MSHA Proposes to Lower Exposure Limits for Silica in Mines
A new rule proposed by the Mine Safety and Health Administration would lower existing exposure limits for respirable crystalline silica in metal and nonmetal (MNM) and coal mines to a permissible exposure limit of 50 µg/m3 for a full-shift exposure, calculated as an 8-hour time-weighted average. MSHA also proposes to establish an action level of 25 µg/m3 for respirable crystalline silica and to replace existing requirements for respiratory protection by incorporating by reference ASTM F3387-19, Standard Practice for Respiratory Protection. The agency describes ASTM F3387-19 as “a standard that reflects the latest advances in respiratory protection technologies and practices.” The proposed rule includes additional requirements related to medical surveillance for MNM miners and exposure sampling. According to MSHA, the medical surveillance requirements are intended to improve early detection of diseases related to respirable crystalline silica.
MSHA’s existing exposure limit for quartz in MNM mines is 100 µg/m3 for a full-shift exposure, calculated as an 8-hour TWA, and there is no separate silica standard for coal mines. The agency explains that coal miners’ exposures to respirable quartz are currently “regulated through reductions in the overall respirable coal mine dust standard.” The new proposed PEL of 50 µg/m3 for both MNM and coal mines is consistent with the PEL set by OSHA’s 2016 final rule on occupational exposure to respirable crystalline silica, which applies to construction, maritime, and general industry workers. It’s also consistent with NIOSH’s criteria for a recommended standard on occupational exposure to crystalline silica that were put forward in 1974, more than 40 years ago: that no worker be exposed to a TWA concentration of respirable crystalline silica greater than 50 µg/m3 as determined by a full-shift sample for up to a 10-hour workday over a 40-hour workweek.
Once the proposed rule is published in the Federal Register, MSHA will open a public comment period and provide information regarding upcoming public hearings that will take place in Arlington, Virginia, and Denver, Colorado. Individuals will be able to attend the hearings in person or participate online.
The text of the proposed rule can be downloaded in PDF format from MSHA’s website. Further details can be found in the agency’s press release and on its silica rulemaking webpage.
Related: Read “Engineering Controls for Respirable Crystalline Silica Hazards,” the cover article for the April 2022 Synergist, to learn about how NIOSH is investigating effective respirable crystalline silica controls. A previous article published in the June/July 2021 issue of the magazine describes the AIHA Mining Working Group’s role in the development of ISO 23875, a standard for mining that specifies performance and design requirements for air quality control systems for operator enclosures and their monitoring devices.