Diesel engines are extensively used to power underground mining machines for applications ranging from transportation of workers and material to ore production. The occupational exposure of workers to solid and gaseous diesel emissions has prompted the Mine Safety and Health Administration (MSHA) to implement exposure limits and to enforce these limits for underground workers over the past several decades. Improvements in mine ventilation are the most commonly used remedy to meet more stringent emission limits. However, increasing ventilation can result in higher costs due to increased power demand and/or the cost of supplemental air shafts. Where it is not practical to increase the ventilation rate to reduce pollutants produced by diesel engines, the implementation of exhaust aftertreatment is one of the strategies that has been pursued. The Diesel Oxidation Catalyst (DOC) was one of the first aftertreatment strategies used in underground mines to curtail gaseous emissions. When DOCs first gained popularity for use on diesel powered mining equipment they were employed primarily to reduce carbon monoxide (CO) emissions on mechanically injected engines. The MSHA eight-hour time-weighted average (TWA) threshold limit value (TLV) for CO is 50 parts per million (ppm). The emission of this pollutant has been of great concern in the underground mine environment where the use of mechanically injected engines under average to extreme engine loads can produce high CO concentrations. A DOC inserted in the diesel engine exhaust system achieves the catalytic conversion of CO into the inert and less dangerous carbon dioxide. The temperature of the DOC substrate plays an important role for the activation of the catalyst. The conversion of CO also is influenced by the DOC inlet temperature. The search for new catalysts, with higher activity toward CO and less problems with aging and deactivation, has sometimes diverted the attention from another important catalyst characteristic, the selectivity. The selectivity of a catalyst is the ability to activate a chemical reaction while avoiding simultaneous sub-reactions. Low selectivity of a DOC means it allows the oxidization of other gaseous species in the diesel exhaust. This lack of selectivity has been exploited to reduce the emission of gaseous unburned hydrocarbons, such as aldehydes and poly-aromatic hydrocarbons (PAHs), but the undesired conversion of nitric oxide (NO) to nitrogen dioxide (NO2) is a major disadvantage. Nitrogen dioxide is a deep lung irritant and MSHA has set a ceiling value (a concentration that shall not be exceeded even instantaneously) of 5 ppm, whereas the TLV-TWA for NO is 25 ppm.2 Although engine operating mode greatly affects the NO2 concentration (at low load engine operation the NO2 concentration is usually higher than at high load operation), the NO2 concentration usually does not exceed a few percent of the total oxides of nitrogen emitted by a mechanically injected diesel engine. The implementation of an active DOC however, can increase the NO2 concentration up to 50% of the total concentration of oxides of nitrogen. This was shown during an isolated zone study at the Stillwater Mining Co., where the National Institute for Occupational Safety and Health (NIOSH) researchers detected a two-to-three fold increase in NO2 concentration for an LHD equipped with different oxidation catalyst devices.3 When a DOC is used in conjunction with an electronically controlled diesel engine, the conversion of NO to NO2 can become a concern. Although the use of a DOC offered significant reduction of CO emissions for older engines, the use of the same DOC on modern combustion technology engines can result in significantly higher NO2 production. The improved combustion technology of these engines induces a reduction of the concentration of CO, unburned hydrocarbons and diesel particulate matter. As a side effect, however, the improved combustion process increases the NO2 emissions at the engine manifold, which can be as high as 15% of the total oxides of nitrogen measured in the exhaust. The advent of alternative fuels such as biodiesel, with a higher organic content than standard diesel fuel, provides a new field of application for DOC control technologies in underground mines. In this role the DOC is used to eliminate the organic fraction of the engine exhaust pollutants, in both gaseous and particulate phase.4,5 However, as with regular diesel fuels, the use of DOCs with biodiesel fuel also results in higher NO2 exhaust concentration. When using DOCs on newer electronically controlled engines, one should review the DOC specifications, especially the potential for increasing NO2 concentrations in the exhaust. This increase can be avoided by reviewing the DOC specifications and matching the DOC you choose with the type of engine you have keeping in mind the temperature and location in the tailpipe could affect the system emissions. Future studies on the optimum DOC formulation for specific applications and temperatures of operation will play a major role in helping to reduce the exposure of underground miners to diesel emissions.