95% of the canopy hoods in use (other than in homes and restaurants) are misapplied and ineffective. The capture range of a canopy hood is extremely limited, and a large volume of air is needed for it to operate effectively. Thus, a canopy hood works best when thermal or buoyant forces exist that will move the contaminant up to the hood capture zone (a few inches below the opening). However, because canopy hoods are generally placed well above a contaminant source so that workers can operate underneath them, they draw contaminants past the worker's breathing zone and then into the exhaust system. If a canopy hood exists in a laboratory, it should be used only for nonhazardous service, such as capturing heated air or water vapor.
Flammable and combustible liquids should be stored only in approved flammable-liquid storage cabinets, not in a fume hood, out on the benches, or in a nonapproved storage cabinet.
Whether or not flammable-liquid storage cabinets should be ventilated is a matter for debate. One view is that all such cabinets should be vented by using an approved exhaust system, because it reduces the concentration of flammable vapors below the lower explosive limit inside the cabinet. A properly designed cabinet ventilation system will do this under most circumstances and results in a situation in which no fuel is rich enough in vapor to support combustion. However, there is still liquid in the cabinet and a source of fresh air provided by the ventilation system. All that is needed is an ignition source. The other view is that in most circumstances flammable-liquid storage cabinets should not be ventilated, because the cabinet is designed to extinguish a fire by depriving it of oxygen and ventilation defeats this purpose.
Both opinions are valid, depending on the conditions. Ventilation is prudent when the liquids stored in the cabinet are highly toxic or extremely odoriferous. Particularly odoriferous substances such as mercaptans have such a low odor threshold that even with meticulous housekeeping the odors persist; hence, ventilation may be desired.
If a ventilated flammable-liquid storage cabinet is used under a fume hood, it should not be vented into the fume hood above it. It should have a separate exhaust duct connected to the exhaust system. Fires occur most frequently in fume hoods. Fire from a fume hood may propagate into a flammable-liquid storage cabinet directly vented into the hood.
If a special-flammable storage cabinet ventilation system is installed, it should employ an AMCA-C type spark-resistant fan and an explosion-proof motor. Most fractional horsepower fans commonly used for this purpose do not meet this criterion and should not be used. If the building has a common fume hood exhaust system, then it is appropriate to hook a flammable-storage cabinet up to it if it must be ventilated.
General ventilation systems control the quantity and quality of the air supplied to and exhausted from the laboratory. The general ventilation system should ensure that the laboratory air is continuously replaced so that concentrations of odoriferous or toxic substances do not increase during the workday and are not recirculated from laboratory to laboratory.
Exhaust systems fall into two main categories: general and specific. General systems serve the laboratory as a whole, and include devices such as fume hoods and snorkels, as codes and good design practices allow.