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Workplace Exposure Limits

At least four standards are used to regulate respirable crystalline silica exposure in the workplace:

The Occupational Safety and Health Administration (OSHA) and Mine Safety and Health Administration (MSHA) currently set the maximum Permissible Exposure Level (PEL) for an 8-hour exposure, 40 hours per week, as the following:

Exposure limit = 10 mg SiO2/m3 ÷ (%SiO2 + 2)

For dust that is 100% silica, the standard would be 0.1 mg/m3 (%SiO2 = 100).  For dust that has no silica, the standard would be 5 mg/m3 (%SiO2 = 0).

However, OSHA is considering lowering its standard by 50%, to 0.05 mg/m3 (50 ug/m3) (

The American Conference of Governmental Industrial Hygienists (ACGIH) has set a recommended exposure limit of 0.025 mg/m3 (25 ug/m3) (

The National Institute for Occupational Safety and Health (NIOSH) has set an occupational standard of 0.05 mg/m3 (50 ug/m3) (


Public Exposure Limits

Safe levels of exposure to the public (sometimes called "environmental exposure limits") to respirable crystalline silica have been established by five states (table from Myers 2010):


The standards from the California Air Resources Board of 0.03 mg/m3 (3 ug/m3) to protect against silicosis, and the Texas Commission on Environmental Quality of 0.00027 mg/m3 (0.27 ug/m3) to reduce cancer risk are particularly well-researched and documented.  (Note that the designation of particle sizes as "PM4" is essentially equivalent to "respirable" particle sizes.)