A chimney stack exhaust treatment unit and an improved exhaust gas treatment system utilizing such a unit are disclosed. The unit comprises a generally spherical outer housing which fits over the mouth of a caustic-resistant chimney pipe and encloses a horizontally mounted squirrel cage fan which is driven by an electric motor situated above it. The fan is situated directly above the chimney mouth so that when it is rotated, exhaust gases are drawn up through the chimney and swirled around the inner walls of the outer housing. Exhaust ports near the top of the housing provide an exit for treated gas. An inner housing encloses a fan motor, isolating it from the interior of the outer sphere. A conduit connects an opening in the outer sphere with an opening in the inner housing, and the inner housing includes a mouth directly above the fan, whereby rotation of the fan also draws outside air through the inner housing to cool the motor.