METHOD FOR EVALUATING EFFECTIVENESS OF SURGICAL MASKS - ASM
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METHOD FOR EVALUATING EFFECTIVENESS OF SURGICAL MASKS
V. W. GREENE AND D. VESLEY
School of Public Health and University Health Service, University of Minnesota,
Minneapolis, Minnesota
Received for publication October 7, 1961
ABSTRACT sample and should control contamination from
sources, such as hair, clothing, etc., that are
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GREENE, V. W. (University of Minnesota,
Minneapolis) AND D. VESLEY. Method for uncontrollable by masks. Hirshfield and Laube
evaluating effectiveness of surgical masks. J. (1941) developed an experimental chamber which
Bacteriol. 83:663-667. 1962.-A portable isola- was a first step toward accomplishing controlled
tion box, provided with a filtered air supply and environmental and quantitative sampling.
a means of access for a test subject's head, was Guyton, Buchanan, and Lense (1956) refined the
attached to an Andersen Sampler and used to techniques for measuring absolute efficiencies
measure orally expelled bacterial contaminants of masking materials as bacterial filters, and also
before and after masking. This technique yielded studied the effectiveness of masks against in-
more detailed quantitative information than was spiring artificially disseminated spores. Their
obtained by either sedimentation plates or data, however, do not yield information relative
Andersen sampling in an unconfined space. to the practical employment of masks as
During talking, unmasked subjects expelled more protectors of the environment against normal
than 5,000 bacterial contaminants per 5 ft; nasopharyngeal expulsions of the wearer.
The recent development by Andersen (1958)
7.2% of the contaminants were associated with of a sampler designed to collect airborne particles
particles less than 4 ,u in diameter. Masked in several categories of decreasing particle size
subjects expelled an average of 19 contaminants/5 suggested the possibility of constructing a modifi-
ft3; 63% were less than 4 ,u in diameter. Mask cation of Hirshfield and Laube's chamber which
efficiencies varied according to particle size of the would: (i) estimate the total contribution of
contaminants. This technique is adaptable for orally expelled bacteriological contaminants in
routine evaluation of an individual's contribution known volumes of air; (ii) estimate the relative
to environmental contamination. proportion of these contaminants associated with
different particle sizes; (iii) and estimate the
Although a great deal of work has been done to relative efficacy of face masks against the organ-
evaluate the efficacy of face masks (Rockwood isms associated with different categories of
and O'Donoghue, 1960), relatively few attempts particle size.
have successfully measured the quantitative
bacterial contribution of nasopharyngeal expul- MATERIALS AND METHODS
sions to the atmospheric environment. Jennison
(1942) reviewed this subject, and attempted to Sampling chamber. The sampling chamber was
enumerate and characterize these expulsions by a plywood box (5 ft X 16 in. X 16 in.) mounted
means of high-speed photography. Most studies, vertically on an angle iron frame (Fig. 1). A high-
however, employed agar plates or glass slides efficiency (> 99%) fiberglass filter formed the
exposed at various distances in front of and below top surface of the box. A fixed metal port pro-
the source of droplets to catch contaminated jected from the tapered bottom of the box, and
particles which either settle or impinge upon served as a connection to the air sampler. A
them. This technique fails to measure the very sliding "guillotine-like" panel with a flexible
small droplets and droplet nuclei which are not plastic collar was provided to permit entry of the
projected any appreciable distance by virtue of subject's head and neck, at a point 4 ft from the
their own kinetic energy (Wells, 1955). Further- sampling port. A glass window was constructed
more, critical tests of mask effectiveness should on one side of the chamber for psychological
exclude normal airborne contaminants from the comfort. The only supply of air during a test was
663664 GREENE AND VESLEY [VOL. 83
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FIG. 1. The sampling chamber. (Left) Subject preparing to enter sampling chamber. (Right) Sampling
chamber in use.
filtered through the fiberglass, and the only 1958). The masks employed in this study were
source of contamination was the subject. When typical of those routinely used in the surgery
proper capping was observed and suitable entry suites of the University of Minnesota Hospitals,
precautions taken, a silent subject contributed and consisted of two layers of thin muslin con-
less than one contaminant/ft3. taining an inner lining of 4-oz outing flannel. All
Test procedure. The panel was closed and the tests were performed in an operating theater at
air exhausted from the chamber for 5 to 10 min 25 C and 51% relative humidity.
by means of the sampler pump, to remove any Supplementary trials. In an adjacent theater,
ambient contamination. Subsequently, the air an identical trial was performed, using the
sampler was attached and a "background" Andersen Sampler in an unconfined space. The
sample was taken. The subject then inserted sampler was located on an instrument tray 18 in.
his head into the chamber, lowered the panel from the subject, who enunciated the words
until the collar was snug around his neck, started "sing and chew" in the same time sequence as
the air sampler which was preset to sample 1 above and directed his speech toward the sampler.
ft3/min, and distinctly pronounced the words In a third theater under similar environmental
"sing and chew" at 10-sec intervals for 1 min. conditions, the subject enunciated the same word
Air sampling was then continued without further pattern, directing his speech through an 18-in.
disturbance for 4 min after talking terminated. trajectory toward a series of exposed blood agar
Samples were collected on blood agar [Trypti- petri dishes with 500 cm2 of surface. The dishes
case soy agar (Baltimore Biological Laboratory) were exposed during the initial 1 min of speech
plus 5% defibrinated human blood] with an and were left open for a subsequent 4 min of quiet
Andersen Sampler; the samples were incubated time.
at 37 C for 24 hr, and then at 20 C for an addi- The subjects participating in these trials were
tional 24 hr. Counts were calculated according taught to enunciate the speech pattern in such a
to the positive hole conversion table (Andersen, manner as to yield consistent and uniform air-1962] EVALUATION OF SURGICAL MASKS 665
TABLE 1. Airborne microorganisms expelled during from contamination. In any event, trials which
talking recovered* on sedimentation plates evaluate masks by means of sedimentation
Subject Masked Unmasked plates necessitate the employment of large
surface areas of test media to compensate for the
Subject 1 variations in speech idiosyncracies.
Average (4 trials) 7 4700 Sampling chamber with Andersen Samplers.
Range 5-11 3900-5700 Table 2 illustrates the usefulness of the sampling
Subject 2 chamber technique. Since the box was
Average (4 trials) 9 2500
Range 5-13 1900-3300 aerodynamically designed to collect both the
Subject 3 heavy droplets, which settle quickly by gravita-
Average (4 trials) 6 4200 tion, as well as the droplet nuclei, which would
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Range 4-11 3500-5400 normally remain suspended in the air, the total
Subject 4 numbers of contaminants recovered are higher
Average (4 trials) 14 1940 than those obtained on the sedimentation plates.
Range 4-23 950-3800 Each of the four subjects, in each of the four
*
Contaminated particles/500 cm2 after 5 min.
trials, expelled more than 5,000 contaminants/5
ft3 of air while enunciating the test sequence of
borne contamination. Preliminary trials served as words. There was also surprising uniformity of
practice sessions to develop uniformity. During
the actual tests upon which this report is based TABLE 2. Airborne microorganisms expelled during
all trials were carried out as uniformly as pos- talking recovered* from sampling chamber
sible by four subjects on each of four separate Total Small particlest
occasions. Masks, when employed, were freshly Subject
changed for each individual trial. Masked Un- Masked Unmasked
masked
RESULTS
Subj ect 1
Sedimentation plates. The numbers of bacteria Average (4 trials) 24 16 371
that are orally expelled during simple speech and Range 13-37 > 5000 6-26 213-555
deposited on 500 cm2 of surface are shown in Subject 2
Table 1. The total contamination ranged between Average (4 trials) 27 21 170
950 to 5,700 colonies from unmasked subjects Range 10-68 > 5000 3-64 42-347
and between 4 to 23 colonies from the same sub- Subject 3
jects wearing fresh masks. These results agree Average (4 trials) 10 4 390
with those reported in the literature, regarding Range 5-18 > 5000 3-5 20-786
both total counts and individual variability. Subject 4
Average (4 trials) 15 7 513
Jennison (1942) considered the contamination Range 8-32 > 5000 2-15 204-832
which results during speaking to be associated Average of all sub- 19 >5000 12 361
with larger droplets than those expelled during j ects and all trials
sneezing or coughing, with the average droplet
being larger than 100 Iu in diameter. However, * Contaminated particles/5 ft3.
he pointed out that the number of smaller t Contaminated particles (less than 4 IA in
droplets expelled, even during talking, is greater diameter) trapped on Andersen stages 4, 5, and 6.
than was previously expected, and that these
particles become airborne and difficult to sample TABLE 3. Apparent mask efficiencies according to
with sedimentation plates. Furthermore, the Andersen data
distribution of contamination on a given series
TotlprtilesParticles
Total particles 5000 361 99.6% 96.7%666 GREENE AND VESLEY [VOL. 83
TABLE 4. Effectiveness of sampling chamber for small number which are actually expelled through
enumerating airborne contaminants the mask.
expelled during talking
DISCUSSION
Andersen sampler results Andersen sampler results The basic problem of surgical mask efficacy
in sampling chamber in unconfined space
Particle size
(contaminants/5 fts) (contaminants/5 ft3) has been under study since the early years of the
century. In recent years the concern with post-
Subjects Subjects Subjects Subjects
masked unmasked masked unmasked operative and other hospital-acquired infections
has intensified interest in masking as part of an
over-all effort to define the role of various environ-
mental and other factors in the epidemiology of
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>8 3.3 >2500 10.4 385
4-8 4.0 >2500 14.3 407 infections. A great deal has been learned about1962] EVALUATION OF SURGICAL MASKS 667
ACKNOWLEDGMENTS tection of contagion masks. Appl. Microbiol.
4:141-143.
The authors wish to acknowledge the coopera- HIRSHFIELD, J. W., AND P. J. LAUBE. 1941. Surgical
tion of R. Klug, Minnesota Mining and Manufac- masks, an experimental study. Surgery 9:720-
turing Co., and of R. P. Singh and P. Pedersen 730.
of the University of Minnesota. JENNISON, M. W. 1942. Atomizing of mouth and
This investigation was supported in part by nose secretions into the air as revealed by
research grant 3019 from the National Institute high speed photography, p. 106-128. In
of Allergy and Infectious Diseases, U. S. Public Aerobiology, Publ. No. 17, American Asso-
Health Service. ciation for the Advancement of Science,
Washington, D. C.
ROCKWOOD, C. A., AND D. H. O'DONOGHUE. 1960.
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LITERATURE CITED
The surgical mask: its development, usage
ANDERSEN, A. A. 1958. New sampler for the collec- and efficiency. A. M. A. Arch. Surgery 80:963-
tion, sizing, and enumeration of viable air- 971.
borne particles. J. Bacteriol. 76:471-484. WELLS, W. F. 1955. Airborne contagion and air
GUYTON, H. G., L. M. BUCHANAN, AND F. T. hygiene. Harvard University Press, Cam-
LENSE. 1956. Evaluation of respiratory pro- bridge, Mass.You can also read
