The U.S. Bureau of Mines recently evaluated a district recirculation system at a Trona Mine in Granger, Wyoming. A 1.52-M (5-ft) diameter, 37.3-Kw (50-hp) vane axial fan was installed in a crosscut between the intake and return airways and recirculated approximately 31.4 M3/s (66,578 ft3/min) of air back into the intake. This amounted to a recirculation factor of approximately 20 pct of the return air. This air was part of the primary ventilation to three different continuous miner sections at operating distances of between 610 m (2,000 ft) and 1,829 m (6,000 ft) from the recirculation fan. The mine environment was constantly monitored for respirable dust, methane, and carbon monoxide in the operating sections and in the main intake and return entries. In addition, air samples were taken with and without the recirculation system during the evaluation and analyzed using gas chromatography. The results of this study showed only small increases in dust, while all other mine contaminants remained at such low levels that it was impossible to determine any relative changes from the recirculation system. Because of this, a brief sf6 tracer gas analysis study was subsequently performed to more precisely determine an increase in a mine contaminant with the recirculation system.