Several velocity readings are taken across the face of the hood, just under the sash. An inert gas is released in the hood. Several tests have been performed by the ventilation industry using these criteria, and surprisingly, a large number of hoods which indicated an acceptable face velocity did not pass the other criteria, i.e. insufficient containment of vapors. There are a number of reasons why hoods actually fail performance tests. Poor lab design, such as aisles in front of hoods, will cause problems.
People walking by can disrupt the air supply to the hood.
Excessive storage in the hood and placing equipment too close to the sash or the air capture slots in the rear of the hood will affect flow. Poor balance of supply and exhaust in the labs may cause entrainment into the air supply duct. This sounds like a pain in the neck, but there are simple ways to do so. The label must be clearly visible, so a regular pen is not the best. You could even pre-print a supply on computer label paper.