The Relationship of Fabric Properties and Bacterial Filtration Efficiency for Selected Surgical Face Masks

Abstract

Surgical face masks are an important component of surgical apparel. The masks are expected to perform as barriers and provide increased protection to the patients and health care workers. In this study, the Bacterial Filtration Efficiency (BFE) of six commercially available surgical face masks was determined for two microorganisms. Fabric characteristics (weight, thickness, pore size, and resistance to synthetic blood strike through) thought to influence the barrier effectiveness were measured and the relationship between these characteristics and BFE was examined. Two challenge microorganisms, Staphylococcus aureus and Escherichia coli were evaluated in this study. For five of the six masks evaluated, the BFE against the challenge microorganism S. aureus was higher than when the challenge microorganism was E. coli. The mask with the lowest mean pore size and lowest maximum pore size had the highest BFE for both microorganisms evaluated, indicating that a relationship exists between pore size and BFE.

Full text

Article Designation: Refereed JTATM
Volume 3, Issue 2,Fall 2003
1
Volume 3, Issue 2, Fall 2003
The Relationship of Fabric Properties and Bacterial Filtration Efficiency for
Selected Surgical Face Masks
Karen K. Leonas, Ph.D. and Cindy R. Jones
Dawson Hall, University of Georgia
Athens, Georgia 30602
ABSTRACT
Surgical face masks are an important component of surgical apparel. The masks are expected to
perform as barriers and provide increased protection to the patients and health care workers. In
this study, the Bacterial Filtration Efficiency (BFE) of six commercially available surgical face
masks was determined for two microorganisms. Fabric characteristics (weight, thickness, pore
size, and resistance to synthetic blood strike through) thought to influence the barrier
effectiveness were measured and the relationship between these characteristics and BFE was
examined. Two challenge microorganisms, Staphylococcus aureus and Escherichia coli were
evaluated in this study. For five of the six masks evaluated, the BFE against the challenge
microorganism S. aureus was higher than when the challenge microorganism was E. coli. The
mask with the lowest mean pore size and lowest maximum pore size had the highest BFE for both
microorganisms evaluated, indicating that a relationship exists between pore size and BFE.
Keywords: surgical face masks, bacterial filtration efficiency, S. aureus, E. coli
Introduction:
Bacterial and viral diseases are spread
through both airborne and blood borne
pathways in the operating theater. Surgical
apparel can minimize the transmission of
disease. The transfer of microorganisms can
be reduced because the protective surgical
apparel creates a physical barrier between
the infection source and the healthy
individual.[1] A medical device intended to
be worn by operating room personnel during
surgical procedures to protect both the
surgical patients and operating room
personnel from transfer of microorganisms,
body fluids and particulate material is
identified as “Surgical Apparel” in 21 CFR,
Part 878.4040. The OSHA Occupational
Exposure to Blood Borne Pathogens: Final
Rule (1991) mandates the principles of
universal precautions, mandates
performance levels, and allows employers to
specify what personal protective equipment
is required and when it must be used.[2, 3]
Surgical face masks are an important
component of surgical apparel. The masks
are expected to perform as barriers and
provide increased protection to the patients
and health care workers. Initially, the
primary purpose of the facemask was to
protect the patient from being contaminated
by bacteria or viral species exhaled or

Article Designation: Refereed JTATM
Volume 3, Issue 2,Fall 2003
2
expelled from the health care worker.
Normal activities such as sneezing,
coughing, shouting, crying, breathing and
speaking may release oral, dermal and
nasopharyngeal bacteria that may cause
post-operative infections.[4] A second
purpose of the mask, that has emerged in the
past decade, is the protection for the health
care worker from exposure to blood borne
pathogens. Skinner and Sutton have
reported studies that show how surgeons
commonly receive blood and/or fluid
splashes to the face during operating room
procedures.[5]
In the past decade, a number of publications
have addressed the development and role of
the surgical face mask in the operating
theater, and its effectiveness in reducing
post-operative infections.[5, 6, 7, 8, 9, 10]
Research has shown that there are numerous
other methods by which bacteria become
airborne and that the microorganisms shed
by the healthcare team are the most
significant contaminating agents, even in
correctly designed operating rooms.[5]
Studies have also shown that the fit of the
mask, the proper positioning and use of the
mask, movement by the wearer, the length
of facial hair and voice level when speaking,
all have a direct bearing on its filtering
efficiency.[11, 12, 13]
Although the effectiveness of the face mask
for reducing surgical site infections has been
controversial, a number of major
organizations have published guidelines for
health care workers to minimize risks of
exposure which include face masks. They
include the Centers for Disease Control
[CDC], Association of Operating Room
Nurses [AORN], Occupational Safety and
Health Administration [OSHA] and the
Operating Room Nurses Association of
Canada [ORNAC]. AORN recommends
that “all persons entering restricted areas of
the surgical suite should wear mask when
open sterile items and equipment are
present” and that masks be worn along with
protective eyewear whenever exposures to
mucous membranes is reasonably
anticipated.[14] The Operating Room
Nurses Association of Canada (ORNAC)
agree with these recommendations.[15]
The CDC guidelines admit that the role of
face masks in reducing the risk of surgical
site infections may be more uncertain than
previously thought. And yet, the same
guidelines support the use of surgical face
masks as personal protective equipment.
In this regard the study of the transmission
of small particles and liquid aerosols
through nonwoven products used in
protective apparel and other filter media is
of importance. This area of study, with
reference to surgical face masks, is of
interest as masks are now expected to act as
protective barriers. In the summer of 2001,
several new ASTM standards specifically
relating to face masks and their evaluation
(ASTM-F2101-01; ASTM-F2100-01) were
approved.[16] In a draft document,
published in 1998, the FDA listed 5 major
categories of tests that are available for
determining the barrier performance and
safe use of a surgical mask. They were 1)
fluid resistance, 2) filtration efficiency, 3)
air exchange pressure (Delta P), 4)
flammability and 5) biocompatibility
testing.[17]
In 1999 Davis reviewed the test methods
used for the evaluation of face masks
effectiveness [18]. Bacterial Filtration
Efficiency (BFE), both in vivo and in vitro,
is a widely accepted method of evaluating
face masks. In these tests, the bacteria
penetrating the face masks are collected,
cultured and counted to determine the
number of Colony Forming Units (CFU’S)
that penetrate the mask. The in vitro test
uses positive and negative controls to
determine the initial number of bacteria.
The challenge bacteria are contained in a
mist, which is produced by aerosolizing the
bacteria with 0.1% peptone water in a
nebulizer. The masks are placed directly

Article Designation: Refereed JTATM
Volume 3, Issue 2,Fall 2003
3
over the opening of an Anderson sampler.
The aerosol consists largely of droplets that
simulate expulsion from the wearer. The
current BFE tests are used with the
microorganism S. aureus. However there
are a number of microorganisms in addition
to S. aureus that are known to cause
nosocomial infections and other serious
health problems. Nosocomial infections,
which are defined as those infections
originating in the hospital or healthcare
center, occur in about 5% of all patients
admitted to the hospital, with 41% being
urinary tract infections, 18% surgical, and
16% respiratory.[19] Postoperative wound
infections occur in up to seven percent of
surgical patients and require patients to
remain in the hospital an average of 7.3
extra days at an additional average cost of
$3,152.[20, 21] Although a variety of
pathogens are encountered in the hospital
environment, a relatively limited number
cause the majority of hospital infections
including Escherichia coli, Pseudomonas
aeruginosa, Enterococcus faecalis, Candida
albicans, and Staphylococcus aureus.[20]
Microorganisms have varying characteristics
that can influence their potential ability to
penetrate the facemask material including
shape, size, and their surface characteristics.
A wide variety of studies have evaluated the
BFE of face masks, however there have
been a limited number of microorganisms
evaluated in these studies.[4,22,23]
Willeke, et. al reported that rod-shaped
bacteria penetrate less than spherically
shaped bacteria of similar size.[22] In
addition, few studies have evaluated the
BFE of the face masks with specifically
engineered fabric characteristics
In this study, the BFE of six commercially
available surgical face masks was
determined for two microorganisms, S.
aureus and E. coli. Fabric characteristics
that influence the barrier effectiveness were
measured and evaluated. Although the fit
of the mask and leaks between the face and
the mask interface are known to be
important performance considerations, they
have not been addressed in this study.
Materials and Methods:
In this study, two components of the FDA
recommended areas were evaluated, 1)
liquid resistance and 2) filtration efficiency.
Six commercial face masks, each from a
different manufacturer, were selected for
evaluation (Table 1). Three of the face
masks (#1-3) were three ply with a pleated
construction, and three (#4-6) were molded
face masks.
Properties that characterize the fabric, such
as thickness, weight, and pore size, were
measured in addition to the liquid resistance
and bacterial filtration efficiency. These
characteristics were determined in
accordance with standard testing procedures
(Table 2). Liquid barrier properties were
measured according to ASTM F-1862-98:
Standard Test Method for Resistance of
Medical Face Masks to Penetration by
Synthetic Blood. This test method is
designed to evaluate penetration of the
masks by synthetic blood under high
velocity. In this project varying degrees of
velocity were examined to determine the
influence of pressure on the level and
mechanism of transmission. Velocity spray
pressures of 80 mmHg, 120 mmHg, and 160
mmHg were selected.
The Bacterial Filtration Efficiency for each
mask was determined in accordance with
ASTM Test Method F2101-01, Evaluating
the Bacterial Filtration Efficiency (BFE) of
Medical Face Mask Materials, Using a
Biological Aerosol of Staphylococcus
aureus. Two bacteria were selected for
evaluation in this study, S. aureus and E.
coli.

Article Designation: Refereed JTATM
Volume 3, Issue 2,Fall 2003
4
Table 1: Face masks Descriptions
Mask Name Description
1 Tie-on Surgical Face Mask
3-ply, pleated rayon outer web with polypropylene inner
web
2 Classical Surgical Mask, Blue 3-ply, pleated cellulose polypropylene, polyester
3 Sofloop Extra Protection Mask
3-ply, pleated blended cellulosic fibers with polypropylene
and polyester, ethylene methyl acrylate strip
4 Aseptex Fluid Resistant Molded rayon and polypropylene blend with acrylic binder
5 Surgine II Cone Mask Molded polypropylene and polyester with cellulose fibers
6 Surgical Grade Cone Style Mask Molded polypropylene
Table 2. Test Methods and Procedures Used to Determine Facemask Properties
Description Method Number Title
Thickness ASTM D1777-96 Standard Test Method for Thickness of Textile Materials
Weight ASTM D3776 -96 Weight Per Unit Area
Pore Size PMI Automated Perm Porometer Operation Manual, Version 6.
Synthetic Blood
Resistance
ASTM F1862-00a
Standard Test Method for Resistance of Medical Face Masks to
Penetration by Synthetic Blood
Bacterial Filtration
Efficiency
ASTM F2101-01
Evaluating the Bacterial Filtration Efficiency (BFE) of Medical Face
Mask Materials, Using a Biological Aerosol of Staphylococcus
aureus.
S. aureus is a gram positive cocci that is
irregular in shape and often in grape like
clusters. Various diseases and ailments
including impetigo, toxic shock syndrome,
food poisoning and pneumonia are attributed
to S. aureus. An average coccus is about 0.5
- 1.0
µm in diameter. E. coli is a gram
negative, rod shaped bacteria and averages
1.1 to 1.5
µm in width by 2.0 to 6.0 µm in
length. E. coli is a leading cause of urinary
tract infections.
The percent BFE was determined as
described in the test method for S. aureus,
and modified for E. coli. The S. aureus was
obtained from American Type Culture
Collection #6538 and E. coli was obtained
from UGA Microbiology Department.
Tryptic Soy Agar was the media used and
Peptone water (Difco Dehydrated 500
grams-Lot #1361000) was used as the
diluting agent as needed for the test method.
Positive and negative controls were

Article Designation: Refereed JTATM
Volume 3, Issue 2,Fall 2003
5
completed for each replication as directed in
the test method. Using the positive control,
it was determined that a challenge delivery
rate of 2200 +/- 500 viable particles per test
was required. This was achieved by diluting
the bacterial stock solution to the
appropriate bacterial concentration. The rate
was determined by the results of the positive
control plates when the aerosol is collected
in the six-stage viable particle cascade
impactor, with no test specimen clamped
into the test system. The exposed plates
were placed in an incubator at 37
o
C for 24
hours. The CFU’s for each plate were
counted using the Protocol Bacteria Colony
Counter, Synopitcs Corporation, V 2.05.
The filtration efficiency percentages were
calculated using the equation provided in the
test method:
100 (C-T) / C = %BFE
where C = average plate count total for test
controls and T = plate count total for test
sample.
Results and Discussion
The fabric characterization results for the
three face masks are presented in Table 3.
The pleated masks had lower pore size
means than the molded masks. Mask #3 had
the lowest mean pore size, 16.9
µm, followed
by Mask #2 with a mean pore size of
19.29
µm, and Mask #1 had the highest pore
size of the pleated masks at 23.97
µm. The
mean pore size of the molded masks were
significantly higher ranging from 31.72
µm
(Mask #6) to 51.0
µm (Mask #5). Although
thickness was not significantly different for
the masks, the basis weight ranged from
58.567 gm/m
2
(Mask #2) to 164.405 gm/m
2
for Mask #6. The molded masks (#4, 5 & 6)
were significantly higher in weight than the
pleated masks (#1, 2 & 3).
The percent Bacterial Filtration Efficiency
for each mask and bacteria are presented in
Table 4. For 5 of the 6 masks (not Mask
#4), the BFE values were higher when tested
with E. coli than for S. aureus. This was
expected as the size and shape of the
microorganisms differ and E. coli is larger
and rod shaped when compared with S.
aureus. S. aureus ranges in size from 0.5 to
1.0 microns and is round in shape. E. coli is
rod shaped and averages 1.1 to 1.5
µm in
width by 2.0 to 6.0
µm in length.
Mask #3 had the highest %BFE for S.
aureus and the second highest %BFE for E.
coli and the lowest mean pore size of the
face masks examined here. This indicates a
relationship between pore size and BFE and
further testing should be completed to
investigate this relationship.
Mask #3, also had the lowest maximum pore
size of 27.19
µm. This is a critical
parameter to measure as this indicates the
largest pore detected in the sample and
therefore particles may be transmitted
through this opening, hence reducing the
BFE. When considering the mean pore size
and the maximum pore size for face Masks
#1 and #2, their order from highest to lowest
is reversed for these two parameters.
Although Mask #2 had a lower mean pore
size than Mask #1, the maximum pore size
was greater than that of Mask #1. This may
help explain why the BFE for the masks is
not in the same order as the mean pore size.
Mask #2 had a slightly lower BFE for E. coli
(98.53%) and S. aureus (88.18%) than did

Article Designation: Refereed JTATM
Volume 3, Issue 2,Fall 2003
6
Table 3: Face Mask Material Characteristics- thickness, weight, pore size, Resistance to
Blood
Pore Size
µm
Synthetic Blood Resistance
(% Passed)
Mask
Thickness
mm
Weight
gm/m
2
Mean
Max. 80 mm
Hg
120 mm
Hg
160 mm
Hg
1 0.3345 66.908 23.97 41.74 70 0 0
2 0.2339 58.657 19.29 43.27 100 100 50
3 0.4417 95.775 16.90 27.19 100 100 100
4 0.6137 140.828 35.06 87.74 0 0 0
5 0.3607 145.760 51.00 146.60 0 0 0
6 0.4742 164.405 31.72 92.12 0 0 0
Table 4. Face Mask Bacterial Filtration Efficiency - Mean and (Standard Deviation)
Mask S. aureus - % BFE E. coli - % BFE
1 91.09
(0.08)
98.53
(0.01)
2 88.18
(0.04)
97.26
(0.01)
3 92.19
(0.03)
99.34
(0.01)
4 90.72
(0.03)
99.10
(0.01)
5 84.82
(0.01)
95.74
(0.03)
6 86.4
(0.05)
99.73
(0.00)
Mask #1 (E. coli, 97.26%; S. aureus
88.18%). The % BFE for Mask #4 for S.
aureus was higher than for Mask #2, which
was unexpected since the mean and
maximum pore size for Mask #2 was lower.
CONCLUSIONS
The BFE of six surgical face masks has been
measured by challenges from two
microorganisms, S. aureus and E. coli.
Although there were no significant
differences between the face masks, the

Article Designation: Refereed JTATM
Volume 3, Issue 2,Fall 2003
7
bacterium did have a significant influence
on the facemask performance. The BFE for
5 of the 6 masks exposed to E. coli was
higher than when exposed to S. aureus. This
was likely due to the size and shape of the
bacteria. S. aureus is round and ranges in
size from 0.5mm to 0.1 mm. E. coli is rod
shaped and is larger, with size ranging from
1.1 to 1.5
µm in width and from 2.0 to 6.0
µm in length. Continuing studies with
different microorganisms and face masks
with varied characteristics will provide
additional information on those factors that
influence facemask barrier performance. In
addition, the relationship between the mean
pore size, the maximum pore size and the
pore size distribution with BFE performance
should also be examined.
ACKNOWLEDGMENTS
This research was funded in part through a
grant from the Georgia Agricultural
Experiment Station, regional project S-1002.
REFERENCES
1. Goldmann, D.A., (1991). Journal of
Hospital Infection, 18 (Supplement A), pp.
515-523.
2. Occupational Safety & Health
Administration. (1991, December 6).
Federal Register
, 56, 64004-64182 (29 CFR
Part 1910.1030), 1991, December 6.
3. Centers for Disease Control. (1987).
Morbidity & Mortality Weekly Report
, Vol.
36, No. 19, pp. 285-289.
4. Chen, S. Vesley, D. Brossuea, L.M., and
Vincent, J. H. (1994) American Journal of
Infection Control, Vol 22, No. 2, pp. 65 -
74.
5. Skinner M.W. and Sutton, B.A.. (2001)
Anesthesia and Intensive Care
, Vol 29, No.
4, pp. 331-338
6 Belkin, N.L., (1997). Infection control
and Hospital Epidemlogy, Vol. 18, Iss. 1,
pp. 49-57
7. Belkin, N.L., (1997). The Guthrie
Journal, Vol. 66, No. 1, pp. 32-34
8. Romney, M.G., (2001). Journal of
Hospital Infection, Vol. 47, No. 4, pp. 251-
256.
9. Tunevall, T.G. (1991). World Journal of
Surgery. Vol 15, pp. 383 - 388.
10. Lipp A. and Edwards, P., (2002). The
Cochrane Library, Volume (Issue) 4, 2002.
11. Koch, F., (1996). Infection Control and
Sterilization Technology, 2, pp 15-17.
12. McLure, H.A., Mannam, M., Talboys,
C.A., Azadian, B.S. and Yentis, M.S.,
(2000). Anesthesia
, Vol. 55, No. 2, pp. 173-
176
13. McLure, H.A. , Talboys, C.A., Yentis,
S.M., and Azadian, B.S., (1998).
Anesthesia,
Vol 53, No. 7, pp. 624-626
14. Association of Operating Room Nurses
(AORN) Recommended Practices
Committee, (1998). AORN Journal,
Vol. 68,
No. 6, pp.1048-1052.
15. Operating Room Nurses Association of
Canada (ORNAC). Recommended
standards for perioperative nursing practice.
4
th
ed. 1998.
16. Annual Book of ASTM Standards,
2002, American Society for Testing and
Materials, West Conshohocken, PA, 2002.
17. “Guidance on the Content and Format of
Premarket notification [510(k)] Submission
Surgical Mask, Draft. Infection control
Devices Branch Division of Dental,
Infection Control and General Hospital
Devices, Office of Device Evaluation Center
for Devices and Radiological Health, Food
and Drug Administration, January 16, 1998.
18. Davis, W. T. (1991) American Journal
of Infection Control, Vol 19, No. 1, pp 16-
18.
19. Beck W.C., (1952) American Journal of
Surgery. Vol 83, No. 2, pp 125 - 126.
20. Belkin, N.L. (1999) The Guthrie Journal
,
Vol. 68, No. 1, pp 16-21.
21. Brock.T, Madigan, M.T., Martinko, JM,
Parker, J. (1994) Biology of
Microorganisms, Prentice Hall, New Jersey.
22. Wileke, K. Qian, Y., Donnelly, J.,
Grinshpun, S., Ulevicius, V. (1996),
American Industrial Hygiene Association
Journal, Vol. 57, No. 4, pp 348 - 355.

Article Designation: Refereed JTATM
Volume 3, Issue 2,Fall 2003
8
23. Qian, Y, Wileke, K., Grinshpun, S.A.,
Donnelly, J., and Coffee, C.C. (1998)
American Industrial Hygiene Association
Journal, Vol. 59, No. 2, pp. 128-132.