key: cord-0977925-bvzc9wh9 authors: Newsom, Richard; Amara, Adam; Hicks, Alexander; Quint, Matthew; Pattison, Chris; Bzdek, Bryan R.; Burridge, James; Krawczyk, Coleman; Dinsmore, James; Conway., Joy title: Comparison of droplet spread in standard and laminar flow operating theatres: SPRAY study group. date: 2021-02-04 journal: J Hosp Infect DOI: 10.1016/j.jhin.2021.01.026 sha: cc3367bd83f2a052f422d6777825becbab021be1 doc_id: 977925 cord_uid: bvzc9wh9 BACKGROUND: Reducing of COVID-19 transmission relies on controlling droplet and aerosol spread. Fluorescein staining reveals microscopic droplets. We used this technique to compare the droplet spread in a standard theatre (ST) and a laminar air flow theatre (LAF). METHODS: We used a ‘cough-generator’ fixed to a theatre trolley at 45-degrees. Fluorescein stained ‘secretions’ were projected onto a series of calibrated targets. These were photographed under UV light and a ‘source detection’ software measured droplet splatter size and distance. RESULTS: The smallest droplet detected was ≅ 120 μm and the largest ≅ 24,000 μm. We detected an average of 25,862 spots in the ST, compared with 11,430 in the LAF (56% reduction). The LAF mainly affected the smaller droplets (<1000 microns). The surface area covered with droplets was: 6% at 50 cm, 1% at 2 m and 0.5% at 3 m in the ST; and 3%, 0.5% and 0.2% in the LAF respectively. CONCLUSION: Accurate mapping droplet spread in clinical environments is possible using fluorescein staining and image analysis. The laminar flow affected the smaller droplets but had limited effect on larger droplets in our AGP cough model. Our results indicate that LAF require similar post-surgery cleaning to those of ST and staff should consider full PPE for medium and high-risk patients. The coronavirus disease 2019 (COVID- 19) pandemic has seen rapid developments in scientific and medical understanding of the SARS-CoV-2 virus. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] Currently UK regulations are changing to keep pace with our scientific understanding 11, 12 but there are gaps in the data 1,2,13 particularly around aerosol generating procedures (AGPs). 1, 8 The NHS and other healthcare systems face a year of severe disruption, from efforts to protect both patients and staff from COVID-19 infection. Around 30-50 % of capacity has been lost in the NHS due to these protective measures. We urgently need to understand the effect of AGPs on droplet and aerosol production within clinical environments to enable us to reduce disease transmission from both patients with confirmed SARS CoV-2 but also from patients and health care workers (HCWs) who may be asymptomatic carriers. [14] [15] [16] [17] It is clear that SARS-CoV-2 can be spread by respiratory droplet splatter and subsequent hand / face contact and aerosols are also infective. 9, [14] [15] [16] [17] [18] The accepted definition is that droplets have diameters >5 μm whereas aerosols have diameters <5 μm. Aerosols remain airborne for prolonged periods of time and can transmit the infection over large distances, 1 whereas droplets fall rapidly to the ground. However, this definition has come under increased scrutiny because particles greater than 5 μm diameter can actually remain airborne for long periods of time and spread beyond two metres. [19] [20] [21] Morawska et al 13 tracheal intubation and extubation. They measured particles with diameters in the range 300nm to 10 μm using a sampling funnel placed at 0.5m away from the patient's face. They found that tracheal intubation produced very low quantities of aerosolized particles at 1.4 particles l -1 whereas extubation produced 21 particles l -1 . They compared these with a volitional cough which produced 732 (SD 418) particles l -1 . They made the point that intubation may not be an AGP at all and that the impact guidance around AGPs have increased waiting times for cancer and other surgeries. 1 However larger droplets > 200 μm are difficult to image and the particle analyser works best in very clean environments. Simonds made a similar finding in patients undergoing non-invasive ventilation / nebulisation and chest physiotherapy. Measuring droplets between 0.3-10 μm they found that there was little aerosol generation and most of the droplets fell to the ground within 1 m. However, in both these trials it was difficult to measure the trajectory of the larger droplets. 22 A standard operating room exchanges the air 20 times per hour and filters air with the removal of 80-97% of particles > 5 μm. Laminar air flow systems equipped with HEPA (High-Efficiency Particulate Air) filters remove 99.97% of particles > 0.3 μm. Current guidelines based on aerosol clearance times recommend a 20-minute theatre clean for a standard theatre and a 2-6 minute clean for a LAF theatre. Public Health England (PHE) guidance is that staff stand over 2m away from a high or medium risk patient. 12 However, the spread of larger droplets in such theatres have not been studied and the importance of a deep clean between patients is unclear. Fluorescent dyes have been used to mark body fluids, [23] [24] [25] [26] and investigate the spread of infection. Matava et al 14 developed a technique to assess the spread of droplets following extubation using a fluorescein dye. They found that a clear plastic drape significantly reduces droplet / spray production from paediatric manikin. There has been a research gap in the area of droplet research as there has not been a sensitive technique available to monitor aerosols or droplets from AGPs, and fomite spread once they have Once the splatter had occurred, we then imaged the targets using a (Nikon DC 800) camera and an F80 lens. The camera was fitted with a UV flash and additional UV illumination was provided with two 30w spotlights (Onforu, Guang Dong, China). Images were saved in numerical order and fresh plates were put out for each run of the experiment. Some images of the cough simulation and of the theatre surrounds were also taken (Figure 1c , 1d). The test was repeated 11 times in a standard theatre (ST) and a laminar flow theatre (LAF). We also calibrated the system using drops of a known volume between 0.1μl, and 2.0μl. These were used to calculate the areas of splatter for a given drop size. Observations were made within two operating theatres. The laminar airflow theatre has an ultraclean, vertical laminar flow ventilation system with high efficiency particulate air (HEPA) filtration. The air under the canopy 'clean zone' is filtered and recirculated at an equivalent of 500- We positioned the plates 445 mm below and 445 mm away from the cough source. We refer to an airborne particle as a 'droplet', and to the region it covers on a detection plate as a 'spot'. The sequence of images from each test were uploaded to the Institute of Cosmology and Gravitation at the University of Portsmouth. The images were initially straightened and de-warped to correct for the position of the camera (Figure 1b ). In these straightened images, one pixel has a width of approximately 85 μm, or an area of 7,225 μm 2 . A source detection algorithm, Source Extractor, 28 which is commonly used in astrophysics to identify objects in telescope images, was then used to detect individual droplet spot on the detection plates. The algorithm was able to identify spots that were an area of 5 pixels or larger, which corresponds to a spot of diameter 200 µm, or to droplet diameters of 120 μm. As well as identifying individual spots, the source detection algorithm also provides the basic properties of the spots, such as their size, position, shape, and orientation. We tabulated all dot size measurements by theatre type, cough, and distance. We compared total numbers of dots captured per cough, and total plate area covered per cough, from each type of theatre, with null hypothesis of equal means. Our alternative hypothesis is that there are on average greater numbers of drops and coverage recorded on the plates in the non-laminar theatre. The J o u r n a l P r e -p r o o f Newsom et al standard deviations of spot counts and areas covered for each cough were of similar magnitude to the corresponding mean counts and areas. We therefore also carried out a randomized permutation test (non-parametric) under the null hypothesis of identical count and area distributions between the theatres, using the difference in means as the test statistic. Tests were run using Statsmodels and NumPy (Python libraries). We calculated the spot size distribution by 'log-binning' spot area values (mm 2 ) from each theatre into a sequence of intervals of exponentially increasing width, and computing distance statistics (mean, variance and standard error) for each bin. A similar method was used to generate a spot area vs distance plot for each theatre. We computed coverage statistics for each plate distance and used this to generate a distance-coverage plot, for each theatre. This research was submitted to and received support from the University of Portsmouth Ethics We measured the mean distance of droplets which result in small (diameter < 1000 μm), medium (1000 μm < diameter < 2000 μm) and large (diameter > 2000 μm) spots. In the standard theatre, small droplets travelled on average 664 mm, medium 924 mm and large 1,282 mm, this contrasts to the laminar flow theatre which was 814 mm, 1,049 mm and 1,503 mm respectively (p<0.01) indicating there was a statistical difference between the theatres at all droplet sizes. The maximum distance travelled in both theatres was over 3.5m. Since it is the smaller droplets that are most affected by the laminar flow ventilation, its effect on total area covered is less pronounced (Figure 2a) . In the laminar theatre we find that the mean plate area covered is A = 8,469mm 2 (SD 3,775) and in the non-laminar theatre A = 11,818mm 2 (SD 3,686). The corresponding p value for the permutation test is p = 0.022. We observed a much slower decline in coverage at larger distances, where spots are typically several times larger than the median. The pattern may be understood by examining Figure 2b , which shows how the distance travelled by droplets depends on their corresponding spot diameter. The error bars in Figure 2b , which give the standard deviation of the travel distance for each spot area, show that the range of smaller droplets (diameter < 1 mm) is constrained to distances < 1.5 m. The variation in the distances travelled by larger droplets is much larger, up to at least 3m. Detailed information about travel distances is essential in order to build particle trajectory models which are consistent with realistic fomite splatter distributions. Our catalogue of spot areas and locations allows us to understand how the rate of fomite contamination varies with distance from the cough. In Figure 2c we show how the mean plate fraction covered by spots varies with this distance. In both theatres we observe a rapid decline in surface coverage. At 0.5m the spot coverage was 5.55% ST and 5.34% LAF (p>0.5), at 1.2m; 2.92% and 1.58% respectively, at 2.1m; 0.82% and 0.56% respectively and at 3.0m; 0.34% and 0.08% respectively. We also detected droplet splatter on the floor, walls and operating theatre lights and evidence of There was a difference in the percentage of surface area affected as well but this was less significant than the drop count, as the total area covered by larger droplets was similar in both theatres ( Figure 2a ). Brown et al 1 investigated extubation in a laminar flow theatre 1 using an optical particle sizer that measured droplets with diameters from 0.3 µm to 10 µm, whereas we measured droplets with diameters from greater than 120 µm with no upper limit. They detected an average of 1,310 smaller particles l -1 during a volitional cough. Perhaps a key difference was that they measured aerosol concentrations, whereas we measured deposited surface area. We know from our results that smaller particles were affected by laminar flow more than larger particles, consistent with their aerodynamic behaviour. Direction is a key determinant of droplet distribution. Our cough model was directed upwards at a 45° angle, typical for extubation. In Brown's paper the patients were supine, and the aerosols sampled at 50 cm from the patient. In a LAF the aerosols could have been affected by the air flow. We limited our data collection to a strip of targets 210 mm wide extending directly in front of the cough model and therefore we are not able to comment on droplets extending sideways from this. While these results were in some way expected, it was surprising that large drops travelled further than smaller drops and could still travel > 3 m within the laminar flow theatre. This is in contrast to previously held views that large droplets fall rapidly to the ground, 21 in our experiment many of the larger droplets had the momentum to travel over 2 m. It was also notable that larger droplets also Our cough model does have limitations, a key area is that it does not measure aerosol production but could be a method to be used in conjunction with aerosol detection of AGPs, or in environments where accurate aerosol measurements are impossible. The respiratory tract has a more complicated configuration in comparison to our model which only had a small (15 mm) external orifice. However, the droplet sizes produced and the distances they were projected were similar to human coughs. Larger droplets are mainly generated in the upper airway and the short, corrugated tube and 90 o angle piece configuration of our model proved effective at generating appropriate particle sizes. The key determinant of droplet projection is velocity and our model reliably produced a clinically realistic cough peak flow of around 300 litres per minute. 30 Smaller droplets (diameter < 120 µm) were not detectable with our technique due to the lack of spatial resolution of the initial imaging techniques. A further criticism is that we used saline 5% which has a different viscosity More research is urgently needed to map droplet spread within hospital environments. Most obviously this applies to areas where AGPs are performed but it is also important to understand the spread that will occur from un-contained coughing in all clinical settings. Combining droplet splatter analysis and optical particle sizing for smaller droplets and aerosols will give us a better understanding of body fluid spread within hospitals. Furthermore, the possibility of creating a mathematical model of droplet spread within a 3D map of each clinical environment may also be vital to predict droplet spread. More research into AGPs is needed, as droplet spread could be wider than previously thought and current guidelines could be reviewed to reduce the potential of hospital infection from high-risk procedures. J o u r n a l P r e -p r o o f Pickering AE A quantitative evaluation of aerosol generation during tracheal intubation and extubation Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy Advice on the use of masks in the context of COVID-19: interim guidance Wearing face masks in the community during the COVID-19 pandemic: altruism and solidarity Turbulent Gas Clouds and Respiratory Pathogen Emissions Potential Implications for Reducing Transmission of COVID Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission Aerosol Generating Procedures and Risk of Transmission of Acute Respiratory Infections to Healthcare Workers: A Systematic Review Led by Science', evidence gaps and the risks of aerosol transmission of SARS-COV-2 COVID-19: protecting health-care workers Airborne transmission of SARS-CoV-2. The world should face the reality Prevention-and-control/covid-19-personal-protective-equipment-ppe. Public Health England. Coronavirus (COVID-19)-what you need to know Size distribution and sites of origin of droplets expelled from the human respiratory tract during expiratory activities Clear plastic drapes may be effective at limiting aerosolization and droplet spray during extubation: implications for COVID-19 Asymptomatic infection by SARS-CoV-2 in healthcare workers: A study in a large teaching hospital in Wuhan SARS-CoV-2 IgG/IgM Rapid Test as a Diagnostic Tool in Hospitalized Patients and Healthcare Workers, at a large Teaching Hospital in northern Italy, during the 2020 COVID-19 Pandemic Modality of human expired aerosol size distributions Modality of human expired aerosol size distributions Trajectories of large respiratory droplets in indoor environments: A simplified approach. Cheng CH, Chow CL Measurements of Airborne Influenza Virus in Aerosol Particles from Human Coughs Lindsley WG Airborne transmission of SARS-CoV-2 Evaluation of droplet dispersion during non-invasive ventilation, oxygen therapy, nebuliser treatment and chest physiotherapy in clinical practice: implications for management of pandemic influenza and other airborne infections Health Technology Assessment Simonds AK Glow gel hand washing in the waiting room: a novel approach to improving hand hygiene education A large-scale assessment of hand hygiene quality and the effectiveness of the 'WHO 6-steps Improving Hand-Washing Performance-A Crossover Study of Hand-Washing in the Orthopaedic Department The use of fluorescein powder for evaluating contamination in a newborn nursery Screening of healthcare workers for SARS-CoV-2 highlights the role of asymptomatic carriage in COVID-19 transmission Software for source extraction The aerosol box for intubation in coronavirus disease 2019 patients: an in-situ simulation crossover study Changes in salivary flow rate, pH, and viscosity among working men and women. Dentristry and Medical research Reid Accurate Representations of the Microphysical Processes Occurring during the Transport of Exhaled Aerosols and Droplets How can airborne transmission of COVID-19 indoors be minimised? With thanks to Rosie Thomas, Dr Paul Smith, Dan McGuigan for photography; for software development, for Prof Chris Luca, Dr Michael Cottrell, Dr Jenny Child for editorial / methodology advice, Dr Andrew Lungren for software development. This work was supported by an impact accelerator grant from the Science and Technologies facilities council. B.R.B. is supported by the Natural Environment Research Council (NE/P018459/1). The Authors declare no competing interests, This work was supported by an Impact Accelerator Account; Science and Technology Facilities Council. AA is supported by the Royal Society Wolfson Fellowship.