Quantifying airborne particle exposure when using aerosol containment devices
It is not unreasonable to say the COVID-19 pandemic caught most of us by surprise. We did not know how infectious the Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2) virus leading to COVID-19 disease was, nor how to best protect the public or healthcare providers. As the virus was so new, much of the knowledge used to determine these risks stems from the literature about the 2003 Severe Acute Respiratory Syndrome-Coronavirus (SARS-CoV) epidemic. SARS-CoV and SARS-CoV-2 are both beta-coronaviruses and share 79.5% of their genetic material1. Both are spread via respiratory aerosols generated by the patient and during medical procedures involving the respiratory tract, of which tracheal intubation is thought to be one of the highest risk procedures for healthcare provider exposure and infection2.
As airway experts, anaesthetists are being called upon to tracheally intubate the most unwell yet most highly infectious patients with COVID-19 disease. With this responsibility comes placing at risk our own health, as well as that of the entire airway team. Since the World Health Organization announced on March 11, 2020 that COVID-19 disease was a pandemic3, what constitutes appropriate personal protective equipment (PPE) during tracheal intubation has varied4,5 while PPE supplies have often been limited, controlled, and changeable6. Self-preservation in the face of perceived PPE scarcity has resulted in a proliferation of ‘aerosol containment devices’ meant to decrease aerosol exposure of the laryngoscopist7. With many initial unknowns about the virus and its spread, ‘something is better than nothing’ has been the approach of many, with tacit legitimacy of the potential protection of such devices published by a high-impact journal8.
Evidence of effectiveness for aerosol containment devices has been mainly in the form of theory,9,10 ‘face validity’10or color indicators8,11,12. We are just beginning to produce objective, high-quality research regarding the effect of aerosol containment devices on time to tracheal intubation, cognitive loading effects, and discover the unintended consequences such as the breaching of PPE with their use13. We are just beginning to appreciate the need for human factors and bioengineering expert collaboration with clinicians to assess whether proposed aerosol containment devices actually fit-for-purpose7 or, as in this important article, actually do contain aerosols14.
Simpson et al quantify airborne particle exposure of the ‘laryngoscopist’ by performing an in-situ simulation of tracheal intubation performed in an intensive care room using five proposed aerosol containment devices. The containment devices included: a vertical plastic sheet; a horizontal plastic sheet; an ‘aerosol box’ (plastic box with open arm holes); and a ‘sealed box’ (sealed plastic box with integrated arm coverings, with and without suction applied). Aerosol generation was produced by nebulising 5 milliliters of saline held below the mouth of the ‘patient’ within the aerosol containment device. Only airborne particles were measured. Airborne particles were measured outside the aerosol containment device being tested, 75 cm above the head of the ‘laryngoscopist’, using a commercial airborne particle counter.
In order to understand the importance of this study, clear definitions of aerosols, droplets and airborne particles are helpful. Aerosols are generated by patients when they cough or sneeze and, to a lesser extent, phonate. Aerosols consist of mucous and water ‘envelopes’ surrounding the virus and are required for viral spread. The SARS-CoV-2 virus consists of a single-stranded RNA and nucleocapsid protein surrounded by a round or oval phospholipid bilayer covered by two different spike proteins1. The virus is 60-100 nanometers (0.06-0.10 micrometers) in diameter and cannot spread on its own. Size of these mucous and water envelopes, defined by their diameter, play a large role in determining the distance of spread. Envelopes can be divided into droplets (>5-10 microns diameter) and airborne particles (<5-10 microns diameter)15. Droplets are divided into large (>60 micron) and small (<60 micron) droplets16. Airborne particles are also called droplet nuclei16. The commercial airborne particle counter used in this study measured particles of 0.3, 0.5, 1, 2.5 and 5 micron diameters.
COVID-19 disease is thought to spread mainly by droplets, either directly from the patient, or by droplet deposition onto fomites where the virus can stay infectious for hours17. Depending on size and their viscoelastic properties (i.e. mucous and water production depending on stage of disease15), droplets can spread 1-8 meters before landing on potential fomites18. Fortunately, simple hand washing and regular cleaning of fomites eliminates the virus19.
Airborne particles remain suspended in the air for a much longer period of time and their dynamics are dependent on multiple factors, of which some may not be obvious or easily predictable. The number of room air changes per hour (ACH) plays a large role (Figure 1), as does whether particles continue to be produced. Size and location of objects in the room, relative humidity, environmental pressure, disruption of air circulation (e.g. opening/closing of doors) and even patient versus ambient temperature differences can affect airborne particle dispersal and elimination dynamics15,18. It is not be unreasonable to assume any proposed aerosol containment device may also modify predicted airborne particle dispersal and elimination.
Although droplets are thought to be the primary means of COVID-19 transmission, this doesn’t mean airborne particle transmission cannot occur. Studying infectious transmission via airborne particles is more difficult than their droplet counterparts. In an elegant study in 2004, Yu et al20 showed SARS-CoV disease transmission could occur via airborne particles. In 2003, the Hong Kong Amoy Garden, a six-apartment building complex, experienced a single index case resulting in 187 confirmed cases in 142 apartment units. Using computational fluid dynamics and multizone modelling, Yu et al showed both wind direction and bathroom and kitchen exhaust fans drove airborne particles horizontally and upwardly infecting inhabitants in these apartments. In the accompanying editorial21, Roy and Milton discussed how elusive it is to prove airborne transmission of infectious diseases. Roy and Milton21 suggested airborne infectious diseases be classified into three categories; obligate airborne (e.g. tuberculosis), preferential airborne (e.g. measles), or opportunistic airborne (e.g. SARS-CoV). It may very well be that SARS-CoV-2, like its SARS-CoV counterpart, is an opportunistic airborne infectious disease22. Airborne particles containing SARS-CoV-2 viruses have been shown to exist in the rooms of patients with COVID-19 disease23.
In this in-situ simulation study, using a standard intensive care room Simpson et al.14 uniquely investigated airborne particle exposure of the ‘laryngoscopist’ by quantifying this exposure in real time using a commercially available airborne particle counter (Lighthouse3016-IAQ). Generally used to assess environmental air quality, this specific particle counter entrains air at 47 cubic centimeters per second, measuring both airborne particle size and concentration using degree of scattered laser light.
The researchers used five milliliters of saline nebulized continuously under the ‘patient’s’ mouth over the 5-minute test period to continuously produce a range of both airborne and droplet sizes. This probably does not accurately represent the droplet and airborne particle production by a patient with COVID-19 disease, as force and amount of aerosol production is not continuous but changes with coughing, sneezing, and type of phonation. However, the standardization of aerosol production in the study protocol allows for relative comparisons of the five aerosol containment devices used in this study. Timing of aerosol containment device placement (e.g. whether during pre-oxygenation or after paralysis) and removal may also play a role in aerosol quantity generation, dispersal and elimination dynamics.
Of the five proposed aerosol containment devices tested, only the sealed intubation box under continuous suction resulted in airborne particles above the ‘laryngoscopist’s head to not increase compared to the baseline room particle count and be more effective at containing airborne particle exposure than no device use. Horizontal or vertical drape use did not decrease airborne particle exposure compared to no device use. Importantly, use of the aerosol box use resulted in significant increase airborne particle exposure at 300 seconds compared to no device use. Why would this be? In our exuberance to create devices to limit aerosol dispersal, we may not be considering the important role air turnover per hour plays in airborne particle elimination. In this study using an enclosed intensive care room had an air changes per hour (ACH) of 18. Approximately 16 minutes is required, without any device use, to eliminate 99% of airborne particles (Figure 1). The size and presence of a semi-open aerosol box meant to contain airborne particles may actually concentrate airborne particles within the box leading to their release over a longer period of time. This may explain why this was only observed at a longer period of time (300 seconds), while also potentially interfering with room air flow and ACT function.
|Air changes per hour||Time (mins) required for removal (99% efficiency)||Time (min) required for removal (99.9% efficiency)|
Figure 1: Time (in minutes) to remove airborne particles based on the air changes per hour (ACH) of a room*
Derived from the CDC: Guidelines for Environmental Infection Control in Health-Care Facilities (2003)
*Assuming airborne particles are NOT continuing to be generated (i.e., after tracheal intubation).
As previously mentioned, unlike continuously nebulized saline as was used in this study, patient generated aerosols are not produced continuously. Coughing and sneezing tend to be the most aerosol generating activities by spontaneously breathing patients24. Therefore, the researchers went on to study the effect of the ‘patient’ coughing. Over the five-minute trial period, the ‘patient’ coughed every 30 seconds. Much to the surprise of the researchers (and this author) only the aerosol box, with the open armholes, showed significant spikes in airborne particle exposure of the ‘laryngoscopist’ of all airborne particle sizes studied. The authors theorize that the transient positive pressure resulting from the cough within the aerosol box resulted in the forceful passage of air out the armholes with entrainment and dispersal of more particle-laden air, via the Bernoulli principle, onto the ‘laryngoscopist’. Of the five devices trialed, only the aerosol box had holes in it allowing for such a phenomenon to occur. It could be argued that an aerosol box would only be placed after the patient is fully paralysed, therefore this would not occur in a patient care situation. However, such devices have been proposed for use during extubation, where patient coughing frequently occurs. Certainly, during the pre-oxygenation phase prior to tracheal intubation, placement of an aerosol box should best be avoided.
Finally, given that once the patient is intubated aerosol containment devices must be removed, the researchers measured airborne particle count 60 seconds after device removal. The authors found this maneuver did not increase particle exposure.
There are a number of questions raised by this study. The researchers did not quantify droplet (>10 microns) or larger airborne particle (5-10 microns) exposure during this study. Given droplets play a large role in COVID-19 spread, this would be an excellent extension of these studies. Another issue that has not received the attention it requires is the droplet and airborne particle exposure of the airway assistant. Multiple COVID-19 airway guidelines state the number of people in the room during tracheal intubation should be limited, however most do recommend an assistant. The assistant usually stands by the side of the patient’s bed, therefore measuring the exposure to both droplet and airborne particles exposure from an assistant’s position is essential prior to adopting any such device. During the SARS-CoV epidemic in 2003, one small intensive care unit found airway assistants were 2.5 times more likely to become infected than their laryngoscopist counterparts25.
In conclusion, the authors should be commended for adding objective evidence to an area that so desperately requires it. Proposed aerosol containment devices should first do no harm7. Such devices have already been shown to increase tracheal intubation time and breech the PPE of the laryngoscopist13, both unacceptable consequences. Simpson et al now show vertical and horizontal drapes are no different than no device use. Most importantly, they have shown that a plastic box with holes for the laryngoscopist’s arms, an idea published in a high-impact journal reproduced in various iterations in social media and in other journals, may in fact increase airborne particle exposure of the laryngoscopist.
Perhaps something may not always be better than nothing.
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- World Health Organization (WHO). Rational Use of Personal Protective Equipment for Coronavirus Disease 2019 ( COVID-19 ). Vol 2019.; 2020. https://apps.who.int/iris/handle/10665/331695.
- Cook TM. Personal protective equipment during the coronavirus disease (COVID) 2019 pandemic – a narrative review. Anaesthesia. 2020;75(7):920-927. doi:10.1111/anae.15071
- Emanuel EJ, Persad G, Upshur R, et al. Fair allocation of scarce medical resources in the time of covid-19. N Engl J Med. 2020;382(21):2049-2055. doi:10.1056/NEJMsb2005114
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- Taiwan doctor’s easy-to-make Aerosol Box embraced in Japan. The Japan Times News. https://www.japantimes.co.jp/news/2020/04/28/national/science-health/taiwan-doctor-aerosol-box/#.XumrLy2z13Y. Accessed June 16, 2020.
- Chan A. Should we use an “aersol box” for intubation? Life in the Fast Lane. https://litfl.com/should-we-use-an-aerosol-box-for-intubation/. Accessed June 16, 2020.
- Lockhart SL, Naidu FJJ, Badh FCS, Duggan FL V. Simulation as a tool for assessing and evolving your current personal protective equipment : lessons learned during the coronavirus disease ( COVID-19 ) pandemic. Can J Anesth Can d’anesthésie. 2020;1239032499(March). doi:10.1007/s12630-020-01638-z
- Brown H, Preston D, Bhoja R. Thinking outside the Box: A Low-cost and Pragmatic Alternative to Aerosol Boxes for Endotracheal Intubation of COVID-19 Patients. Anesthesiology. 2020;(Xxx):1-2. doi:10.1097/ALN.0000000000003422
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- Simpson JP, Wong DN, Verco L, Carter R, Dzidowski M, Chan PY. Measurement of airborne particle exposure during simulated tracheal intubation using various proposed aerosol containment devices during the COVID-19 pandemic. Anaesthesia. 2020. doi:10.1111/anae.15188
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