The underground use of diesel-powered equipment in hard-rock mines is well established, and the use of diesel equipment in underground coal mines is greatly increasing. Studies have shown safety advantages of the equipment based on tons of ore produced. However, as the number of diesel units and their power ratings increase, concern over the health effects of the exhaust emissions becomes more significant. This Bureau of Mines report reviews the underground use of diesel equipment, and presents comprehensive explanation and evaluation of an advanced air quality monitoring methodology that provides an assessment of the underground atmosphere. The methodology involves the following two phases: (1) establishing pollutant characteristic curves for specific diesel equipment that illustrate the relationship between the concentration of the exhaust pollutants and the concentration of co2 measured at the same location and over the same period of time, and (2) using an air quality index to establish a single co2 concentration at which the other diesel pollutants are considered below harmful levels. Evaluation of the methodology was carried out on the 7,100-ft level of Homestake Gold Mine, Lead, South Dakota.