Analysis of Regulations and Risk due to Silica Exposure - Analysis for the US

For every company, the health of the employees is very important. For this reason, a careful analysis of the working environment with adequate awareness allows to create an as safe as possible environment and to prevent even the most serious diseases.

Definition of dust 

Dust is broadly defined as small solid particles created by the breaking up of larger particles. Depending on their size, these particles can become hazardous to worker health, particularly when suspended in air. The largest size particle that can be suspended in air for long periods of time it is about 60 μm, which is about the thickness of a human hair.

Particles ranging from about 60 to 2,000 μm can also become suspended in air, but they only reach heights up to approximately 0,9m above the ground before they fall back to the surface.

Particles larger than about 2,000 μm generally creep or roll along the surface due to wind velocity acting upon them [EPA 1996]. These larger particles of dust can affect the nasal passages, causing an irritated and congested nose, and might also cause an irritant cough should they deposit in the throat.

Smaller airborne particles of dust, which can remain suspended in air for hours, pose a greater risk to the respiratory system when inhaled. In general, the smaller the aerodynamic diameter of the inhaled dust particle, the more likely it will be deposited more deeply in the respiratory tract.

Safety guide

The safety and health regulations for the U.S. mining industry are enforced by the U.S. Mine Safety and Health Administration (MSHA). The Federal Mine Safety and Health Act of 1977 created MSHA, giving it the authority to develop and revise improved mandatory health and safety standards to prevent injuries and protect the lives of those working in mines.

If potential exposure to silica dust is suspected, a respirable dust sample is obtained, and x-ray diffraction following NIOSH Analytical Method 7500 [NIOSH 2003] is used to quantify crystalline silica content. The three most common forms (polymorphs) of respirable crystalline silica are quartz, tridymite, and cristobalite. If the sample contains greater than 1 per cent quartz, a respirable dust standard is calculated with the following equation:

Enforcement of a respirable standard based upon this equation effectively limits respirable quartz 3 exposure to less than 100 µg/m^3.

NIOSH has a recommended exposure limit (REL) for 3 respirable crystalline silica of 50 µg/m , as a time-weighted average for up to a 10-hour day during a 40-hour week [NIOSH 1974]. The exposure limits for tridymite and cristobalite are limited to 50 percent of the value calculated from the formula for quartz.

What are the potential risks for health?

As reported in "NIOSH Hazard Review: Health Effects of Occupational Exposure to Respirable Crystalline Silica," occupational exposure to respirable crystalline silica dust can have several adverse health consequences, including silicosis, tuberculosis, chronic bronchitis, emphysema, and chronic renal disease [NIOSH 2002].

When workers inhale respirable dust, the particles can penetrate the body's defense mechanisms and reach the alveolar region of the lungs. Crystalline silica particles that deposit in the alveolar region can stimulate an inflammatory and toxic process that can ultimately develop into clinically recognizable silicosis. Depending on the concentration of respirable crystalline silica and duration to which they are exposed, workers may develop any of several forms of silicosis [NIOSH 2002]:

• chronic—resulting from long-term excessive exposures, and first clinically apparent 10– 30 years after first exposure;
• accelerated—resulting from exposure to higher concentrations of crystalline silica, first clinically apparent 5–10 years after the initial exposure;
• acute—resulting from exposure to unusually high concentrations of crystalline silica, clinically apparent within weeks to 5 years after the initial exposure.

REFERENCES

• EPA [1996]. Air quality criteria for particulate matter, Vol. 1. Research Triangle Park, NC: National Center for Environmental Assessment, Office of Research and Development, Environmental Protection Agency.
• ACGIH [2007]. 2007 Threshold limit values for chemical substances and physical agents and biological exposure indices. Cincinnati, OH: American Conference of Governmental Industrial Hygienists
• Davis GS [2002]. Silicosis. In: Hendrick DJ, Burge PS, Beckett WS, Churg A, eds. Occupational disorders of the lung: recognition, management, and prevention. W.B. Saunders, pp. 105–127.
• EPA [1996]. Air quality criteria for particulate matter, Vol. 1. Research Triangle Park, NC: National Center for Environmental Assessment, Office of Research and Development, Environmental Protection Agency.
• 54 Fed. Reg. 2521 [1989]. Occupational Safety and Health Administration: air contaminants; final rule; silica, crystalline-quartz. (Codified at 29 CFR 1910.)
• NIOSH [1974]. NIOSH criteria for a recommended standard: occupational exposure to crystalline silica. Cincinnati, OH: U.S. Department of Health, Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 75–120.
• NIOSH [2002]. NIOSH hazard review: health effects of occupational exposure to respirable crystalline silica. By Schulte PA, Rice FL, Key-Schwartz RJ, Bartley DL, Baron P, Schlecht PC, Gressel M, Echt AS. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2002–129
• NIOSH [2003]. NIOSH manual of analytical methods, 4 ed., 3 supplement. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2003– 154
• NISA [2010]. Occupational health program for exposure to crystalline silica in the industrial sand industry, 2nd ed. Washington, DC: National Industrial Sand Association.
• Dust Control Handbook for Industrial Minerals Mining and Processing