The laboratory hoods found at the National Institute of Environmental Health Sciences were quantitatively tested and evaluated in terms of containment performance. The hoods were challenged with a tracer gas. Sulfur Hexafluoride, and peak concentrations leaking from the hood were measured in the breathing zone of a mannequin. The tests proved that laboratory hood performance is subject to baffle position, face velocity, tracer gas challenge height and bottom slot obstructions. These factors had statistically significant effects, P <0.0001, on overall containment efficiency. The results helped to identify the parameters which could be manipulated by the employee, the Health and Safety Branch and facility engineers to provide optimum performance and reduction of hood leakage to below 0.1 ppm.