PURPOSE:  Develop controls that reduce silica dust exposures for operators of continuous mining machines and roof bolting machines to prevent the development of silicosis.

RESEARCH SUMMARY:  In underground coal mines, the two occupations with the highest risk of excessive exposure to respirable silica dust are the continuous mining machine operator and the roof bolter operator. The Mine Safety and Health Administration (MSHA) analyzed nearly 31,000 personal samples for these occupations during the period 1999-2003. Nearly 12% of these samples exceeded the federal dust standard of 2 µg/m3 while 20% of these samples exceeded a silica dust concentration of 100 µg/m3; the maximum allowed under MSHA regulations. Overexposures continue to occur with miners still contracting coal workers’ pneumoconiosis and silicosis, both debilitating and fatal lung diseases.

To lower the silica dust exposures of roof bolter operators, NIOSH developed a canopy air curtain that reduced dust levels under the air curtain by approximately 50% in laboratory testing. The canopy air curtain was modified for retrofit onto a roof bolter and tested at an operating underground mine. Results indicate that dust levels for the operator working beneath the drilling canopy were reduced but not to the level found in the lab. NIOSH is working with the bolter manufacturer and mine operator to modify the size and flow characteristics of the tested air curtain to improve its dust reduction potential and application to other bolting machines.

Wet head continuous miners introduce water via sprays located behind each cutting bit on the cutting drum instead of via a spray bar positioned back from the cutting drum. This places sprays closer to the cutting with the potential to reduce airborne dust levels. NIOSH has completed an evaluation of a wet head miner that was operated with exhaust curtain ventilation. Some improvements in dust levels at the operator were measured. Future studies are planned to evaluate wet head technology for miners that are operated with blowing face ventilation.

Airflow from the dust scrubber on the continuous miner can be redirected to the face through a series of louvers in an attempt to contain and capture more of the dust generated at the face, thus reducing the dust exposure of the miner operator. NIOSH worked with a coal company to test redirected scrubber flow. Results showed that the dust exposure of the shuttle car operators was reduced over 50%. The impact on the exposure of the miner operator was much less, indicating a need to research and optimize the balance between the redirected scrubber flow and the face ventilation flow. Laboratory tests will be completed to develop guidelines regarding the use of redirected scrubber exhaust for control of respirable dust, with special attention given to balancing redirected and primary airflows.