleaning methods.
  • Establishing and implementing a written exposure control plan that identifies tasks that involve silica exposures and methods used to protect workers. Control plans may include such topics as substitution of safer materials, limiting access to dusty areas, use of engineering controls and personal protective equipment.
  • Offering medical exams — including chest X-rays and lung function tests — every three years for workers exposed at or above the action level for 30 or more days per year. Working with local occupational health clinics equipped for respiratory testing should be utilized.
  • Training workers involved in operations that result in silica exposure on ways to limit exposure. Training should be repeated periodically and at the time of hire for new employees. Supervisors should also participate in the training and enforce compliance.
  • Keeping records of workers’ silica exposure and medical exams. Periodic review of these records should be conducted by a competent person.
  • Offering medical surveillance to employees who will be exposed at or above the action level for 30 or more days a year starting on June 23, 2020. (Medical surveillance must be offered to employees who will be exposed above the PEL for 30 or more days a year starting June 23, 2018.)
  • Implementing engineering controls to limit exposures to the new PEL by June 23, 2021 for any hydraulic fracturing (fracking) operations in the oil and gas industry.

1 https://www.atsdr.cdc.gov/toxprofiles/tp.asp?id=1483&tid=290#bookmark06

2 https://www.osha.gov/dsg/topics/silicacrystalline/

By: Gary N. Crawford, CIH, FAIHA; Principal
For more information contact him at RHP Risk Management (gcrawford@rhprisk.com) or visit . www.rhprisk.com

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