30 C.F.R. § 60.2
The following definitions apply in this part:
Action level means an airborne concentration of respirable crystalline silica of 25 micrograms per cubic meter of air ([MICRO]g/m3 ) for a full-shift exposure, calculated as an 8-hour time-weighted average (TWA).
Respirable crystalline silica means quartz, cristobalite, and/or tridymite contained in airborne particles that are determined to be respirable by a sampling device designed to meet the characteristics for respirable-particle-size-selective samplers that conform to the International Organization for Standardization (ISO) 7708:1995: Air Quality-Particle Size Fraction Definitions for Health-Related Sampling.
Specialist means an American Board-Certified Specialist in Pulmonary Disease or an American Board-Certified Specialist in Occupational Medicine.
30 C.F.R. §60.2