In operating rooms (ORs), laminar airflow (LAF) systems create a homogenous airflow with very little turbulence from the air conditioner outlet to the inlet. Particularly, in ORs with a vertical LAF system, downward airflows from air-conditioner outlets on the ceiling are exhausted via air-conditioner inlets near the floor. Contrarily, non-laminar airflow (nLAF) systems do not produce homogenous airflows; then, a mixed or turbulent flow is passed into the OR, where air, aerosols, and particles can be homogenized. However, cost-effectiveness analyses found that LAF systems are more expensive compared to nLAF systems. Cacciari et al. reported that LAF systems increased building costs by 24% and annual operating costs by 36% compared to the respective nLAF system costs. 1 To ensure a low infection rate in an ultra-clean air environment, the usage of LAF as an air conditioning system has been recommended when performing surgery. [3] [4] [5] [6] The rate of reoperation or wound infection after artificial joint surgery in an LAF-equipped OR was reported to be smaller than that in an nLAF-equipped OR. 7 Contrarily, Lidwell et al. reported that the use of prophylactic antibiotics was a more effective prevention method for development of joint sepsis after surgery than ventilation systems in the OR (1.6% under ultra-clean air without antibiotics versus 0.8% under a conventional airflow system with antibiotics). 4 Thus, perioperative prophylactic antibiotics are thought to be the most potent prevention factor for the development of surgical site infections (SSI). However, in gastrointestinal surgery, the effectiveness of prophylactic antibiotics varies among different types of surgery, and some studies reported that antimicrobial prophylaxis was not observed. 6, 8, 9 Additionally, the efficacy of LAF for SSI prevention was demonstrated under the gastric or vascular surgeries. 5, 6 On the other hand, LAF operating theaters, long operating times, high hospital care volume, antibiotic-laden cement, and forced air warming (FAW) systems have been reported as surgical risk factors for SSIs. 10 Additionally, the global guidelines of the world health organization (WHO) for SSI prevention indicate that the use of LAF was not necessarily recommended for patients undergoing total arthroplasty surgery. These WHO 2016 guidelines were based on previous reports that showed that surgeries performed in LAF-equipped ORs resulted in increased SSIs and expenditures. This led to a reevaluation of the cost-effectiveness of this method and the risks of SSIs. 2, 11, 12 It is expected that the number of ORs employing nLAFs will increase in the future. However, the benefit of using an LAF system for preventing SSIs remains controversial.
The safety of FAW systems is also controversial, although the active warming of patients during surgery is indisputably beneficial. 13,14 In our previous study under an LAF system, the upward FAW airflow, above the head area, was completely counteracted by the LAF, and the FAW did not cause a meaningful airflow in the surgical site. 15 However, the effect of the FAW airflow in an nLAF-equipped OR has not been reported to date. Therefore, we investigated the impact of FAW on the airflow and sanitation quality of an nLAF-equipped OR.
The study was performed in an OR within the Munakata Medical Association Hospital, which is equipped with a vertical nLAF system. The airflow speed at the diffuser 
An upper-body manikin was placed in the supine position; a lower-body warming blanket was placed underneath it and was covered with a surgical drape ( Fig 1D) . A 3-dimensional ultrasonic anemometer (WA-790; Sonic Corporation, Tokyo, Japan) was used to measure the direction and speed of the airflow at intervals of 300 mm at 324 points (9 × 6 × 6): 9 points on the x-axis (in the right/left direction at 300 mm intervals with the cross-sectional point placed at point X4), 6 points on the y-axis (in the head or foot direction at 300 mm intervals with the head at point Y6), and 6 points on the z-axis [at 100, 300, 500, 800 (operating table height), 1,100 and 1,400 mm from the floor] (Fig 1B, C) . A "meaningful airflow" is defined as an airflow under 10 cm/s.
The sanitation quality was evaluated with a cleanliness recovery test during the use of the lower-body blanket. For this test, dust was artificially generated in the surgical field with the nLAF turned off; then, the air conditioning was activated and the dust removal process was assessed. 16 We performed measurements at the center and lateral edges of the operating table at the Y3 level ( Fig 1B) ; three measurements were made at each height (800 mm and 1,500 mm) for 6 points. Using a particle counter (KC-03A; RION Co., Ltd., Tokyo, Japan), suction sampling was performed at a rate of 1 ft 3 /min to measure the number of dust particles with a grain size of ≥ 0.5 µm. The air conditioning was turned on with an initial dust particle load of approximately 50,000 per cubic foot, and transitions in the number of dust particles were measured at 1 min intervals for 10 min. A comparison between the FAW running group [FAW (+)] and the FAW stop group [FAW (−)] was performed.
The presented data are expressed as means and standard deviations. An unpaired t-test was conducted to compare the control levels between the FAW (+) and FAW (−) groups.
A comparison between the two groups was performed via a two-factor repeated-measures analysis of variance (Fig 4B) . The statistical analysis was performed using the Prism 6 software (GraphPad Software: La Jolla, CA, USA), where a p-value of p < 0.05 is considered statistically significant.
Examination of the operating table from the right and foot sides showed that the nLAF-induced downward air current moved along the floor toward the operating table causing an upward air current on both sides of the table (Fig 2A) . Contrarily, the FAW caused no meaningful airflow above or around the table (Fig 2B) , except for the airflow around the head area.
At the X5 point, an upward air current was detected directly above the manikin's head in the area separated by the anesthesia screen (9-30 cm/s) (Figs 3A and B, top row) , regardless of the presence of the nLAF. The FAW caused no meaningful airflow above or around the operating table (Figs 3B, top and bottom row) .
At the Y3 point, an air current (16 -28 cm/s) was detected moving along the floor from the outside towards the operating table (Fig 3A, bottom row) . An upward air current (18 -34 cm/s) was detected at the edges of both sides of the operating table (Fig 3A, bottom row).
There was no significant difference in the number of dust particles per cubic foot before activation of the air conditioner, regardless of whether or not FAW was present [FAW (+): 52650 ± 1694 vs. FAW (−): 51289 ± 3667, p = 0.48]. The number of particles per cubic foot 10 minutes after air conditioner activation was 17.2 ± 11.1 for FAW (+) and 9.8 ± 4.5 for FAW (−). There was no significant difference in the cleanliness recovery rate between the FAW (+) and FAW (−) groups (Fig 4) .
This is the first study to investigate the FAW airflow under an nLAF system. It was found that FAW caused no meaningful airflow or disturbance of airborne cleanliness in the surgical field under the nLAF system. However, the FAW caused an upward air current above the head of the operating table, and nLAF could not counteract this upward air current. Our previous study revealed that the upward air current caused by the FAW was completely counteracted in the LAF-equipped OR, where the air-conditioner outlets were fully covered by the head area. 15 In the OR used in this study, although vertical down airflow from the ceiling above the surgical field was not detected, the sanitary standard of the OR was adequate. This cleanliness in this OR could be mainly due to air circulation cycle times per hour.
The nLAF system used in this study created an upward air current on both sides of the operating table flowing along the floor, operating table, and surgical drape. A downward air current from the air-conditioning inlets generated this upward air current on the ceiling. This phenomenon could be owing to the location or number of the air-conditioning inlets ( Fig   1A) . The OR used in this study has only two air-conditioning inlets in the corner of the room.
Thus, the airflow from the ceiling creates neither a homogenous flow nor a laminar airflow and becomes stagnant. These results indicate the possibility of contamination of the surgical field from surrounding operating beds and the importance of the layout of the air-conditioner outlets.
Many interventions have been suggested to affect the micro-organisms in ORs. avoided. However, if the micro-organisms and bacteria are of considerable size, the contamination of large surfaces, such as floors, and the particle transfer throughout the surgical area should be measured and evaluated. The 0.5 micron size is the important standard size for determining if the environment is below or above the accepted level of particles for cleanroom standard. Additionally, most commonly bacteria size is over 0.5 micron, so particle size smaller than 0.5 is not critical issue, then we measured particles over the 0.5 micron. The sanitary standard of the OR used in this study was adequate, but potential contamination from the floor was expected because an air current from outside the operating table was detected at an airspeed between 16 and 31 cm/s. In almost all existing reports in which the air-conditioning type was found to be a risk factor for SSIs, the air-conditioning outlet layout or airflow in the OR was not discussed. If the airflow in the surgical field had been evaluated as a contributing factor, instead of the air-conditioning type, the conclusions in these reports might have been different.
A limitation of this study is that we could not evaluate the effect of upward airflows on both sides of the operating table and head area on the number of airborne particles in the surgical field. The detection of point-to-point particle movement is practically difficult because artificially generated dust particles are quickly diffused. Generating a fixed number of particles is also difficult. Belani et al. generated the illuminate neutrally buoyant detergent bubbles into the head area separated by anesthesia drape using by bubble generator and shifting bubbles were counted in the surgical site by photography. 20 They reported that the number of shifting bubbles into the surgical field originating from the head area was higher when the FAW was operative, but did not discuss the ventilation situation. However, the FAW-induced upward airflow detected in this study could possibly transfer the airborne from the head area to the surgical field or spread respiratory droplets to non-intubated patients.
Nevertheless, further investigation is needed to address these concerns. Further, neither the operating staff nor the equipment around the operating table were included in this study.
These factors could play a significant role on the airflow.
In this study, we demonstrated that the FAW caused no meaningful airflow (> 10 cm/sec) and did not diminish the sanitation quality in the surgical field. However, above the head area, the upward FAW-induced airflow was not counteracted by the nLAF system, which caused an upward airflow at the edges of the operating 