Exhaust fumes from various laboratories in a building are found to be reentering the building through the fresh air inlets through a cross connection resulting from exhaust vents being flush with the roof and fresh air inlets being above the roof. The suggested correction method is a combination of the following: raising exhaust outlets above roof level by means of short stack extensions, discharging exhaust fumes upward at high velocity, and reducing amount of fresh air taken in at roof level.