APPLIED MICROBIOLOGY, Sept. 1970, p. 474-479Copyright © 1970 American Society for Microbiology

Vol. 20, No. 3Printed in U.S.A.

Evaluation of the Edgegard Laminar Flow HoodLEWIS L. CORIELL AND GERARD J. McGARRITY

Institute for Medical Research, Camden, New Jersey 08103

Received for publication 11 February 1970

In a horizontal back-to-front flow high-efficiency particulate air-filtered laminarhood, it is shown that a Blake bottle obstruction to the air flow causes a downstreamcone of turbulent air which can draw microbial contamination into the work area ofthe hood. In controlled experiments, contamination with T3 coliphage was re-

duced by a series of perforations around the open edge of the hood which eliminatesthe cone of turbulent air. The average reduction in phage counts was 90.75, 86.79,91.12, and 98.92%, depending upon the site of nebulization. The phage counts were

reduced in 48 of the 51 tests.

Many medical applications have been found forfiltered laminar airflow first developed by Whit-field (7). Laminar airflow and modifications of ithave been shown to reduce airborne microbialcounts in medical research laboratories (3)1 surgi-cal amphitheaters (4), intensive care units (1), andanimal colonies (5); a laminar flow hood forhighly infectious materials gives better than 99.9%containment (2). In these studies, performancehas been documented by low and high volumebacterial air sampling, persistence of concen-trated artificially generated aerosols, and fre-quency of contamination in daily use, i.e., num-ber of contaminations or infections in a laminarflow facility compared to the number in the samearea with conventional ventilation.The best criterion of efficacy of laminar flow

installations is the absence of contaminationduring routine use. This assumes proper tech-niques are used, because neglect of maintenance,misuse, or both, can reduce the efficiency of anysystem. Holes or breaks in the high-efficiency par-ticulate air (HEPA) filter or air leaks in the sys-tem may allow dirty air to enter. Poor executionof aseptic procedures can negate the effect oflaminar flow benches. For example, any objectplaced in the airstream of laminar airflow willcreate downstream turbulence. The larger theobject, the larger the area of turbulence. If a suf-ficiently large object is located near the front of ahorizontal laminar flow cabinet, contaminantsfrom the room or the operator can be drawn intothe work area behind the obstruction. In a recentreview (8), Whitfield and Lindell stated that thearea of turbulence extends downstream approxi-mately three times the width of the obstruction.This holds true when the airstream travels aroundboth sides of the obstruction. If the airstream

travels around only one side of an obstruction,downstream turbulence will extend approximatelysix times the width of the obstruction.The purpose of this report is to record the

effect of the turbulent cone of air created by anobstruction in a laminar flow airstream on aero-sols of T3 bacteriophage, delivered just outsidethe hood across the tip of the turbulent cone, andto study the effect on this of a series of perfora-tions around the front edge of the hood throughwhich air is drawn by the fan that drives the airthrough the filter. It has been claimed that theseperforations reduce turbulence of the airstreamafter it passes over an object (6).

MATERIALS AND METHODSLaminar flow hood. A horizontal laminar flow hood

(Edgegard Hood, Baker Co., Inc., Biddeford, Me.)was used in these studies. The working area of thishood was 3 ft wide, 2 ft deep, and 22 inches high(91.44 by 60.96 by 55.88 cm). Air passed from backto front through a HEPA filter which removed99.97% of dioctyl phthalate particles with a diameterof 0.3 ,m. Air left the filter at an average velocity of100 ft (30.48 m)/min and traveled horizontally throughthe hood with minimum turbulence, or "laminar" airmovement. Air exited at the front of the hood, towardthe operator. In the hood under study, some of theexit air was drawn in through perforations locatedacross the bottom and two sides of the hood. Theseperforations were 0.25 inch wide by 1 inch long (0.635by 2.54 cm) and numbered 16 per linear ft. Additionalmake-up air was drawn in through a grill beneath thehood in front of the knees of the seated operator, pre-filtered through a coarse filter, and mixed with the airdrawn through the perforations before it passedthrough the HEPA filter. During the present studies,air in the center of the working area of the hoodmoved at 85 a 5 ft/min (25.9 i 1.5 m/min) withthe perforations open. With the perforations coveredwith masking tape, air speeds were 80 + 5 (24.38 +

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EDGEGARD LAMINAR FLOW HOOD

1.5 m/min). Observations on air turbulence and con-tamination were made first with the perforations openand then repeated with the perforations covered withmasking tape. All other conditions were kept identical.

Measurement of airflow. The direction and turbu-lence of airstreams inside the hood were determinedby observing smoke. Smoke was generated by exposingswabs dipped in titanium tetrachloride to air. Airvelocities were measured with a hot wire thermoane-mometer (Alnor Corp., Chicago, Ill.).

Test procedure. To evaluate the effectiveness of theperforations around the open edges of the hood, T3bacteriophage was nebulized in a DeVilbiss no. 40nebulizer at various sites near the open edge of thehood, and air samples were collected in all glass im-pingers (AGI-4), which sampled 0.43 cubic ft (12.5liters) of air per min (9). Phage was assayed by plaquecounts as previously reported (2).A Blake bottle measuring 12 by 4 by 2.5 inches

(30.48 by 10.16 by 6.35 cm) was placed in the air-stream (Fig. 1). Two locations of the nebulizer andthe location of the impinger are illustrated. Experi-ments were performed with the nebulizer 6, 9, and12 inches (15.24, 22.86, and 30.48 cm) above the worksurface at location 1. Each experiment consisted ofnebulizing the phage for 1 min at a prescribed heightwith the Blake bottle in place and with the perfora-tions open. The impinger was replaced with a fresh

one in precisely the same location, all perforationswere covered with tape, and the nebulization and col-lection were repeated. Generally, two experimentswere performed per day. For experiments releasingphage outside the hood, the nebulizer was positionedat location 2, 1 inch outside the hood, 5 inches (12.7cm) to the right of, and 6 inches (15.24 cm) abovethe impinger. The Blake bottle was 8 inches (20.32cm) inside the hood. In control studies without theBlake bottle, no phage particles were detected, regard-less of whether the perforations were open or coveredwith masking tape. These controls were included foreach experiment recorded below.

Studies with a thermoanemometer indicated thatcovering the perforations with masking tape did re-duce the average air speed slightly, from 85 ft/min(25.9 m/min) to 80 ft/min (24.38 m/min) in thecenter of the hood. Control studies with no obstruc-tion showed that the slightly lower velocity with theperforations covered did not account for the aspira-tion of phage particles into the sampler.

RESULTSAirflow patterns can be illustrated by the path

of smoke from a swab with the perforations openand taped (Fig. 2). On the left of Fig. 2, the per-forations are open and smoke from a swab

FIG. 1. Diagram showing location ofnebulizer, impinger, and Blake bottle in Tables 1, 2, 3, and 4. The cone ofturbulent air downstream from the Blake bottle and paths of nebulized phage are shaded. The location of the im-pinger within this zone is shown by a black circle.

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CORIELL AND McGARRITY

FIG. 2. Smoke patter/Is in horizontal flow laminar flow hood. No turbulence is evident when the perforationsare open with no obstruction (left) or with a tissue cultureflask (center). Turbulence does occur in the lee ofatissue culture flask when the perforations are taped (right).

streams directly out of the hood in a laminarpattern. Placing a tissue culture flask obstructionupstream of the swab has no effect on the direc-tion of the smoke when the perforations are openas shown in the center; however, when the perfor-ations are covered with masking tape, the smokecan be observed flowing into the hood toward theobstruction as shown on the right.To determine if this control of turbulence be-

hind an obstruction by the perforations wouldproduce more efficient protection against micro-biological contamination, T3 coliphage was re-leased at various locations near the open edge ofthe hood with samples obtained inside the hoodat a location where routine laboratory work wouldbe performed. The sites of phage release andsampling outlined above were selected to simulatecontamination that might be present at the frontof the hood from room air or from the clothing,arms, and hands of the operator. The number ofphage particles released was many logs greaterthan expected normal contamination to permitquantitative recoveries and reproducibility. Theresults of nebulizing phage at heights of 6, 9, and

12 inches (15.24, 22.86, and 30.48 cm) above thework surface are presented in Tables 1, 2, and 3,respectively. These tests were repeated 34 times:13 times with the nebulizer positioned 6 inches(15.24 cm) above the work surface, 14 times at9 inches (22.86 cm), and 7 times at 12 inches(30.48 cm) above the work surface. The resultswith the nebulizer setting at 6 and 9 inches weresimilar, i.e., the number of phage particles recov-ered in the impinger was markedly reduced in the27 tests when the perforations were open. In 16(59%) of these tests, the reduction was greaterthan 99%, and in 23 tests (85%), the reductionwas greater than 90%. The average reduction was98.92% with the nebulizer 6 inches above thework surface (Table 1); with the nebulizer 9inches above the work surface, the average reduc-tion was 91.12% (Table 2). When the nebulizerwas placed 12 inches above the work surface, areduction of phage particles recovered in the im-pinger was observed in six out of seven experi-me,nts, and the average reduction in the six testswas 86.79% (Table 3). At the latter nebulizersetting (12 inches above the work surface), the

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EDGEGARD LAMINAR FLOW HOOD

TABLE 1. No. of bacteriophage particles (PFU)recovered per cubic foot of air sampled in the

lee of a Blake bottle obstruction in anEdgegard Hood with the perforations

open or closed with tapea

No. of PFU/ft7 with perforationb

Taped

8,15011,98515,65069,500100,000100,000100,00072,150

245,000102,50050,000

613, 500465,000

Open

<50<50<50<5050

<50<50950450<50<50

54,3509,400

cloud of phage particles is in approximately thegeographic center of the open face of the hood,largely above the turbulent air behind the Blakebottle; therefore, the perforations have a de-creased but still perceptible effect.

TABLE 3. No. of bacteriophage particles (PFU)Reductionc recovered per cubic foot of air sampled in the

lee of a Blake bottle obstructioni in anEdgegard Hood with the perforations

nf%lirl open or closed with tapea99.3999.5899.6899.9399.9599.9599.9598.6899.8299.9599.9091.2098.00

a Nebulizer 6 inches (15.24 cm) above worksurface (position 1 on Fig. 1).

I In experiments 1 and 2, the impingers werelocated 4 inches inside the hood. All other studieswere done with impingers 6.5 inches inside thehood.

c Reduction calculated: 100 - (open/taped).Average reduction was 98.92%.

TABLE 2. No. of bacteriophage particles (PFU)recovered per cubic foot of air sampled in the

lee of a Blake bottle obstruction in anEdgegard Hood with the perforations

open or closed with tapea

No. of PFU/ft3 with perforations

Taped Open

581,500446,500

7,3156,3154,8753,365

8651,7654,5353,2156,1007,8255,9001,750

3,815<50<50<50150115<50250100

1,2851,465<50365400

Reductionb

99.3499.9999.3299.2196.9296.5994.2285.9097.8060.0375.9899.3693.8177.14

No. of PFU/ft7 with perforationsExpt Reductionb

Taped Open

28 1,900 <50 97.3629 16,850 3,585 78.7930 35,650 2,900 91.8731 4,500 6,700 (+150)c32 2,350 650 72.3533 37,150 3,665 90.1434 66,750 6,515 90.24

a Nebulizer 12 inches (30.48 cm) above worksurface (position 1 on Fig. 1).

b Reduction calculated: 100 - (open/taped).Average reduction was 86.79%.

- Not included in average.

TABLE 4. No. of bacteriophage particles (PFU)recovered per cubic foot of air sampled in the

lee of a Blake bottle obstruction in aniEdgegard Hood with the perforations

open or closed with tapea

Expt

3536373839404142434445464748495051

No. of PFU/ft- with perforations

Taped

24,000<50

1,9853,6854,5352,7001,135

11,3503,1653,95013,00018,35018,85015,500

315250<50

Open

<50<50<50485

1,187<50<50365<50

1,4751,415

50<50<50<50<50<50

a Nebulizer outside the hood, 5 inches (12.7cm) to the right and 6 inches (15.24 cm) abovethe impinger (position 2 on Fig. 1).

b Reduction calculated: 100 - (open/taped).Average reduction was 90.75%.

Expt

12345678910111213

Expt

1415161718192021222324252627

Reduction6

99.79097.4586.8473.8398.1595.6096.7998.4062.7089.1099.7399.7399.0084.1380.000

a Nebulizer 9 inches (22.86 cm) above the worksurface (position 1 on Fig. 1).

b Reduction calculated: 100 - (open/taped).Average reduction was 91.12%.

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CORIELL AND McGARRITY

Experiments presented in Table 4 simulate thegeneration of contaminants in the room air, orfrom the clothing of the operator, because thenebulizer was placed 1 inch (2.54 cm) outside thehood and 6 inches (15.24 cm) above the work sur-face. With the perforations taped and the im-pinger placed behind an obstruction, phage par-ticles were recovered in the impinger behind anobstruction in 15 out of 17 experiments, and withthe perforations open the number of phage par-ticles recovered was reduced by 62.70 to 99.79%,with an average reduction of 90.75%.When all conditions were kept the same as re-

corded in Table 4 except for moving the nebulizeran additional 5 inches to the right, i.e., 10 inchesto the right of the sampler, no phage could bedetected in any test with the perforations open.With the perforations closed, phage particles weredetected in each test as follows: 135, 650, 1,285,250, and 50 particles/ft3 of air.

DISCUSSIONIt is easy to observe the airflow patterns in a

laminar flow hood by following the distributionof smoke released at various points in the hood.It is also easy to see the cone of turbulent air onthe downwind side of an obstruction in the air-stream and to observe that smoke moves in a cir-cular pattern within this turbulent area which ex-tends downwind three to six times the width ofthe obstruction (8). By means of an automaticdust particle counter (Dynac Corp., Portland,Me.), it can readily be shown that this circulareddy of turbulent air aspirates dust particles intothe hood if an obstruction is placed so that thecone of turbulence extends outside the front ofthe hood. These observations made with smokeand dust particles are significant, but they do notshow whether viable organisms such as bacteriaor viruses could also be aspirated into the hoodbehind an obstruction. In continuous use, the airin the hood and in the room is almost free of vi-able organisms, so it is very difficult to get a sta-tistically reliable test for the efficacy of the perfo-rations. However, in routine use, small numbersof organisms are constantly shed by the operatorimmediately outside the hood and by his manipu-lations inside the hood. It is, therefore, desirableto know whether the perforations confer someprotection against accidental infection in a hori-zontal laminar flow hood and whether this canbe demonstrated with viable organisms in a con-trolled experiment which is reproducible, albeitan exaggeration of what might occur in routineuse of the hood.

Other attempts have been made to control cre-ation of turbulence in the lee of obstructions in

horizontal laminar flow cabinets. Converginghorizontal flow has been recommended to reduceturbulence and provide eddy-free purging behindobstructions. We have not evaluated convergingflow units and, to the best of our knowledge, theirmicrobiological significance has not been quanti-tatively determined.T3 coliphage was used in these experiments be-

cause it is relatively stable in an aerosol, can bequantitatively recovered from the air, and is non-pathogenic for personnel. In each of the experi-ments reported here, the tests were carried outwith the hood in operation and the filtered airmoving from back to front of the hood. A tech-nician was seated in front of the hood in all tests.The nebulizer was placed so the cloud of aero-solized phage was carried across the tip of thecone of turbulent air outside the hood. The ex-periments were designed to determine whetherthe phage particles were drawn back into thehood and, if so, whether this could be reducedby the perforations. The phage studies show thatthe number of phage particles recovered was re-duced in 48 of 51 tests when the perforationswere open. In one test, the number was increased(test 31), and in two tests no difference was de-tected (tests 36 and 51). The average reductionshown in Tables 1 to 4 ranged from 84.10 to98.92%, depending on the conditions of the test.It should be noted that with the perforationsopen, the least contamination was observedwhen the phage was nebulized 6 inches above thework surface. The usual work area of a hood is6.5 inches inside the hood and 4 to 8 inches abovethe surface. The present studies provide evidencethat the perforations are desirable under the con-ditions of the tests. Obstructions of other sizes,shapes, and locations were not tested, but theconsistency of the results obtained suggests to usthat the perforations are a good addition to anyhorizontal flow microbiological hood. If a hoodwithout perforations is used, it is important toreduce obstructions to a minimum and to haveno obstructions between the surface of the filterand the site of aseptic procedures.

ACKNOWLEDGMENTS

This work was supported by grants from the John A. HartfordFoundation, by Public Health Service grants CA 04953-10 fromthe National Cancer Institute and FR 5582 from the GeneralResearch Support Branch, and by State of New Jersey grant-in-aid L-35.

The authors thank Judi Sarama and Arlene Valloreo for tech-nical assistance.

LITERATURE CITED

1. Bodey, G. P., E J. Freireich, and E. Frei. 1969. Studies ofpatients in a laminar flow unit. Cancer 24:972-980.

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2. Coriell, L. L., and G. J. McGarrity. 1968. Biohazard hood to

prevent infection during microbiological procedures. AppI.Microbiol. 16:1895-1900.

3. Coriell, L. L., G. J. McGarrity, and J. Horneff. 1967. Medicalapplications of dust-free rooms. I. Elimination of airbornebacteria in a research laboratory. Amer. J. Pub. Health Nat.Health 57:1824-1836.

4. Coriell, L. L., W. S. Blakemore, and G. J. McGarrity. 1968.Medical applications of dust-free rooms. IT. Elimination ofairborne bacteria from an operating theatre. J. Amer. Med.Ass. 203:1038-1046.

5. McGarrity, G. J., L. L. Coriell, R. W. Schaedler, R. J. Mandle,and A. E. Greene. 1969. Medical applications of dust-free

rooms. III. Use in an animal care laboratory. Appl' Micro-biol. 18:142-146.

6. Useller, J. W. 1969. Clean room technology. NASA SP-5074.National Aeronautics and Space Administration, Office ofTechnology Utilization, Washington, D.C.

7. Whitfield, W. J. 1962. A new approach to clean room design,p. 529. Annu. Proc. Inst. Environmental Sci.

8. Whitfield, W. J., and K. F. Lindell. 1969. Designing for thelaminar flow environment. Contamination Control 8:10-21.

9. Wolf, H. W., P. Skaliy, L. B. Hall, M. M. Harris, H. M.Decker, L. M. Buchanan, and C. M. Dahlgren. 1964.Sampling microbiological aerosols. Public Health Mono-graph no. 60. Government Priniting Office, Washington,D.C.

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