inhibit drainage, and the spray action is not as effective on the top and sides of the duct. Any washdown piping, which is located outside must be protected from freezing. A drain and waste valve on the water supply piping that allows it to drain when not in use is helpful. Route the drain lines carefully to prevent the creation of traps that retain water. Write special operating procedures to cover the washdown procedure for these types of hoods. The exhaust from a perchloric acid hood should not be manifolded with that from other types of chemical hoods.
9.C.2.10.6 Radioisotope Laboratory Chemical Hoods
Design chemical hoods used for work with radioactive sources or materials so that they can be decontaminated completely on a regular basis. A usual feature is a one-piece stainless steel welded liner with smooth curved corners that can be cleaned easily and completely. The superstructure of radioisotope hoods is usually made stronger than that of a conventional hood to support lead bricks and other shielding that may be required. Special treatment of the exhaust from radioisotope hoods may be required by government regulations to prevent the release of radioactive material into the environment. This treatment usually involves the use of HEPA filters (see section 9.C.4.2).
Another practical way to handle radioactive materials that require special exhaust treatment is to use a containment chamber within a traditional chemical hood. Several safety supply companies offer portable disposable glovebag containment chambers with sufficient space to conduct the work and then dispose of them in accordance with applicable nuclear regulatory standards.
9.C.2.10.7 Clean Room Laboratory Chemical Hoods
Chemical hoods in clean rooms are generally no different than traditional chemical hoods, except that they are usually made of polypropylene or thermoplastics. Some have hinged sashes rather than sliding sashes. Most require separate chemical hoods for acid work and solvent work.
Polypropylene hoods burn easily, melt quickly, and may become fully involved in a fre. There are fre-retardant polypropylene and other thermoplastics available, but they cost more. Alternatively, an automatic fire extinguisher may be installed inside.
9.C.2.11 Laboratory Chemical Hood Exhaust Treatment
Until recently, treatment of laboratory chemical hood exhausts has been limited. Because effluent quantities and concentrations are relatively low compared