Silica

What is Silica?
Silica is a mineral found in the Earth's crust. It can be found in many forms such as crystalline silica- quartz, cristobalite and tridymite. It also has amorphous forms, such as fused silica, where the crystalline structure has been changed. In the industrial setting, the term silica usually refers to quartz.
Where is Silica Found?
In the industrial work place, silica is used as a raw material in concrete, brick, glass and ceramic products. It has also been used as an abrasive blasting agent. Silica media can be used to filter contaminants for water purification. Silica gel is used as a filter media for some industrial hygiene air sampling applications.
What are the Health Effects from Silica?
Silicosis is one of the primary health effects associated with exposure to silica. There are several different types of silicosis that are dependant upon the level and duration of exposure. Chronic silicosis is the most common and occurs from long term, low level exposures. Acute and accelerated silicosis occur from higher level, short duration exposures. Silicosis is a fibrotic lung disease that can progress even after removal from exposure. It is the respirable fraction of silica, less than 10 microns in diameter, that is of medical significance.

Since OSHA's inception, the number of deaths attributed to silicosis has declined to approximately 250-300 per year.  Many believe the decline in silicosis deaths occurred as a result of improved controls that were implemented after OSHA was created. Many of the silicosis deaths that have occurred since OSHA's inception may have been caused by silica exposures that occurred prior to OSHA. The decline in the number of US manufacturing facilities has also been listed as a possible reason for the decline in silicosis deaths.

It should be noted that silicosis numbers may be misleading for several  reasons. There may be inconsistencies in reporting on death certificates so the number of fatalities may be under-reported. In addition, there may be silicosis cases that are undiagnosed or misdiagnosed so the actual number of cases reported may be underestimated. The quality of life may be significantly reduced for those with this lung disease.
What Exposure Standards Apply to Silica?
The OSHA PEL for General Industry is based upon a formula that was adopted when OSHA came into existence in the early 70's. The formula for respirable silica is found in Table Z-3 for Mineral Dusts in 29CFR 1910.1000. The respirable silica PEL formula is listed below:              

 (10 mg/m3) รท (% silica + 2)

As the percentage of silica increases, the PEL decreases. The PEL formula essentially  limits the respirable silica concentration to 0.1 mg/m3.

Since OSHA's inception in the early 70's, there has been periodic interest in establishment of a substance specific Silica standard, similar to those present for Lead, Cadmium and others in Part 1910 Subpart Z. Currently, OSHA has efforts underway to establish a substance- specific standard for Silica which contains a PEL at a fixed level of 0.1, 0.075 or 0.05 mg/m3. The draft proposal for a Silica PEL also contains specific requirements for air sampling, respirators, hygiene facilities and medical surveillance. It should be noted that those proposed requirements are not enforced as law; however, it would be wise to become familiar with those requirements

The OSHA PEL for silica in Construction is currently based upon an obsolete sampling method that counts dust particles. To protect construction workers from exposures to silica, controls should be implemented to the General Industry PEL (as a minimum) although arguments could be made that the General Industry PEL is not enforceable for the construction industry.

The MSHA PEL for respirable silica uses the same formula as the OSHA General Industry PEL listed above.

The ACGIH TLV for respirable silica-quartz is 0.025 mg/m3 for an 8-hour time-weighted average (TWA) exposure (2007). Since 1974, the NIOSH REL for crystalline silica has been 0.05 mg/m3.