An apparatus for rotary dumping of rail cars, including a backside airflow diverter, a backside hood for removing contaminated air from the back side of the pit and at least two baffles on a rotational frame, the baffles each presenting a leading edge that contacts the backside airflow diverter during at least a portion of rotational motion. At least one air intake is positioned to pass through the upper surface of the backside diverter into the backside hood at a location where pressure in the backside hood is maintained at a substantially neutral or negative level even when pressure in the material receiving pit is increased by a down rush of air caused by the exit of the load from the railcar.