Published in the December 2006 American Longwall Magazine
Hydrogen sulfide exposure is a double-edged sword: while the gas is quickly and easily detected through its foul odor (similar to rotten eggs), workers can suffer from olfactory paralysis, allowing one’s senses to no longer detect its presence while it continues to build in the environment and in miners’ systems.
Make no mistake – the gas is just as deadly as any other, according to US Mine Safety and Health Administration toxic agents chief Michael Valoski. High levels of exposure can result in a coma, convulsions or respiratory failure, and can even be deadly.
While hydrogen sulfide is not considered a typical gas, it is found naturally in the air of some western US mining operations. Because of that, MSHA developed a permissible exposure limit, or PEL, for the gas through its Coal Mine Safety and Health division: an enforced eight-hour time-weighted average of 10 parts per million and a 2 Excursion Factor.
Similarly, the National Institute of Occupational Safety and Health outlined its own exposure thresholds, recommending a maximum of 10ppm but also outlining 100ppm as its Immediately Dangerous to Life or Health Level.
Unfortunately, the odor threshold for the gas – .13ppm – is well below the NIOSH and CMS&H PEL. Workers, as a result, literally stop smelling the rotten eggs odor and are then unable to identify any increase in concentration of the gas.
Valoski visited a western longwall for his technical research, which he presented at the 11th US/North American Mine Ventilation Symposium at Penn State University in June 2006. He opted to evaluate readings from five separate locations at the face, spanning the last open crosscut before the face, to the mouth of the return on the panel.
Through the multi-day, multi-shift study, Valoski found the headgate to have the lowest intensity of hydrogen sulfide, while the tailgate’s exposure levels were the highest. The particular mine studied had peak readings of 280 and 310 cubic feet per minute on each of the two days the readings were taken.
The third day, on which he noted his study took place through a maintenance shift, he returned results for the gas concentration that were very low. His determination – “hydrogen sulfide is contained within the coal”
As a result of his work, the mine developed and implemented a hydrogen sulfide exposure control process. “The approach uses personal monitoring, administrative controls, and engineering controls,” he noted.
Specifically, every mine worker, inspector or visitor to the face at the operation must don a hydrogen sulfide detector to track exposure levels. If levels increase and become hazardous, the unit (a T82 by Industrial Scientific) will activate a dual alarm for sight and touch to alert the individual; the alarm is set to sound at 10ppm, Valoski said.
At the end of each shift or visit to the face, all units’ memories are downloaded to a computer for storage and evaluation.
Administratively, Valoski said, the mine enforces a regulation that no individual remain in the tailgate area of the shearer when it is active. Additionally, all workers are required to have an MSHA-approved respirator on their person at all times.
From an engineering control perspective, Valoski said ventilation is a vital component. Added to that is a shearer regulator process that will automatically slow its speed if concentrations go above 15ppm, a required action that the shearer operator cannot override. By doing so, the ventilation portion of the process can be allowed to do its job.
Hydrogen sulfide is a hazard that will not disappear, Valoski said, so finding the most efficient, effective way to manage it for the safety of workers was key. “Using a multi-faceted approach of engineering and administrative controls, the exposure of coal miners to hydrogen sulfide can be limited.”
