in the Chemical laboratory Edifed by N O R M A N V. STEERE, School o f Public Health, University of Minnesota, Minneapolis, Minn., 55455
II. VENTILATION OF LABORATORY OPERATIONS Part One \'entilation in laboratories is frequent,ly not adequate t o prevent exposure of personnel to vapors, gases, dusts, and aerosols which may bbe toxic. Many lahoratory hoods and exhaust systems are poorly designed, poorly maintained, and t , w small to meet the needs of increased lahorirt,ory use and advanred technology. Protection of the health and safety of lahorntory personnel requires that inadequnt,e ventilation systems he improved and that designs for ventilation in new lalxmt,ories be fully adequate. Improvements can be made economically if at,t,ention is given to some basic principles of laboratory ventilation. To simplify the consideration of laboratory exhaust ventilation, the functions of erl~austsystems are grouped into the rntegories of capture, transport, and disposal. Capture will be discussed in this inst:dlment, transport and disposal in the next. Air supply to laboratories will not he discussed except to mention that e f f e c hive operation of the exhaust system depends nn a balanced supplg of air, and t,llltt the quality of results in the laboratory may depend on an air supply of high quality(l4). Substituting less hazardous materials would he appropriate means of minimizing the needs for ventilation and minimizing the hazards if ventilation systems stop working (as is the ease when power fails). Using toluene instead of benzene, and S+ instead of Sr' would be two good examples of substitution. Another desirable sohstit,ution would be replaring carbon tetrachloride with l,l,l-trichloroethane, or another solvent if lesser volatility is acceptable.
Laboratory Hoods
A hood is no good unless it c ~ p t u r e and s retsina the atmospheric aontaminxnts generated within it. A h o d is not intended to capture contaminants whirh became air-borne elsewhere in the isboratory, nor is x hood generally designed to contain explosions. Successful performance depends primarily on the velocity of air moving through the hood. Factors which affect the face velocity and air movement, through the hood are cross-currents, entrance shapes, thermal loading, meehlmianl action, exhaust slot design, and ohstruetions. Successful performance of a hood mav also depend upon its ability t,o confine s fire, t o withstand corrosion (4), to he readily cleanable if contaminat,ed, and tc, collect certain eontaminmts surh as radiw isotopes and pathogens ( 8 ) before they enter the exhaust system. We believe a laboratory hood intended for generd use should be appropriate for use of flammable liquids and gases, and should be constructed of material whirh will withstand fire for several minutes so that the h o d enclosure can maintain its integrity and confine the fire until i t can he extinguished (7). Design of hoods which may become contaminated and heve to he cleaned of radioactivity may be based on one of two current philosophies. Constmrtion of stainless steel with welded joints and eoved corners hm been the generally arcepted philosophy. A recent view is that equivalent results a t lower cost can be obtained from ordinary hood materials rovered with drippxhle coatings. We feel that stainless steel ia best for college Capture of Air-borne Contaminants and university hoods for radioactive service, because we do not believe that stripChemicals and micro-organisms whirh prthle coatings can be maintained efferarp t,oxic or mthoeenie. -~ . or whose toxi- t i d y or eeonomioally where lahorntories v . ~ I . , ~ i pnqwrtws:,rr v nor knlla.n. should Ijr are not numerous or roncentmted or wella am1r~dl4 8,s !h:a~pwqde in the I : d ~ t m t ~ ~ r vstatTed. do not absorb, ingest, or inhale the maDesign criteria heve been estahlished terials. I n order to prevent inhalation of for hoods in radioactive servire (13) vapors, gases, and perticulates (fumes, inoluding provisions for scrubbers (11) smokes, dusts, and aerosols) the contamiand filters with pressure drop gauges. nants must he contained or captured. Scrubbers and filters may be required to Methods used include hoods, enclosures, limit release to the atmosphere to permisand spot ventilation. sible quantities, or to prevent oontsmine; tion of ducts and fans which can rompliGreat flexibility st law initial investcate maintenance. ment and low operating costs can be Safe use of hot concentrated perehhric provided by enolosures and spot ventilaaeid requires prevention of vapor contact tion. T h e ~ etwo methods may be the with organic materials or vapor condenssonly possible or economical means of imtion where organic material may later provingexistinglabor~atorie~, and they may come in contact. Safe use of perehlnric he the best mean8 of building ventilation acid may be accomplished by use in a flexibility into new lrtboratories.
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separate hood with n duet wash-down system and no exposed organic coating, sealing compound (41, or lubricnnt; safe use may also be accomplished by use of a special scrubber unit (11). Chemical safety data sheets and the ACS Monograph on Perchlorates outline other precautions for safe use of perrhloric aeid, including the prohibition of organic materials (gas and oil) for heabing.
Hood Face Velocity The velocity of air entering a hood a t its face determines whether or not the hood will he safe, since an adequate face velocity is the basic requirement for c a p ture and control of contaminants generated within a hood. A minimum face velocity of 100 lineal ft per min for general laboratory hoods is s recommendation (1, 10, 18, 18, 15) with which we agree. Hoods for highly toxic materials require face velocities ranging from 125 to 200 f t per miu.
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Figure 1. Obrtructions and poor slot design reduce hood performance.
Although face veloaities much greater than 100 lineal f t per min may disturb gas flames and fine powder in conventional hoods, the disturbances can be offset by use of electric heat, screening, or a hood design in which half of the air does not pass through the bottom slot. Obstruction and poor design of hood exhaust slots are illustrated in Figure 1, while Figure 2 shows an arrangement of three exhaust slots designed to prevent obstructions and provide more uniform flow of air.
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Volume 41, Number 2, February 1964
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oniform flow of air out of an air flow bench in a clean room provides the best ezclmion of contaminants. Laminar air flow is expected to make a, substantial contrihution t o the techniques of contamination control in dean rooms (S),and laminar flow could contribute to cmtaminntion control in laborstories. Because cross-aurrent,s outside a hood can divert or nullify sir flow ink, a hood and its capture ahility, i t is important to locate h , ~ , d s to minimize air currents fn,m drwrs, windows, and air supply grilles.
One good and two poor locetions for lahorntory hoods are illustrated in Figure 3-hood A will have a better chance of working correctly because i t is in a. loeation with minimum cross-currents from windows, doors, and pedestrian traffic. Pedestrian traffr past a hood should be ns lit,tle as possible since a walking rate of one mile an hr c;mses a cross-current velocity of 88 ft per min, and two miles an hr 176 it per mi". Figure 4 a and b show how vapors eddy near the front of an ordinary hood and are easily d r s v n out by a passer-by. Figure 4 c shows how a n sir-foil added to a hood will prevent eddies by providing a sweepine, flow of : ~ i rxcmss the bottom of the hood. Even wlml a hood has a n adequate face velority, a n investigator aitn inhnle toxic materials hrought back h y the eddies around his body. CI:trke (4) cites a n instanre in nhiah n person received a, critiral e p m m f n m toxic ehemiritls in a hood with face velorit)- over 100 f t per min. Figure 5 shews how such a n exposure can owur and how it can be prevented hy 8 shield or by n section of horizontal-sliding sash. Improvement of Existing Hoods
Figure 2. Three eihoud slots provide mare uniform air flow. (Not drawn to rcolel.
Uniform flow of air into and through laboratory hoods will provide the hest enpture of contaminants ( l o ) , just as
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Figure 3. LocoBon of hoods. A is in the best location; B ond C are poorly located.
Existing hnt,ilntion," i t h ed., Edwsrds Brothers, Inc., Ann Arbor, hlich., 1962. (14) "hlrdiwl School Facilit,ies," Puhlir H d t h Service Publirntion No. 875, Washington, 1). C., 1961. (15) hlirhignn Ilepnrttnent f Health, dlichiqnn's Oceupntionnl Health, 4, No. 4 , 1 ( I W ) ) . The second installment of this two-part zrtielr will npponr in March.
Sul,srquent nrtirlrs in this series will d e d with: Fire-Proterted Stwage for Records 2nd Clremimls Good Labelling I'm~.tices Flnmrnahle Liquids in the Laboratory 1,aboratory Equipment Hneards Protert,ive Equipment for 1,aborntory Persunnrl Lnbamtory Design Considerations Safe Handling of Compressed Gas Cylinders Monitoring Lahamtory Atmosphere Fire Extinguishers and Estinguishing Systems Respiratory Protection ior Lnboratory Emergencies Emergency Cnrc of Injuries in the 1,nboratory Fire Emergency and Rescue l'roredures llisneter Conl,rd Planning
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