mfety in the chemical laboratory
edited by MALCOLM M. RENFREW University oi ldaho MOSCOW. ldaho 83843
Good Hood Practices for Safe Hood Operation William G. Mikell a d Frank H. Fuller E. I. du Pont de Nemours & Co., Wilmington. DE 19898
Laboratory hoods are part of the equipment chemists and technicians use t o protect themselves and others from exposure to abnormal events t h a t produce vapors, fumes, and dust and from potential explosions and runaway reactions while conducting experiments. T o do this safely means proper hood design and proper work practices. But safe operation depends on many other things as well: advance planning, including checking the literature, designing safe experiments, assembling equipment, materials, and personal protection, including shields if there is any chance of explosion, and, finally, setting up the right equipment for safe operation inside the laboratory hood. High hood face velocity will not automatically compensate for incorrect or careless work practices. Laboratory safety is jeopardized when there is a disproportionate emohasis an hiaher face velocities and little or no emphasison work practices or hood design, or room supply air distribution. The Du Pont laboratory standards require a minimum face velocity of 60 fpm with equal
Wlillam G. Mlkell is the EnvmmWntBi Conlro Manager at me Du Ponl Company Exper rnenta Stat on n Vd mlnglon. DE Tn.5 1ac.loty nouses a w g e numoer of laooratories engaged in s wide variety of research and development activities for Du Pont He was active with the National Research Council Committee on Hazardous Substances in the Laboratory engaged in developing their report on "Prudent Practices for Handling Hazardous Chemicals in Laboratories". Frank H. Fuller, a registered proferslonai engineer, is a Senior Consultant in the Engineering Department of E. I. du Pont de Nemours & Company. He specializes in evaluation and control of the industrial environment and has expertise in ventilation for fume and dust control. air conditionino. heat stress, and laboratory hwd perfor-
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author of numerous publlcatms and technml reports and is certiiled can Board of lndustr$alHygle
A36
Journal of Chemical Education
emphasis on safe work practices, correct room supply air distribution, and proper hood design. To insure the health and safety of h w d users, installations are tested with a modification in geometry of the performance test developed by Caplan and Knutsonl for the American Society of Heating, Refrigerating and Air ConditioningEngineers. Inthis test, a mannequin simulates a chemist standing a t the face of the hood. A mixture of air and Refrigerant 12 is diffused into the hood a t about 12,000 ppm. Concentrations in the breathing zone are picked up by a probe and analyzed by an infrared analyzer. Results of this test are used to determine the hood index2 (HI) as ameasure ofthe h w d performance. hood index = lrreathing - -zone . . c. .m r m t r a t i m -log emlsswn concentration 'This test is very useful in 5tudying hmd performance; hwever, it shwld he recog. nixed that it does not enconnpass all the potential situations that can arise in hood usage. Using this test method, the importance of placing emission sources well behind the sash was demonstrated. A small increase in distance between the source and the ehemist resulted in a large reduction in exposure concentration. Hood performance was Less sensitive t o face velocity than to distance behind the sash. Results show that a t a source distance of 2 in. behind the sash (about 8 in. from the breathing zone of the chemist), the hood index was 3 for a 60 ft/ min hood. At 8 in., the index increased t o 6, a millionfold decrease in concentration between inside and outside. Our standard specifies that sources be located a t least 6 in. behind the sash. This is 12 to 13in. from the operator's nose for s well designed hood with entrance airfoils and bevels. The importance of face velocity and sash opening was also determined. At the 2-in. distance (8 in. from the operator's breathing zone), there was no improvement in performance with an increase of face velocity from 60 t o 100 ftlmin. The hood index remained a t 3. When the velocity was increased t o 140 ftlmin, there was less then one order of magnitude improvement in hood index. On the other hand, reducing the sash opening by lowering a vertical sliding sash, another work practice, provided over three orders of
magnitude improvement, much greater than that due to the resulting velocity increase alone. The effects of work practices on some typical laboratory operations using a 9-min procedure involving the pouring, transfer, and filtration of acetone were also investigated. A procedure was standardized and goad work practices defined as summarized a t the end of this article. All sources were kept a t least 8 in. inside the plane of the sash. The sashes were closed as much as practical, and the operator kept his face outside the plane of the sash. In contrast, "bad" work practices consisted of locating the acetone source only 2-3 in. inside the hood, having all the sashes open, and the operator leaned into the hood for two 15-s periods durine the 9-min ooeration. Usine., .. mad work prartices, no detectalde concentra. tlons (dnecrion level O.U1 ppm) xere uhrained at the breathing rune of the operator. Good work practices provided an average hood index of greater than 6, about two orders of magnitude better than poor work practices. Results of all tests underscored the prime importance of good work practices. Like any tool, the hood has to he used properly if it is going to do the job i t was designed to do. A laboratory hood is not a waste disposal unit. Appropriate scrubbers should be provided to prevent worker expasure and emission of toxic materials to the environment. Small quantities of chemicals should he used whenever possible. Shields and catch pans should be used to provide protection against unexpected releases or spills. Even the best hoods cannot overcome improper work practices by the user. A tour through any laboratory complex will reveal many bad examples. However, there are some straightforward modifications in hood design and installation that can make it easier far the chemist to arrange his apparatus better, making it easier to practice good techniques. With a hood able to aceommodate modern equipment such as analytical instruments, ovens, and computer controls, it becomes easier for the chemist to employ safe work practices. Likewise, gas control valves for gases and electrical outlets are usually placed outside of the hood.
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Hood design changes are sometimes necessary to provide more space and encourage good practices. For example: Hoods having bench sections that can be installed or removed according t o project needs can provide more fleaibility to accommodate equipment of various types and physical dimensions as well as t o enable computer-controlled or instrument-run operations. Bench width was increased 8 in. and hoods were equipped with an instrument strip suitable for feeding signals to a minicomputer in the laboratory for remotely controlling operations in the hood. This is especially useful for long or overnight experiments. These hoods can aecammodate equipment of different sizes without disrupting hood air flow. Electrical cables and plugs do not hang out of the hood face and sashes can be fully closed. Another modification is aimed a t providing more working room for t h e chemist without undertaking major airhandling renovations. Modifications are often limited by the building heating and ventilating systems. By converting a 10-ft vertical sashed hood to a 15-ft hood with horizontal sashes, a50% increase in bench space was achieved while maintaining the same total air volume through the hood. The extra space allows additional service outlets, electrical receptacles, and ventilated storage beneath the bench top. This additional bench space should result in a less crowded hood and produce better adherence t o good work practices. Finally, this is a summary of good hood practices: Sash openings should be kept to a minimum. Sources of emission should be kept a t least 6 in. inside the hood. Users should keep their faces outside the plane of the haod sash. Storage in the hood should he kept to a minimum. Exhaust portsfrom haod andsupply air vents to room should not be blocked. Traps, scrubbers, or incinerators should he used to prevent toxic and noxious materials from being vented into the hood exhaust system. Remain alert to changes in air flow. Prepare a plan of action in case of an emergency. T o save energy, turn off hlowey and close sashes when hood is not in use if installation is engineered t o permit it.
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I Caolan. K. J.: Knutsan. G. W. "DeveloDment of Crlrsrm lor Des gn Se ecrson and n place Tesung of .aboralary F ~ m ehoods and -aboratov Room Venl laloon Arr Supply F nal Repon AShRAE RP-
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70. March 1978.
2Fuller, F. H.; Etchells. A. W. 'The Rating of Laboratory Hood Performance"; ASHRAE J 1979, October.
Volume 65
Number 2
Februaw 1988
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