Underground miners may be exposed to hazardous quantities of toxic gases such as carbon monoxide (co) generated from mine fires or explosions. Every underground miner is required to carry a filter self-rescuer (fsr) that, when operated, will remove CO from the miner's breathing air. In addition, all underground miners must have near their work site self-contained self-rescuers (scsr) that, when activated, will supply breathing oxygen. In many situations with fires, miners do not know when to don either self-rescuer, since they have not been informed of a fire in a different mine location. Usually miners do not carry the gas instrumentation necessary for the detection of the toxic, colorless, and odorless fire product carbon monoxide. If each miner carried a personal CO alarm that responds to life-threatening concentrations of CO, the miners would then be alerted to don either the fsr or scsr and exit the mine. For this purpose, a miner's personal CO alarm prototype called pemcoal has been developed by the U.S. Bureau of Mines. The pemcoal unit is small enough to be carried on a miner's belt, has a flashlamp visual alarm, requires no calibration for use, and uses a disk sensor that changes color by reaction with carbon monoxide Pemcoal was tested at concentrations of CO to 1,000 ppm, at temperatures from 5 to 40 deg c, and with several potential mine gas interferents. The pemcoal alarm times are sufficiently fast to warn miners before they are exposed to hazardous quantities of carbon monoxide.