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Let's Talk About Silica

In industries such as manufacturing and construction, dust is a common work by-product and it can be harmful.

At times, dust may contain crystalline silica, a naturally occurring mineral. Crystalline silica is most commonly found in quartz form, but also it can be found in cristobalite and tridymite.

What you may not realize is that crystalline silica is known to produce silicosis and lung cancer. Silicosis is a lung disabling disease, mainly associated with lung scarring and fibrosis, and its symptoms include breathing difficulty and chronic cough. There are three categories of silicosis: chronic silicosis, accelerated silicosis and acute silicosis.

Luis Pieretti

Luis Pieretti, Ph.D.

Safety and Health Consultant

Chronic silicosis--sometimes referred to as classic silicosis--is the result of chronic, long-term (usually periods of 15 to 20 years) exposure to low amounts of silica. Accelerated silicosis results from high exposures to respirable crystalline silica after 5 to 10 years of exposure. Acute silicosis can occur in as quickly as a few months or up to two years after exposure to extremely high concentrations of respirable crystalline silica.

The concentration of respirable crystalline silica is related to the size of the inhalable particles. In fact, particle size is the most important variable to consider when talking about particles in general; as it affects both the behavior of aerosols and how deep they can reach inside the respiratory system (deposition mechanisms include interception, inertial impaction, diffusion, gravitational settling and electrostatic attraction). Particles small enough to reach the alveolar region are in the respirable fraction. Respirable crystalline silica, therefore, refers to the concentration of crystalline silica--measured using a size-selective sampler--that can reach deep in the respiratory system.

The concentration, or Permissible Exposure Limit (PEL), for crystalline silica depends on the percentage of silica found in the sample obtained and if it is in the respirable fraction or not. These PELs are found in 29 CFR 1910.1000 Z-3. For respirable crystalline silica, the PEL can be calculated using the following formula:

10 mg/m 3
%SiO2+2

The percentage of crystalline silica in quartz form found in the sample analyzed is referred to as %SiO2 . If cristobalite is found instead of quartz, half the value calculated from the count or mass formula for quartz is used. The same applies for tridymite. If the sample analyzed contains 100% quartz crystalline silica, the OSHA PEL would be 0.098 mg/m3. If the sample analyzed does not contain silica, then the OSHA PEL would be 5 mg/m3, which is the same permissible exposure limit for particulates not otherwise regulated in the respirable fraction.

One of the many complimentary services provided by the USF SafetyFlorida Consultation Program includes an industrial hygiene survey to help employers to identify if exposure to silica and other agents is occurring. If you would like to determine your employees' exposure to the different chemical and physical agents (such as noise) in your workplace, call us or request a free consultation through our website, www.usfsafetyflorida.com. Remember, our consultation services are free and confidential.

References:

Hinds, W. C. (1999). Aerosol Technology. Properties, Behavior and Measurement of Airborne Particles, (2nd ed.). New York: John Wiley & Sons, Inc.