The NIOSH environmental assessment of diesel exhaust in two fire stations found that concentrations of gas-phase components of diesel exhaust were well below their respective evaluation criteria prior to the installation of engineering controls. Personal exposures to diesel exhaust particulates in these two fire stations were also low, because the quantities of Ce generated were moderate, and because . firefighters and paramedics did not spend much time in the apparatus bay. Concentrations of diesel soot (measured as Ce) in the bays were reduced by 76 and 91 percent in Stations 3 and 5, respectively, after the ceramic diesel exhaust filters were installed. NIOSH researchers concluded that, in stations with pull-through apparatus bays, the ceramic filters are effective at reducing the emission of diesel soot into the apparatus bay. Further study is needed to determine the effectiveness of this and other controls for reducing diesel exhaust at stations where the apparatuses are required to back into the station. Although this study suggests that gas-phase components of diesel exhaust do not build up to unhealthful levels in closed apparatus bays without the filter controls, additional study may be warranted to determine the effectiveness of this and other engineering controls at limiting gas-phase components in environments where they may accumulate.