The Air Quality & Climate Division (AQCD) requires that emissions from most regulated air pollution sources exhaust through an unobstructed, vertical stack of sufficient height to ensure appropriate upward dispersion of the exhaust, thus minimizing the ground level concentration of air pollutants where individuals may be exposed. An unobstructed vertical stack is crucial to ensure the velocity momentum of the exhaust exiting the stack, as well as any exhaust heat induced buoyancy, may optimally disperse the pollutants upward.

Stack obstructions such as the older “conventional” style rain caps, as well as horizontal stack discharges, restrict upward velocity momentum and reduce potential buoyancy dispersion and can increase ground level exposure to pollutants in the exhaust.