key: cord-0824697-d1c8dmh1
authors: Sperna Weiland, Nicolaas H.; Traversari, Roberto A.A.L.; Sinnige, Jante S.; van Someren Greve, Frank; Timmermans, Anne; Spijkerman, Ingrid J.B.; Ganzevoort, Wessel; Hollmann, Markus W.
title: The Influence of room ventilation settings on aerosol clearance and distribution
date: 2020-10-23
journal: Br J Anaesth
DOI: 10.1016/j.bja.2020.10.018
sha: 6053b309cc00340dee41b590310d56413f5a384d
doc_id: 824697
cord_uid: d1c8dmh1

nan

healthcare workers involved in aerosol-generating procedures such as tracheal intubation or bronchoalveolar lavage were at increased risk of becoming infected. 1 For the current COVID-19 pandemic caused by SARS-CoV-2, several international guideline committees have recommended that these procedures be performed in airborne isolation rooms. [2] [3] [4] These rooms typically have a negative pressure relative to the adjacent hallway and a relatively high air exchange rate. However, because they are limited in most hospitals, it is inevitable that, in the context of a pandemic, aerosolgenerating procedures in SARS-CoV-2-infected patients take place in other hospital environments such as operating theatres or general ward rooms.

Ventilation system properties differ in various hospital settings and this could influence aerosol behaviour, potentially compromising healthcare worker safety. For instance, ventilation systems in operating theatres are not designed for airborne isolation, but to protect the surgical field from contamination using a positive pressure system. In ward rooms, air exchange rates are much lower than in an airborne isolation room or operating theatre. To our knowledge, no previous study has compared the relative influence of room pressure and air exchange rate on aerosol behaviour in different hospital settings. We aimed to quantify this to identify potential risks associated with different working environments. The results could guide specific recommendations that may help in choosing optimal working environment to protect healthcare workers performing aerosol-generating J o u r n a l P r e -p r o o f procedures.

We performed a simulated aerosol-generating procedure on six different single-patient hospital rooms (data supplement A1 to A5) with varying air exchange rates (1 to 91 change(s) h -1 ) and pressure gradient towards the adjacent hallway (P room -P hallway , measured with a needle micromanometer (Accubalance 8380, TSI, Shoreview, MN, USA). One of the rooms with a low air exchange rate was equipped with an air purification unit (City Touch, Camfil, Stockholm, Sweden) recirculating 600 m 3 h -1 through an efficiency particulate air (EPA) filter with a 99.5% particle removal efficiency to improve air exchange rate.

Aerosols were dispersed from a test-fluid (Durasyn 164/Emery, INEOS, London, UK) using a nebulizer (ATM 226, Topas GmbH, Dresden, Germany) positioned at the head-end of the bed (1 m above ground level). Two particle counters (Solair 3100, Lighthouse, Boven-Leeuwen, The Netherlands) sampling air 1.5 m above ground level, were placed in the room's periphery and in the hallway next to the closed door. All equipment was remotely operated to avoid room disturbance; doors remained closed during the entire measurement sequence. The study protocol measurements consisted of a 15-min baseline and 15-min of particle dispersal followed by a wash-out time recording of 60 min.

Data were stored digitally and processed off-line (Matlab R2018b, The MathWorks Inc., Natick, MA, USA). After triplicate measures on each hospital room, data were synchronised and particle oncentration counts were averaged per minute. Because SARS-CoV-2 aerosols appear in two peak concentrations with aerodynamic diameters of 0.25 to 1.0 µm and >2.5 µm, we analysed particle size ≥0.5 µm to assess room ventilation efficacy. 5 In all situations, baseline aerosol concentration was 0-0.6 x 10 6 m -3 , which increased to 10-92 x 10 6 and 0.2-10 x 10 6 m -3 after aerosol dispersal in the hospital rooms and in the hallway, respectively. Aerosol washout was modelled as the fitted natural exponential decay function (R 2 >0.95 for all situations) as proposed by the US Centers for Disease Control and Prevention (data supplement A6). 6 We classified the six rooms according to their ventilation system properties with air exchange rate (high vs low) and pressure gradient towards the hallway (positive vs. neutral vs. negative). Results are summarised in table 1 and in the on-line data supplement. There was considerable variation between the rooms with the 99% removal time of aerosols ranging between 7 min and 307 min depending on air exchange rate. On the room with the lowest air exchange rate, the addition of an air purification unit improved air exchange rate from 1 to 11 change(s) h -1 and 99% removal time of aerosols from 307 to 47 min. Aerosol distribution to the hallway, calculated as the ratio of areas under the curve (AUC hallway /AUC room ), was associated with pressure hierarchy. We found significant J o u r n a l P r e -p r o o f distribution (4-15%) on positive pressure rooms, detectable distribution (1%) on neutral pressure rooms and unmeasurable (0%) on negative pressure rooms. For each pressure gradient, higher ventilation rates seemed to reduce hallway exposure (data supplement A7).

These results highlight the importance of ventilation system settings on aerosol clearance and distribution in various in-hospital settings. In this study, aerosols remained airborne for more than 5 h in a room with low ventilation rate. These rooms should be considered 'contaminated' for extended durations after aerosol-generating procedures have been performed in SARS-CoV-2-infected patients, since it has been shown that airborne SARS-CoV-2 remains viable for at least 3 h. 7 Addition of a recirculating air purification unit in these rooms improved aerosol washout dramatically; this could be a simple and inexpensive solution to improve safety for healthcare workers.

Air exchange rate in the operating room was very high and therefore the time needed to remove 99% of all aerosols was very short. Although this may vary between hospitals, 8 in our setting, aerosol distribution to the hallway could not be neglected for a positive pressure gradient. Whereas reduced hallway exposure was found in neutral and negative pressure environments, healthcare workers in the room benefit most from high air exchange rates to reduce the amount of aerosols quickly. It is important to assess the local situation when deciding on the best location for aerosolgenerating procedures in SARS-CoV-2 infected patients.

NSW: initiation of the project, study design, data collection, data analysis, drafting the manuscript, revising the manuscript RT: study design, data analysis, revising the manuscript JS: data collection, data analysis, revising the manuscript FSG: study design, revising the manuscript AT: initiation of the project, revising the manuscript IS: data analysis, revising the manuscript WG: initiation of the project, revising the manuscript MH: initiation of the project, study design, revising the manuscript. All authors approve the final version of the manuscript. Detailed descriptive information on included rooms and their ventilation system settings. Rooms were classified according to their ventilation system properties with air exchange rate (high vs low) and pressure gradient towards the hallway (positive vs. neutral vs. negative). Main outcome measures for the study were aerosol clearance, expressed as the time necessary to remove 99% of aerosols after a simulated aerosol-generating procedure and relative hallway exposure, expressed as the ratio between hallway and room exposure. Lower 99% contaminant removal times were found on rooms with higher air exchange rates. We found significant aerosol distribution (4-15%) in positive pressure rooms, detectable distribution (1%) in neutral pressure rooms and unmeasurable distribution (0%) in negative pressure rooms. Higher ventilation rates seemed to reduce hallway exposure. APU, air purification unit; Pa, Pascal; AUC, area under curve.

SARS among Critical Care Nurses

Consensus statement: Safe Airway Society principles of airway management and tracheal intubation specific to the COVID-19 adult patient group

Perioperative Management of Patients Infected with the Novel Coronavirus

COVID-19 Information for Health Care Professionals -Recommendations

Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals

Guidelines for Environmental Infection Control in Health-Care Facilities

Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1

Are aerosol-generating procedures safer in an airborne infection isolation room or operating room?

A8 Aerosol (≥0.5 µm) concentrations in rooms and hallway J o u r n a l P r e

The authors wish to thank Wilco van Nieuwenhuyzen, Henk Prins, Matthijs de Wit and Marco Scholten for their technical assistance with the measurements.