A statistical study of available data regarding frictional ignitions in the United States from 1983 to 2005 was conducted by the National Institute for Occupational Safety and Health (NIOSH) to determine and define conditions controlling their occurrence (MSHA, 2006a; MSHA, 2006b; and MSHA, 2006c). Frictional ignitions are defined as the ignition of a flammable mixture of methane and air that is initiated by frictional heating. Ignitions created through the addition of energy from open flames and exposed electrical circuits are not included in this analysis. Energy released in a roof fall that creates a spark and ignites a methane mixture would be considered a frictional ignition. Frictional ignitions represent the majority of all ignitions in underground coal mines. Over the study period of 1983 to 2005, a total of 1,993 ignitions were reported in underground bituminous coal mines in the United States, of which 1,589 were frictional ignitions. Reportable ignitions to the U.S. Mine Safety and Health Administration (MSHA) are any unintentional occurrences of flame underground regardless of duration. Continuous miners represented the largest source for frictional ignitions with 1,090 (68.6% of all frictional ignitions). Longwall mining operations represented the highest concentration of frictional ignitions during this study period with 1,365 frictional ignitions (85.9% of all frictional ignitions). From 1983 to 1991, underground productivity and the number of frictional ignitions increased. However, productivity continued to increase from 1991 to 2000 while total frictional ignitions fell, thereby disproving this connection between productivity and frequency of frictional ignitions. Ninety percent of all frictional ignitions occurred in underground coal mines that liberated more than 393 L/s (1.2 MMcfd million cubic feet per day) of methane through the main ventilation system. Although methane production is not the determining factor for the occurrence of frictional ignitions, it is a good indicator of the anticipated frequency.