This Bureau of Mines report investigates the feasibility of a monitoring methodology that uses carbon dioxide (co2) as an indicator of mine air quality. For the methodology to be valid, the pollutant-to-co2 ratios must remain constant if engine operating conditions do not significantly change. However, because of the complex dynamics of the fuel injection system, the fuel-air combustion process, and the engine speed-load governing system, the pollutant-to-co2 ratios may vary during repetitive, but transient engine speed and load operation. These transient effects were investigated. In addition, the influence of changing engine conditions due to engine maladjustment, and a practical means to evaluate engine condition were investigated to advance the methodology. The laboratory investigation determined that co2 is an effective indicator of engine exhaust pollutants. It was shown that the exhaust concentrations of carbon monoxide, carbon dioxide, oxides of nitrogen, hydrocarbons, and particulate matter do not significantly vary among repetitive, but transient engine speed and load duty cycles typical of in-service equipment. Based on an aqi and threshold limit values, particulate matter exhibited the greatest adverse effect on air quality. Particulate mass was separated into volatile (organic soluble fraction) and nonvolatile (insoluble carbon fraction) components.