





























EVIDENCE IS INCONCLUSIVE THAT FORCED AIR 
WARMING DEVICES INCREASE SURGICAL SITE 
CONTAMINATION OR INFECTION

Authors

Ashley Bonner, BSN, RRNA 
School of Nurse Anesthesia 
Harris College of Nursing and Health Sciences, 
Texas Christian University, Fort Worth, Texas. 
Correspondence: a.bonner@tcu.edu  
 
Mark Welliver, CRNA, ARNP, DNP 
Associate Professor of Professional Practice 
School of Nurse Anesthesia 
Harris College of Nursing and Health Sciences, 
Texas Christian University, Fort Worth, Texas. 
Correspondence: m.welliver@tcu.edu 

Acknowledgements 
This review was conducted in partial fulfillment of the require-

ments for the Doctor of Nursing Practice School of Nurse 
Anesthesia at Texas Christian University. I want to acknowledge 
Mark Welliver, CRNA, ARNP, DNP, Faculty of TCU School 
of Nurse Anesthesia for expert advice and guidance in this 
assignment.

AbstrAct 
The greatest degree of heat loss in surgery is during the first 

hour after induction of general anesthesia. Intraoperative hypo-
thermia poses great risks for the patients and their recovery. 
The use of forced air warming devices has been well studied 
and shown to maintain patient normothermia. There is concern 
that forced air warming disrupts operating room airflow and 
contaminates the sterile field leading to surgical site infections. A 
literature search was performed using Embase, Web of Science, 

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Clinical Key and Nursing at Ovid Joanna Briggs Institute (JBI).
Five articles were found comparing the contamination risk 
of forced air warming with other warming technologies. The 
synthesis review found insufficient evidence to suggest delayed 
or discontinued use of forced air warming. The studies’ lack of 
data showing patient surgical site contamination and inability 
to conclude that the forced air warming devices actually caused 
surgical site infections due to intraoperative contamination do 
not support a change to clinical practice. As the greatest amount 
of patient heat loss is during the first hour of anesthesia, the 
use of forced air warming devices at this time is supported as 
opposed to delaying use due to unsupported concerns of surgical 
site contamination. 

keywords 
Forced air warming, intraoperative, contamination, 
infection, normothermia 

IntroductIon

Hypothermia is an issue during the perioperative period and 
is caused by various factors. Anesthetics such as volatile agents, 
intravenous (IV) agents, and spinal/epidural anesthesia impair 
the patient’s thermoregulation capabilities.1 Within the first 
hour of administering anesthesia, the patient’s core temperature 
drops significantly due to redistribution of body heat.1 Core 
temperature redistribution is due to peripheral vasculature vaso-
dilation and subsequent transfer of body heat to the environ-
ment. Radiation of body heat is the primary mechanism of heat 
loss followed by convection, conductance, and evaporation. 
As with all diffusions, the greater the difference in concentra-
tion, pressure, or heat energy, the greater the net movement. 
Peripheral vasodilation also increases the surface area exposure 
of body heat promoting net diffusion. Cold operating room (OR) 
temperatures, infusion of cold IV fluids and open body cavities 
contribute to heat loss experienced by the patient. Perioperative 
hypothermia is associated with postoperative mortality, bleeding, 



platelet inhibition or dysfunction, shivering, and a decrease in 
neutrophil activity causing infections.1 Active air warming is 
known to be superior to passive warming in preventing intraop-
erative hypothermia. Forced air warming (FAW) has shown to 
be most effective compared to other warming or heat conserving 
methods in maintaining core body temperatures, reduced shiv-
ering, reduced morbidity caused by infection and bleeding and 
shorter length of hospital stay overall.1 

Evidence supports the use of FAW preoperatively and intra-
operative, especially during the first hour after general anes-
thetic induction, a critical point of heat redistribution from the 
core body to the periphery. During this time, the surgical site is 
exposed and prepped for a sterile field. The OR air is prudently 
directional and filtered during surgery to prevent the contamina-
tion of sterile equipment and surgical sites. Unidirectional down-
ward airflow is used in many operating suites, specifically ortho-
pedic, to deter unwanted pathogens from surgical sites and sterile 
areas.2 There is concern that FAW systems may increase the risk 
of infection by airborne-microbial emissions directly from the 
device or by disrupting laminar airflow in the OR.2 Therefore, 
it is important to evaluate active warming devices to deter-
mine if one contributes to increased contamination. Does FAW, 
compared to other warming devices, increase intraoperative 
infection risks by interrupting OR ventilation and/or releasing 
microbial emissions resulting in a contaminated surgical site? 

methodology

A literature search was conducted using Embase including 
Medline, Web of Science™, Clinical Key and Nursing@Ovid 
(Joanna Briggs Institute) to find articles discussing intraoperative 
contamination possibilities of forced air warming systems and 
its counterparts. The keywords used were “forced air warming,” 
“operating room*,” “contamination” and “infection.” Keywords 
were combined using OR and AND to limit the results. The final 
search was “(forced air warming) AND (operating room*) AND 

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(contamination OR infection).” Nine out of 16 articles found in 
EMBASE were relevant to the topic. Seven were excluded because 
they addressed only the use of an active warming device for the 
prevention of perioperative hypothermia and discussed the reason 
for laminar airflow and its proper use in the OR. Three addi-
tional studies were excluded because of unavailable data.

The Web of Science database provided 9 total articles using 
the same keywords; however, only 2 were unique. Four of the 
9 were repeats from the previous EMBASE search, and 3 were 
excluded after reviewing the abstracts for relevance. In Clinical 
Key, a search using the terms yielded a total of 120 articles. After 
reviewing these abstracts, it was determined that there were no 
new articles. Two additional studies were obtained from the bibli-
ographies of appropriate articles. A total of 10 articles were used 
for this review.

lIterAture revIew

Legg et al compared two warming devices against a control of 
no warming device.2 The devices, a FAW blanket (Bair HuggerTM) 
and a radiant warming blanket (HotDog® were applied to a 
volunteer’s torso.2 The purpose of the study was to determine if 
these devices increased the temperature and the number of simu-
lated bacterial particles present at the surgical site.2 A controlled 
simulated operation was set up with one surgeon present in the 
theatre.2 Temperature measurements were performed before and 
30 minutes after warming and a HandiLaz® counter was used 
to count the number of particles.2 The HandiLaz counter is a 
handheld device that measures particles in the air using laser and 
photo detector technology, the particles are counted and sorted 
according to size.2 Each mode of measurement was repeated 5 
times using each device and the control.2 The operating theatre 
was unable to simulate a true surgical environment because it 
lacked the usual influence of equipment, personnel, and their 
movements during normal working conditions. Although the 
experiment resulted in a significant mean increase in temperature 



(1.1°C vs. 0.4° C, P<0.0001) and concentration of particles 
(1,038.2 vs. 274.8, P=0.0087) when using a FAW device, conclu-
sions cannot be made that this would cause an increased intraop-
erative risk of surgical site infections.2 

A replicated full factorial design was performed by Belani et 
al to determine whether a FAW blanket (Bair HuggerTM Model 
540; Arizant Heathcare) or a heat-conducting fabric blanket 
(HotDog® Model B110; Augustine Temperature Management) 
positioned over the torso disrupted ventilation in the OR 
causing surgical site contamination.3 The conducting fabric was 
powered using low voltage electricity. Neutrally buoyant deter-
gent bubbles, with a 4-mm average diameter, were produced 
using a generator specifically designed for the purpose of air 
current visualization.3 An operative theatre was set up for a 
total knee replacement in an orthopedic OR with laminar down-
ward ventilation airflow.3 An anesthesia practitioner was placed 
at bedside with an anesthesia drape at high or low position. 
Neutrally buoyant bubbles, which simulate skin cell fragments 
and free-floating bacteria, were introduced at the head of the 
mannequin to track air movement under the sterile drape. Time-
lapse photography was used at 10-second increments to measure 
bubbles present at the surgical site.3 FAW had a large increase in 
bubbles measured at the surgical site compared to conduction 
fabric (132.5 vs. 0.48, P=0.003) and the controlled condition of 
no warming device (0.01, P=0.008).3 Conduction fabric did not 
have a significant difference in bubble measurement compared to 
the control (P=0.87).3 Although the simulation environment of 
this study found air current disruption with FAW a causation or 
correlation to surgical site contamination or infection is unable 
to be made. A conflict of interest existed with this study as the 
authors received research funding, consultant fees, and salary 
from Augustine Temperature Management, the company that 
manufactures the non-FAW device conductive fabric blanket used 
in the experiment.3



A simulation study by McGovern et al also used neutrally 
buoyant bubbles to identify disruption of airflow comparing a 
FAW device (Bair HuggerTM Model 540; Arizant Healthcare) and 
the HotDogTM conductive fabric blanket (Augustine Temperature 
Management).4 This study found the similar results as Belani 
et al.3 Of the two devices, the FAW device resulted in higher 
measurements of bubbles (68 vs. 0, P< 0.001) in the simulated 
surgical site area of the mannequin. In addition, this study 
included retrospective collection of data from joint replacement 
surgeries during a 2.5 year period.4 Data included infections that 
presented within 6 months after surgery. The study found that 
1,066 patients had surgery using the FAW device and 371 used 
the conductive fabric.4 A higher incidences of joint infections 
was found when FAW was used (odds ratio 3.8, P=0.024) and a 
significant reduction in infection rates was found with conductive 
fabric versus FAW (0.8% vs. 3.1%, P=0.024).4 The major weak-
ness in the joint infection data collection was that the prophy-
lactic antibiotic regimen changed multiple times over the 2.5 year 
time period.4 Incomplete recording of important predictors of 
deep infections such as blood transfusions, obesity incontinence 
and patient fitness level before surgery further weaken the study. 
This is the first evidence found that links (albeit weakly) FAW to 
surgical site infections.

Sessler et al conducted a study in 2011 using FAW in a two 
separate simulated OR environment to determine disruption 
of air quality. The Bair HuggerTM FAW model 522 upper body 
blanket and model 635 under- body blanket were tested in three 
conditions: baseline with the blowers off, blowers on with cool 
ambient air, and blowers on with heat.5 A conscious volunteer 
was positioned on the OR table and 6 heated mannequins were 
positioned around the OR to simulate OR personnel. The use of 
nonmoving heated mannequins allowed convection currents but 
prevented airflow disturbance by human movement in an effort 
to isolate FAW airflow disruption. 



A particle detector was placed over the volunteer’s abdomen 
and a vapor generator allowed for visualization of airflow.5 The 
equation for the log reduction of particle concentration at the 
test point in relation to the particle load in the room was PEx = 
-log(Cx/Cref).

5
 Log reduction of 2.0 was indicated as the threshold 

minimum value by Deutsches Institut für Normung (DIN) stan-
dards developed in Germany to calculate the function of laminar 
airflow. The worst of 5 measurements were calculated for each 
device. By comparing the 3 different testing conditions to the 
baseline particle load in the OR, FAW was found to have no 
compromising effect on laminar airflow (3- to 5-log reduction).5 
A supporting author of the study received research funding from 
Arizant Healthcare, manufacturer of Bair Hugger.5 

In 2011, Albrecht et al evaluated the Bair Hugger model 505 
and included the evaluation of 2 generations of intake filters by 
the same manufacturer, the newest 200708D model, and the 
current 200708C model.6 Five of each filter model were tested 
according to industry standards using a sodium chloride aerosol.6 
Fifty-two of the FAW models used in real OR settings were 
assessed for intake filter retention efficiency, performance in the 
OR, FAW blower-generated particles, and air path microbial 
colonization of the internal distal hose.6 

The newer model filter’s efficiency was lower than that of the 
current model, resulting in FAW blower contamination emissions 
of 61.3% vs. 93.8%.6 Buildup of internal airborne contamina-
tion largely depends on the environment in which the device 
is used; 58% of the 52 blowers from 11 hospitals produced 
contaminants.6 This was the first study to evaluate the relation-
ship of intake filter performance with FAW blower contamina-
tion. Microbial-contaminated emissions in the OR are a potential 
clinical risk. 

Recently, Reed joined the authors of the Belani et al3 study and 
conducted a similar experiment on the most current and widely 

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used FAW device. The Bair Hugger FAW model 750 was evalu-
ated on four categories: intake filter efficiency, intake filter perfor-
mance in the OR, production of airborne contaminates, and 
internal air path microbial colonization.7 We tested the intake 
filter’s efficiency by counting sodium chloride particles the filter 
captured during a 10-minute challenge.7 Laser particle counts 
were calculated from 23 FAW blowers after hours in the OR of a 
hospital to test the filters’ efficiencies.7 We replaced the filters on 
the same 23 FAW devices, measured the particle counts down-
stream, and obtained swabs from the internal air path surfaces to 
determine contamination emission and microbial colonization.7 

The filter challenge revealed only 63.8% efficiency and, by 
removing and challenging all 23 intake filters, it found only 
minute differences in filtration efficiency and showed that the 
intake filters were performing to specifications in the OR.7 
Furthermore, 96% of the FAW devices produced contamination 
emissions, and 100% revealed the presence of microorganisms 
from the distal end of the blower.7 The study found that FAW 
devices inadequately filter particles, allowing microbial coloniza-
tion distal to the filter. 

In a nonrandomized comparison study, Dasari et al compared 
the temperature warming effects of the Bair Hugger, an under-
body resistive mattress (IndithermTM, Inditherm Medical, 
Rotherham, UK), and the HotDog conductive fabric system.8 The 
hypothesis was that increased temperatures caused by warming 
systems can disrupt laminar flow ventilation by creating convec-
tion currents at the surgical site. To test this hypothesis, warming 
devices were placed on the lower body of a mannequin, and 
temperature measurements were recorded in 60-second intervals 
using 24 thermostats at various heights above the OR table.8 In 
an environment simulating an orthopedic OR, temperatures were 
measured during 3 distinct periods over 5 different areas of the 
mannequin: the right and left shoulder, abdominal area, and right 
and left knee.8 The “control” time was the 20-minute period 



before warming, the “transition” was the 10-minute period after 
the device turned on, and the “steady state” was the 20-minute 
period after the device was turned on, when the device had ther-
mally balanced.8 This technique was performed for each device. 

The data was used in an analysis of variance model to formu-
late results.8 FAW caused the greatest temperature increase from 
the control period versus the conductive blanket (+2.73 (0.7)°C; 
P<0.001) or resistive mattress (+3.63 (0.7)°C; P<0.001) at the 
mannequin’s abdominal surgical site.8 At shoulder level, there 
was no significant difference in temperature among any of the 
devices.8 The effect of increased heat at the surgical site in terms 
of infection remains unknown. 

Huang et al conducted an experiment during 16 abdominal 
vascular prosthetic graft insertion procedures using Bair Hugger 
FAW systems.9 Bacterial counts in the air and wound specimens 
were collected and compared at the start and end of surgery.9 
The mean number of colonies of microbial growth from the OR 
air and the exhaust fan near the axillae of the patient decreased 
from the start of the operation to the end (mean reduction, 
36.4%; P<0.01).9 These results could have been due to the move-
ment of OR staff and turbulent air circulation at the beginning 
of the procedure compared with the end. A 6-month follow-up 
visit confirmed that no patients experienced postoperative or 
graft infections during that time.9 The author concluded the Bair 
Hugger did not contribute to bacterial contamination of the 
operating environment or the surgical field.9 

Moretti et al conducted a study similar to Huang in that 
microbial samples were collected on agar plates using the Active 
Surface Air System in the OR of a procedure instead of a simu-
lated environment. Samples were collected in 3 different points: 
A1, A2, and A3, around the operating table in the empty OR 
immediately before surgery, when the patient arrived, and when 
FAW was applied.10 Twenty noncemented hip implantations 

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were performed; no patient had postoperative hypothermia-
associated complications or surgical infection in a 6-month 
period.10 However, a significant bacterial load was observed after 
the patient was placed on the OR table and FAW was applied 
(A1=17.8±14.5 vs. 79.2± 52.2 cfu/m3, P<0.001; A2=19.4±17.5 
vs. 61.2±38.8 cfu/m3, P<0.001; A3=19.2±17.7 vs. 69.1±56.8 cfu/
m3, P<0.001).10 The results are not quantifiable but suggest that 
FAW does not contaminate the surgical field. Medical staff and 
movement in the OR better explain the significant increase in 
bacteria colonization. 

In 1997, Avidan et al conducted four experiments using 10 
FAW devices (9 Bair Huggers, Augustine Medical [now 3M], and 
1 Warm Touch [Mallinckrodt Medical, St. Louis, MO]).11 The 
first specimens were collected from the airflow of 10 blowers 
using agar plates on sterile towels in an unused OR.11 Control 
plates were used during the 5-minute intervals between turning 
on the blowers.11 Four the 10 (40%) agar plates grew organisms, 
including Staphylococcus epidermidis, Corynebacterium, and 
Cryptococcus albidus.11 

The second experiment sought to determine if blowing warm air 
through a perforated blanket reduced microbial contamination. 
The plates were placed below the blankets for 30 minutes with 
the blower on, and control plates were collected for the same 
time with the blowers off.11 Neither the control nor the experi-
mental plates grew organisms.11 Next, we used sterile swabs to 
collect samples from both sides of the internal filter and the inside 
of the proximal and distal end of the hose. The inside of the 
internal filter lacked growth, while the outside grew organisms 
such as Aspergillis fumigatus and Bacillus sp; both the proximal 
and distal hose swabs had similar growth.11 The final experi-
ment consisted of comparing the microbial growth of the direct 
airstream from three blowers—with and without an attached 
microbial filter at the distal end of the hose. The microbial filter 
prevented growth after it was applied to the same blowers that 



grew Acinetobacter lwoffii and Staphylococcus epidermidis on 
the plates placed directly in the airstream of the warmers.11 The 
author states that, after this study, their hospital facility changed 
its policy to require the attachment of a perforated blanket when 
using FAW in the OR.

synthesIs

Studies in this review compared airflow disruption and micro-
bial contamination of FAW with that of other warming devices 
to identify infection risk associated with these devices. Ten of 
the studies 2-8,11 were experimental designs, 2 involved human 
subjects,9,10 and 1 included retrospective hospital surgical infec-
tion rates.4 Although the studies by Albrecht, Reed, and Huang 
found that microbial contaminates may be present in FAW 
device hoses, its clinical significance has been questioned by 
the lack of data showing surgical site contamination, causative 
increased infection rates, and a specific study that showed that, 
despite microbial presence at the FAW device distal hose, no air 
contamination from the perforated warming blanket required 
for patient use was found.11 Only a retrospective study found 
an increased infection rate over a 2.5-year period when FAW 
devices were used versus a convection device, but the causative 
variables were not isolated, and prophylactic antibiotic regimens 
were not controlled. Considering that high-efficiency particulate 
air filtration improves microbial entrapment and that microbial 
growth on FAW device hoses occurs, regular changing of FAW 
device filters may be beneficial but cannot be recommended. 
Hose decontamination is also a consideration, but further study 
is necessary to determine its efficacy. Randomized controlled 
studies in multiple facilities comparing FAW and other patient-
warming devices should collect data on surgical site or implant 
infection rates and correlate it with the causative pathogen.

conclusIon 
The studies in this review primarily focused on the disrup-

tion of OR airflow and microbial contamination and emissions 

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of FAW devices and could only infer potential effect on surgical 
site contamination. No studies have demonstrated that FAW 
contributes to a greater risk of surgical site infection. In addition, 
most studies were funded or conducted by researchers affiliated 
with two competing device manufactures, introducing bias. The 
clinical decision to avoid FAW during the immediate period after 
induction of anesthesia and during surgical prep must be weighed 
against the well-documented risk of core temperature redistribu-
tion, which is at its worse during this time. 

Warming of the patient during the first hour of anesthesia is 
important, and maintenance of normothermia has become a stan-
dard of practice. Hypothermia has been associated with increased 
infection rates.15 This review found limited data to support 
limiting FAW perioperatively. If FAW is the only method available 
to maintain patient temperature, its avoidance at the beginning of 
the operative period is not advised. If other warming methods are 
available and effective, they may be substituted. Evidence does 
not support terminating the use of FAW in the OR environment, 
nor does it support delaying the use of FAW until surgical drapes 
are placed. Because it may take as long as an hour to prepare and 
drape a patient for surgery, the heat loss during this time can be 
significant. Until definitive evidence is found to advise otherwise, 
the use of warming devices, including FAW, is recommended 
without change from current practice.



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