What Does 50 Micrograms of Silica Dust in the OSHA PEL Mean?
The OSHA Standard requires that workers exposed to silica dust have their exposure levels measured at least once every two years. This measurement is called the permissible exposure limit (PEL). A worker’s exposure level may not exceed the PEL if they are wearing protective clothing and using personal protection equipment such as respirators.
In order to calculate the amount of silica dust that would cause silicosis, it is necessary to know how much silica dust is inhaled into your lungs over time. To do this, scientists use a formula known as the “effective concentration” or “concentration factor.” The concentration factor is calculated by dividing the total amount of silica dust inhaled into your lungs over time by the total amount of silica dust present in the air.
For example, if there were 10 milligrams of silica per cubic meter of air, then a worker exposed to 100 mg/m3 would need to breathe 1,000 mg/m3 for 20 minutes before being diagnosed with silicosis. This is because the concentration factor (100 mg of silica inhaled into the lungs over twenty minutes) divided by the concentration in the air (1,000 mg/m3) equals 10 minutes.
The concentration factor is used to calculate the silica dust exposure for various activities in different scenarios. The table below shows the exposure levels for a worker performing each of the activities listed under the “Activity Name” column.
Once you know the concentration factor for each work activity, you then need to compare these values to the PEL. The PEL for all workers is 50 micrograms of silica dust per cubic meter of air over an 8-hour workday.
When working with concentrations at or below 50 micrograms per cubic meter of air for an eight-hour workday, workers are less likely to develop silicosis.
The table below shows the exposure level for each activity, the time of exposure in hours and minutes, as well as the concentration factor:
How Long Does Silica Dust Stay in the Air?
The next question, and perhaps the most important, is how long does silica dust stay in the air? In another article on this blog, we discuss what does 50 ug/m3 mean. To answer this question, it is necessary to know the properties of the substance. Silica dust, also known as silicon dioxide or quartz, is an inorganic, aerosol-particle solid. Specifically, it is a glass-like material created by the combustion of sand. This material can be hazardous when it is not handled correctly. The United States Occupational Safety and Health Administration (OSHA) has implemented extensive safety measures to help prevent unnecessary exposure to this substance.
The properties of inorganic solids often include relatively low melting and boiling points, high heats of fusion and vaporization, as well as a lack of magnetic response and low conductivity. Also, most inorganic solids will become energized upon impact.
These properties all relate directly to the fact that silica dust is a solid particle.
This means that when silica dust is airborne, it may have a relatively short lifespan. In fact, many studies show that most airborne particles of silica dust settle out of the air quite rapidly (in matter of seconds or minutes).
When it does settle out of the air, it is commonly deposited close to its original source. This source could be a point-emission source (such as when sand is directly burned in a flame), or it could be a non-point source (such as when silica sand is present in soil or sandstone).
When airborne, the particle size of silica dust ranges from 0.09 to 1.6 microns.
Particles smaller than 0.09 microns, which are classified as respirable size, can pass through the upper respiratory tract and enter the lungs. Of course, when these particles are deposited in the lungs they can lead to severe health complications (such as silicosis or even lung cancer). The larger particles of silica dust are not likely to pose the same health risks when inhaled, but they can still cause harm when large quantities of dust are ingested. It is important to note that, while silica dust can lead to severe complications, it does not cause the inflammatory response in the body that other microparticles such as silicates do.
Aerosolized and respirable silica dust falls into the “other” category when it comes to health effects and can pose a risk to an individual based on the concentration that they are exposed to as well as the duration and frequency of exposure.
The next time you go to the beach and touch sand, be thankful that you don’t have to worry about harmful chemicals being absorbed through your skin. Better yet, when you’re done playing in the sand, do us all a favor and shower!
Sources & references used in this article:
- Regulation of Silica: Will Lowering the Exposure Level Cost Jobs or Improve Public Health (EAG Geltman – W. Va. L. Rev., 2017 – HeinOnline)
- AT LAST, THE SILICA STANDARD. WHAT NOW? (AB Kaelin – Journal of Protective Coatings & Linings, 2016 – search.proquest.com)
- MITIGATION OPTIONS FOR RESPIRABLE CRYSTALLINE SILICA: ENGINEERING CONTROLS VS. PERSONAL PROTECTION (K Billy, R Vent – 79th Conference on Glass Problems, Ceramic …, 2019 – Wiley Online Library)
- Laboratory Quantification of Environmental and Quality Risks Associated with Pneumatic Transfer of Frac Sand (I Kidd, T Fuss-Dezelic, J Shi, C Wilcox… – SPE Eastern Regional …, 2016 – onepetro.org)
- Regulation of Silica: Will Lowering the Exposure Level Cost Jobs or Improve Public Health? (EA Glass Geltman – West Virginia Law Review, 2018 – researchrepository.wvu.edu)
- A PILOT STUDY TO ASSESS THE IMPACT OF THE PROPOSED CRYSTALLINE SILICA STANDARD IN THE CONSTRUCTION INDUSTRY (K Lyons – 2015 – digitalcommons.mtech.edu)