formaldehyde xylene a approach to systemssolved. Toachieve this, andto save space in the lab-oratory, the processors have been housed in cup-boards under the benches (Fig. 6). These

J Clin Pathol 1984;37:401-408

Removal of formaldehyde and xylene fumes fromhistopathology laboratories: a functional approachto the-design of extraction systems

FP EDWARDS, AR CAMPBELL

From the Department of Histopathology, North Manchester General Hospital, Crumpsall, Manchester M8 6RB

SUMMARY There have been many reports of the relative hazards of formaldehyde, xylene, and hotparaffin wax fumes in pathology laboratories. In contrast, there have been comparatively fewdescribing efficient extraction systems for their removal. When the opportunity presented to designa new laboratory the efficient removal of these fumes was given a high priority. In designing theextraction systems described consideration was given to the physical properties of the fumes and thedegree of freedom of use of the bench, with the object of achieving sub-threshold limit values foreach area of exposure. The efficiency of removal of the formaldehyde and xylene was measured usinginfrared spectroscopy.

Increasing awareness of the health hazards associatedwith chemicals commonly used in histology depart-ments has stimulated both equipment manufacturersand medical laboratory scientists to improve the lab-oratory environment by reducing the release of toxicfumes. Closed circuit tissue processors and theabsorption of fumes by charcoal filters are both exam-ples of developments by manufacturers.The installation of an extraction system in a labora-

tory is not easy for a variety of reasons. The installa-tion is usually undertaken only when a new laboratoryis built or when an existing block is converted. Thelatter may well present difficulties in installing extrac-tion. The unavailability of proven systems and oftenthe lack of liaison between the laboratory staff andplanning team can result in the installation of aninefficient system.

In 1982 the histopathology department at NorthManchester General Hospital moved into a new lab-oratory which was formerly a ward. The extractionsystem installed was based on the original conceptsproposed by the laboratory staff. These often rudi-mentary ideas were developed by the architect andheating consultant to give an extraction system whichwas not only very efficient but designed with the userin mind.

Design objectives

Three basic objectives were set:1 To achieve sub-threshold limit values for eachAccepted for publication 20 December 1983

chemical used in the laboratory.2 To keep each working area free from obstructionssuch as hoods, canopies, cabinets.3 To remove the traditional odours associated withhistology laboratories.A balanced warmed air heating system was chosen

to provide the general ventilation. This providesfiltered warm air to the laboratory at four to six airchanges each hour. The system is supplemented byindividual local extraction modules built into eachbench where toxic agents are used. These are con-nected directly to the external environment at the sec-ond storey level of the building.The design considerations for the extraction mod-

ules were:I Threshold limit value of the agent. Each modulemust not exceed, under normal working conditions,the figures of 2 ppm for formaldehyde and 100 ppmfor xylene. These figures were used as maximumexposure limits not as time weighted averages.2 Accessibility to the bench. Where an area is con-stantly used or the whole depth of the bench is neededor where articles are often moved on or off the benchthe extraction must be achieved without hindrance tothe worker. Bench areas considered to be in this cate-gory were: specimen dissection (cut up), tissue dis-posal, wax embedding, staining, and slide mounting.Areas where total freedom of access was not necessarywere: tissue processing, staining machines, andfixative preparation.3 Physical characteristics of the toxic vapour.(a) Inflammability-fans with spark proof motors

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are essential when exhausting xylene. Installing fanswith motors outside the air current was an addedprecaution. (b) Vapour density-the direction ofextraction was a prime consideration. Removal of theagent away from the worker providing the greatestprotection. Consideration of the density of the agentand its extraction along the direction of its naturalpath will give more efficient extraction and enhanceprotection. Formaldehyde being only slightly heavierthan air, with a relative vapour density of 1 07, tendsto remain static at the liberation point, while xylene(relative vapour density 3 67) will fall naturally. Thecapture velocity necessary will vary according to thedensity of the agent.

Formaldehyde extraction

Four areas were identified where contact withformaldehyde was such that local extraction wasnecessary.

SPECIMEN DISSECTION AREAThis consists of a one piece stainless steel unit 2000 x600 mm available commercially as a Grantham SlopHopper unit (Dent & Hellyer Ltd, Wallworth Road,Andover, Hants). It incorporates a flat dissectionarea, 800 x 480 mm, a central sink, 520 x 400 x220 mm, and a sluice, face opening 350 mm diameter(Fig. 1). The unit has been modified to accommodatea sparge pipe to supply a cleansing sheet of water overthe dissection area. Also fitted is a hand held lowpressure water spray, which is used to cleanse a raisedCorian (Corian Distribution, Richmond House, Blen-heim Terrace, Leeds) cutting table 500 x 360 mm,sited on the dissection area. The sink is used to open

Edwards, Campbell

fluid filled cysts and so on, and the sluice to open andclean lengths of bowel.

Removal of formaldehyde is through a singleextract slot, 22 x 1880 mm, raised 40 mm over thebench surface. The extract slot runs the full length ofthe unit immediately behind all three work areas. Thepoint of extraction can be altered by using the valvesin the trunking (Fig. 2) and any combination of allthree areas can be ventilated. The fan is sited beneaththe unit and has a variable speed control. Air flowmeasurements using a hot wire anemometer at a point25 mm in front of the extract slot gave the figuresshown in the Table.Removal of the formaldehyde is from front to back,

the raised cutting table allowing air to pass under aswell as over the specimen area. The dissection area isregularly irrigated using the hand spray and spargepipe to remove residual fixative.The efficiency of the extraction system was mea-

sured by monitoring formaldehyde concentrationsusing a Miran (Wilkes Scientific Co Ltd, 64 BurnersLane, Kiln Farm, Milton Keynes) infrared gas ana-lyser at wavelength 3 58 gm coupled to a flat bed chartrecorder. Air was sampled at chest height con-tinuously through two typical cut up sessions conduc-ted by the same pathologist. Fig. 3 shows theformaldehyde concentrations reached without extrac-

Airflow velocity measurementsfor the constituent areas of thedissection benchAirflow velocity (mls)Sluice Sink Cut up area1-778 m/s 2 032 rn/s 2-79 rn/sVent closed 2-54 rn/s 3 302 rn/sVent closed Vent closed 4 064 m/s

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Fig. 2 Specimen dissection unit showingdetails of construction of extraction trunkingandfan. Note valves in trunking to controlpoint of extraction.

Fig. 3 Formaldehyde concentrations during acut up session with no extraction. The fan wasswitched on after about 40 min whenconcentrations reached 13ppm (off scale).

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Fig. 4 Formaldehyde concentrationsduring a cut up session with extractionfan on.

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tion and Fig. 4 shows values reached when the fan wason throughout the cut up session.

Noise level from the unit was 57 dBA, which is notconsidered excessive (threshold limit value 90 dBA).This level does not prohibit dictation of specimendescriptions to secretarial personnel or tape recorder.

SPECIMEN DISPOSALThe formaldehyde released during tissue disposal andwashing of specimen pots is removed by extractionvents sited above and to the rear of the disposalmachine hopper (Haigh Tissue Disposal Machine,Model 55B, Haigh Engineering Co Ltd, Alton Road,

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Fig. 5 Extraction vents andducting over wax embedding units.

Ross on Wye) and the wash up sink. The vents, sited600 mm over the washing areas, are angled at 450 toimprove vapour capture. Air flow velocity through thevents is 1-016 m/s. Formaldehyde concentrations

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Fig. 6 Tissue processor housed in extraction cupboardsunderneath the benches and mounted on mobile platforms.The extraction fans can be seen on the back wall and smokesensor on roof of cupboard.

monitored throughout an entire tissue disposal ses-sion indicated that the system maintains for-maldehyde concentrations below the threshold limitvalue at all times.

FORMALIN FIXATIVE PREPARATIONA closed circuit pumped system was constructed toprepare the large quantities of formalin fixative1needed to supply several hospitals and postmortemexamination rooms. The apparatus consists of a 1251closed polythene tank. Mounted over this, on a gan-try, are two peristaltic pumps, a paddle mixer, and awater inlet solenoid valve controlled by an automaticfluid level detector. The whole unit is housed in acupboard from which air is constantly extracted at arate of 0 638 m3/s. This system enables 1201 of fixativeto be prepared and dispensed without contaminatingthe laboratory atmosphere.

STORAGE OF SPECIMENSSpecimens stored in polythene containers on openshelves can contribute to the general concentration offormaldehyde in the atmosphere. To remove thissource of contamination all specimens are kept incupboards which have constant extraction at0 319 m3/s. The shelves are perforated to allow airmovement within the cupboard. Opening the cup-board doors for a few minutes produces a concen-tration of formaldehyde of 1-4 ppm in the laboratoryadjacent to the cupboard.

Paraffin wax fume extraction

The hazards associated with paraffin wax fumes arelargely unidentified as the composition of the fumescan vary with the wax constituents, additives,

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Fig. 7 Staining and sectionmounting area composed of twomounting modules, pre- and post-staining dish modules on either sideof the sinks. The individual fanspeed controllers are mounted onthe rear wall over each unit.

impurities, solvents, and also the degree of heating. Inrecognition of the possible dangers we decided toremove paraffin wax fumes from the laboratory.

EMBEDDING CENTRESMost of the fumes generated from embedding centrescome from the hotplate and wax reservoir area. Theyrise in the hot air and are captured by siting angledextraction vents 200 mm directly over the hotplateand reservoir area (Fig. 5). The air flow is 2 032 m/sthrough the vents to provide complete capture asshown by smoke pencil tests.

TISSUE PROCESSORS (CAROUSEL TYPE)Here, in addition to hot wax, formalin and xylenehave to be considered. The mixture of fumes of vary-ing vapour density, the flammability of the reagents,and the physical size and complexity of the machinesmake efficient fume extraction by air flow alone animpossibility. To enclose the processor is the onlypractical solution if the problem of accessibility can besolved. To achieve this, and to save space in the lab-oratory, the processors have been housed in cup-boards under the benches (Fig. 6). These haveconstant extraction at 0 319 m3/s, alarm protectedfans, and heat and smoke sensors. The processorsstand in drip trays mounted on runners, allowingthem, to be pulled out smoothly for cleaning andchanging; loading and programming can be donewithout moving the processors.

Xylene vapour extraction

Protection against xylene fumes was necessary for themanual staining bench, the section mounting area,and around the two staining machines.

MANUAL STAINING AND SECTION MOUNTINGThese two areas were combined as both demanded asimilar degree of freedom on the bench-thus an

Fig. 8 A staining dish extraction module with traypartially removed to show lower chamber construction andextract port position. Note rim around port to containspillage.

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Fig. 9 Perforated bench top extraction modules for sectionmounting.

enclosed extraction system was ruled out. A flow pro-tection system was decided on, with the extraction ofxylene downwards aided by its natural fall owing to itshigh vapour density. Due attention was given to itsflammability in the type of fans and position ofswitchgear and so on. Much thought was given to thechoice of material for the bench construction.Resistance to the commonly used staining reagentsand to delamination by water together with the abilityto construct the extraction modules and sinks withoutleakage problems were all important considerations.The staining and section mounting bench is a one

piece unit measuring 5520 x 750 mm. It incorporatesfour down draught extraction modules and two sinksmoulded into the bench (Fig. 7). The modules eitherside of the sinks are trays sunk into the bench top tohold the xylene and alcohol dishes before and afterstaining. The trays measure 450 x 450 mm and holdnine 120 x 120 mm glass staining dishes. Extractionis through 20 mm diameter holes in the sides of thetrays and down through the centrally placed extrac-tion port (Fig. 8). The fans, situated directly beneatheach unit in the cupboards, are vented directly

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a similar construction to the staining dish modules,except that a top cover replaces the staining dish trays.The cover, which is flat and fits flush with the benchtop, is perforated with 10 mm holes equally spaced at33 mm centres over the whole area. Sections aremounted at the centre of the perforated area, theworker being protected by a curtain of air flowingdown through the perforated bench top.The whole of the bench top is constructed of

Corian. This is a mineral based compound bondedinto a homogeneous non-porous material whichsatisfies all the construction criteria detailed above. Itsuse in this laboratory has enabled the bench top,extraction modules (trays, top covers, and lowerchambers), and sinks to be moulded into a jointlessunit.

Xylene concentrations were measured using aMiran gas analyser at wavelength 13 19 gm. The sam-ple point was 150 mm over the xylene dishes andmidway between the person's face and the slide beingmounted. Xylene concentrations over the dishesbefore and after staining increased rapidly in still airwhen the covers were removed. Within two and a halfminutes of switching on the fans, xylene concen-trations fell from 75 ppm to nearly zero (Fig. 10).Extraction tates of similar efficiency were achievedthrough the section mounting modules. A figure ofunder 10 ppm was achieved with the fan on comparedwith 75 ppm with no extraction (Fig. 11).

STAINING MACHINESThe intermittent use of the staining machines used inthe laboratory determined that enclosure extractionwas adopted.

Poststaining

Fig. 10 Xylene concentration overthe dishes. The glass lids wereremoved at the start of the traceand extraction fans switched onafter 2 min (arrows).

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Fig. II Xylene concentration duringsection mounting. The extraction fanwas switched off after mounting12 slides (arrow). A further 12 slideswere then mounted with no extraction(5 -.8 minutes).

Carousel type machinesTo contain and remove xylene and alcohol vapour

emanating from a Shandon 23 stage stainer, used forPapanicolaou staining, a free standing perspex cover

760 x 740 x 680 mm was provided. The front isdetachable and has a slot at its base to allow air entry.The cover is shaped to fit the contour of the back wall,through which an extraction fan removes the fumes.The cabinet size is large enough to permit dailymachine operation and light enough to be removedwhile cleaning the machine and changing reagents.The fan is kept running during these operations.

Linear staining machineOwing to the location of the SKI linear stainer in thelaboratory and the accessibility needed, it was impos-sible to extract this equipment without unsightlytrunking and loss of access. A Shandon Fumeguard(Shandon Southern Products Ltd, Astmoor,Runcorn) with its xylene filtration system was pur-chased. Preliminary tests indicate a poor level ofextraction with appreciable escape of xylene from thereservoir end of the machine. The position of theextract fans relative to the reservoir is the main reasonfor the failure to capture a vapour as heavy as xylene.An improved model (Shandon Linistain Retro FumeHood) has recently been introduced featuring a morecompact design.

Discussion

The Health and Safety Executive and its Americanequivalent, the National Institute for OccupationalSafety and Health, both present a formidable array ofreferences to published work on the health hazards offormaldehyde.23 These include acute toxicity in awide range of experimental animals,4 - 6 eye

irritancy,7 skin sensitisation,8 some evidence ofmutagenic effect,9 10 and carcinogenicity in rats andmice. 112 In man there are reported acute toxic effectsdue to ingestion'3 and inhalation,'4 skin'5 and eye'6

irritancy, and skin1 7 and possibly lung18 sensitisation.There is no evidence that exposure to formaldehyde

has produced cancer in humans. The weight of experi-mental evidence is such, however, that the AmericanInstitute has recommended that although humans andanimals may differ in their susceptibility to specificchemical compounds, any substance that producescancer in animals should be considered a risk tohumans.3The hazards associated with the use of xylene are

not as extensively reported. They include acute respi-ratory distress in humans,'9 but xylene is given amoderate toxicity rating, with headache, nausea, lossof appetite, lassitude, and impairment ofcoordinationall being quoted.20

Faced with the considerable range of effects fromthese two commonly used reagents in our laboratory,the senior staff were determined to provide a safeworking environment. The initial step in our designwas mentally to enclose each bench operation and itscontaminant and then make such openings or changesin the enclosure as were dictated by the operationtherein. The air flow then compensates for the open-ings and design compromises. We soon realised thatso much access was needed for the specimen cut up,tissue disposal, and staining and section mountingbenches that enclosure by hoods or cabinets wasimpracticable as well as undesirable for the desiredclean design of the laboratory. The only alternativewas to provide protection for the staffby removing thecontaminant by efficient capture using air flow alone.The success of this depends on the efficiency of thesystem and is achieved in three main ways. Firstly, byway of the forced air heating system to provide gen-eral ventilation. This also provides slight positive airpressure in the laboratory which ensures each localextraction module is never starved of air. Secondly,noting the density of the fumes, it was proposed to use

the direction of their natural flow. More efficient cap-ture can then be achieved with lower air velocities.Finally, each extraction vent was positioned as near as

possible to the contaminant point compatible with the

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freedom of movement on the bench.The resultant local extraction modules described

are very efficient as can be seen by the gas analysertraces. At no time during normal working are thethreshold limit values for formaldehyde or xyleneexceeded during specimen cut up, tissue disposal, orsection staining and mounting. In fact these figuresare achieved with the fan speeds set below their maxi-mum values. This gives latitude to deal with abnormalloads (gross spillage) or any future reduction intlhreshold limit values. As the efficiency and air veloc-ities for each module on installation are known, thesecan be measured yearly to monitor the performance ofthe systems.The complexity of tissue processors and staining

machines makes the design of an open air flow extrac-tion system virtually impossible. The limited need foraccessibility and the fact that when spillage occursliquids may enter the equipment, making cleaningdifficult, prompted us to choose enclosure type extrac-tion for the carousel tissue processors and stainingmachines. The need to reach the rear of the equipmentfor cleaning and reagent replacement was met in thecase of the processors by placing them on mobileplatforms; for the staining machine a cabinet lightenough to be easily removed was constructed.

Experience in using the Corian staining bench forthe past year has endorsed the decision to use thismaterial. Its excellent moulding and jointingproperties have resulted in a smooth leak free unit. Itsrelative resistance to stains keeps cleaning to aminimum and surface damage, should it occur, can beremoved by simple abrasives. The designs have givencomplete freedom of use and when not in use forsection mounting the area is available for general use.By considering the ergonomic requirements, the

freedom of movement needed, and the physical prop-erties of the fumes liberated modular extraction unitshave been designed which impose few restrictions anddifficulties in their use. This has given a more efficientlaboratory in which the staff benefit from a saferworking environment.

We acknowledge the invaluable help of Mr BarryRansom and Mr Roy Evans, members of the planningteam, Dr D deKretser and Mr DJ Jenkins for theirassistance in the preparation of this manuscript, andMiss J Perry for the photographic assistance.

Edwards, Campbell

References

I Carson FL, Martin JH, Lynn JA. Formalin fixation for electronmicroscopy. A re-evaluation. Am J Clin Pathol 1973;59:365-73.

2 Health and Safety Executive. Toxicity review, 2: Formaldehyde.London: HMSO, 1981.

3Robbins A, Bingham E. Formaldehyde: evidence of carcino-genicity. NIOSH/OSHA, Current Intelligence Bulletin 34,Cincinnati, Ohio: NIOSH Publications, 1980.

4 Salem H, Cullumbine H. Inhalation toxicities of some aldehydes.Toxicol Appl Pharmacol 1960;2:183-7.

1 Kilburn KH, McKenzie WN, Thurston RJ. Effect of formaldehydeas vapour and absorbed on carbon, on hamster airways. Am RevResp Dis 1976;113:238.

6 Feldman YG, Bonashevskaya TI. On the effects of low concen-trations of formaldehyde. Hygiene and Sanitation 1971,36:174-80.

7 Grant WM. Toxicology of the eye. 2nd ed. Springfield, Illinois:Charles C Thomas, 1974:502-7.

8 Maurer Th, Thomann P, Weirich EG, Hess R. Predictive evalu-ation in animals of the contact allergenic potential of medicallyimportant substances. II Comparisons of different methods ofcutaneous sensitisation with weak allergens. Contact Dermatitis1979;5: 1-10.

9Rosenkranz HS. Formaldehyde as a possible carcinogen. BullEnviron Contam Toxicol 1972;8:242-4.

°Wilkins RJ, Macleod HD. Formaldehyde induced DNA-proteincrosslinks in Escherichia coli. Mutat Res 1976;36:11-6.

"' Kerns WD. Long term inhalation toxicity and carcinogenicitystudies of formaldehyde in rats and mice. Reported at the thirdCIIT conference on toxicology. Raleigh, North Carolina: 1980.

12Swenberg JA, Kerns WD, Mitchell RI, Gralla EJ, Pavkov KL.Induction ofsquamous cell carcinomas of the rat nasal cavity byinhalation exposure to formaldehyde vapour. Cancer Res1980;40:398-402.

'3 Rathery F, Piedelieure R, Delarue J. Death by absorption offormalin. Annales de Medicine Legale, de Criminologie et dePolice Scientifique. 1940;20:201-9.

4 Fassett DW. Aldehydes and acetals. In: Patty FA, ed. Industrialhygiene and toxicology. 2nd ed. New York: Interscience,1963:1970-2.

I Proctor NH, Hughes JP. Chemical hazards in the workplace. Phila-delphia: Lippincott, 1978;272-4.

16 Kelecom J. Corrosive eye damage with formalin. Arch Ophthalmol(Paris) 1962;22:259-62.

1Marzulli FN, Maibach HI. The use of graded concentrations instudying skin sensitives: Experimental contact sensitisation inman. Food Cosmet Toxicol 1974;12:219-27.

'8Sakula A. Formalin asthma in hospital laboratory staff. Lancet1975;ii:816.

9 Bush CL, Nelson CE. Xylene (a warning of its use in the histologyand cytology laboratory). Medicine 1977;8:16.

20 Sax NI. Dangerous properties of industrial materials. 5th ed. VanRostrand Rheinhold: New York, 1979:1094-5

Requests for reprints to: Mr FP Edwards, Department ofHistopathology, North Manchester General Hospital,Crumpsall, Manchester M8 6RB, England.



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formaldehyde xylene a approach to systemssolved. Toachieve this, andto save space in the lab-oratory, the processors have been housed in cup-boards under the benches (Fig. 6). These