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) (http://www.afsinc.org/content/view/1226/).
The American Conference of Governmental Industrial Hygienists (ACGIH) has set a recommended exposure limit of 0.025 mg/m3 (25 ug/m3) (http://www.oildri.net/Downloads/MSDS_1200000.pdf).
The National Institute for Occupational Safety and Health (NIOSH) has set an occupational standard of 0.05 mg/m3 (50 ug/m3) (http://www.reginfo.gov/public/do/eAgendaViewRule?pubId=201104&RIN=1218-AB70).
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 Qualilty 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.)