122 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



economize, the more expensive stainless-steel construction need not be 

 used, since protective coatings can be employed. Strippable films are 

 very helpful in reducing acid corrosion of the exposed areas of the hood. 

 Also it is advisable to coat the interior of the duct work with acidproof 

 paint before it is put in place. In practice, there is usually more difficulty 

 from acid corrosion of metal parts than from contamination with radio- 

 active materials. 



A hood should not be placed near doorways or windows or in the vicin- 

 ity of strong air currents, which may tend to draw fumes from the hood. 

 All the hoods in any one room should be controlled by the same switch. 

 This will avoid the chance that air flow from an operating hood will bring 

 contamination into the room from a nonoperating hood. The exhaust 

 air should be discharged so as not to contaminate surrounding facilities; 

 a point of discharge 5 to 10 ft above the roof is usually satisfactory. The 

 blower should be mounted so that the motor does not become contam- 

 inated. Also the blower should be located near the top of the exhaust 

 duct so as to minimize the escape of active material caused by positive 

 pressure within the duct. Usually it will not be necessary to filter the 

 hood output. The performance of the hood may be readily evaluated by 

 setting off commercial smoke bombs and observing the distribution of 

 smoke both within the hood and from the hood stack. 



For work in the gas phase, particularly with carbon 14, a specially 

 designed hood is desirable which permits ready access to a vacuum rack 

 (see Fig. 4-6). 



hood. E, easily cleanod-up sinks, located near the front of the hood, are recom- 

 mended in place of less accessi])le trough sinks located at the back. F, an air velocity 

 of 50 to 80 linear feet per minute at the face of the hood, with the .sash wide open, is 

 recommended; an exhaust-duct damper can be set as required. G, the lintel behind 

 the sash contains an air bypass opening which becomes exposed when the sash is 

 lowered; this bypass prevents excessive air velocity at the work surface when the 

 sash is nearly closed. H, the counterbalanced hood sash may be made of tempered 

 safety glass. /, the withdrawal of both light and heavy fumes may be achieved by 

 the proper adjustment of a movable section of the back baffle. /, in accordance with 

 general practice, a trough is located behind the bottom edge of the lintel to carry 

 any condensate to the ends of the hood. A', turbulence of the air entering the hood 

 should be reduced as much as po.ssible; this may be accomplished by the use of a 6-in. 

 "picture-frame" airfoil at the sides and bottom of the hood. L, the air-flow char- 

 acteristics will be improved by the use of a vent space between the bottom airfoil 

 and the hood body. 71/, service outlets (gas, water, vacuum, etc., as required) should 

 be located near the front of the hood so that the operator will not have to reach into 

 the hazardous zone to make hose connections. A'^, service handles and electric outlets 

 should be located outside the hood. 0, there should be plenty of 110-volt electric 

 outlets, connected to two or more laboratory circuits to minimize the likelihood of 

 circuit overloading. P, the base structure should be strong enough to support a ton 

 of shielding, but it is not recommended that shielding be incorporated in the hood 

 structure. {From Donald R. Ward, Design of Laboratories for Safe Use of Radio- 

 isotopes, AECU-2226, November, 1952, Courtesy of Atomic Energy Commission.) 



