The perchloric acid hood, with its associated ductwork, exhaust fan, and support systems, is designed especially for use with perchloric acid and other materials that can deposit shock-sensitive crystalline materials in the hood and exhaust system. These materials become pyrophoric when they dry or dehydrate (see also Chapter 5, section 5.G.6). Special water spray systems are employed to wash down all interior surfaces of the hood, duct, fan, and stack, and special drains are necessary to handle the effluent from the washdown. The hood liner and work surface are usually stainless steel and are sealed by welding all seams. Perchloric acid hoods have drains in the work surface. Water spray heads are usually installed in the top of the hood, behind the baffles, and in the hood interior. The water spray should be turned on whenever perchloric acid is being heated in the fume hood. Welded or flanged and gasketed fittings to provide air-and water-tight connections are recommended. The ductwork, fan, and stack should be fabricated of plastic, glass, or stainless steel and should be fitted with spray heads approximately every 10 feet on vertical runs and at each change in direction. Horizontal runs should be avoided because they inhibit drain-down, and the spray action is not as effective on the top and sides of the duct. The washdown piping, located outside, should be protected from freezing. A three-way valve on the water supply piping that allows it to drain when not in use is helpful. Care should be used when routing the water lines to prevent the creation of traps that will retain water. Special operating procedures should be written 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 fume hoods.
Hoods used for work with radioactive sources or materials should be designed 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, which can be cleaned easily and completely. The superstructure of radioisotope hoods is usually made stronger than that of a conventional hood in order to support lead bricks and other shielding that may be required in the hood. Special treatment of the exhaust from radioisotope hoods may be required by government agencies to prevent the release of radioactive material into the environment. This usually involves the use of high-efficiency particulate air (HEPA) filters (see section 8.C.8.4).