COVID-19 : the aerosol box confer minimal to no benefit in containing aerosols and may increase airborne particle exposure


Aerosol boxes meant to protect health care workers when they intubate COVID-19 patients may actually increase their exposure to airborne virus particles, an Australian study warns.

Intubation is done when patients are placed on a ventilator.

Aerosol boxes have been touted as a quick, simple way to protect workers, but their effectiveness and safety were never clinically tested. No international guideline on personal protective equipment has endorsed their use.

Even though experts have raised concern that the devices may put health care workers at risk, they are in use worldwide.

In this study, researchers tested the effectiveness of five aerosol containment methods (aerosol box; sealed box with and without suction; vertical drape; and horizontal drape) compared with no intervention.

Volunteers held a bottle of harmless fluid just under their mouth, and coughed every 30 seconds. Detection devices were used to count different-sized airborne particles and assess particle spread for five minutes.

Compared with using no containment device, exposure to airborne particles of all sizes was higher when an aerosol box was used, the study found.

If COVID-19 particles act the same way as the fluid used in this simulation, the results suggest the aerosol box increases exposure to virus particles – in some cases by a factor of five times or more, according to findings published July 9 in the journal Anaesthesia.

The investigation was conducted by a team at Intensive Care and Anaesthesia Specialists at Eastern Health in Melbourne. Peter Chan and Joanna Simpson, who led the effort, said the research team was surprised by the findings.

“Spikes of airborne particles were clearly seen, coinciding with patient coughing. We believe that these represent particles escaping from the arm access holes in the aerosol box,” the researchers reported.

The race to provide protective equipment has resulted in a variety of devices, but evidence of their safety and effectiveness has been lacking, the study authors said.

“This study demonstrates that devices such as the aerosol box we tested – which is typical of designs used worldwide – confer minimal to no benefit in containing aerosols during an aerosol-generating procedure and may increase rather than decrease airborne particle exposure,” the authors concluded.

In a journal news release, Chan said if the aerosol box were sold as a product and regulated, it would probably need to be recalled immediately due to the potential infection risk for health care workers.

“Unfortunately, because these devices have been donated and are not regulated in any way, health care workers might be continuing to increase their exposure to COVID-19 while thinking they are protecting themselves,” he added.

The coronavirus 2019 (COVID‐19) pandemic has highlighted the urgent widespread necessity for adequate personal protective equipment (PPE) for healthcare providers.

Coronavirus 2 (SARS‐CoV‐2), the virus that causes COVID‐19, is found at it’s highest concentration in sputum and upper airway secretions.

Although the SARS‐CoV‐2 coronavirus itself ranges in size from 0.075–0.160 microns, it requires a water and mucous envelope to spread [1]. Respiratory secretions, consisting of mucus and water, provide these envelopes.

The size, accumulation and volume of virus‐containing envelopes determine the size of respiratory droplets. Droplets can be categorised as either large (> 60 microns in diameter) or small (10–60 microns in diameter). Large droplets tend to fall on surfaces closer to the patient (< 2 m) and small droplets, with thinner mucus and water envelopes, tend to fall on surfaces further away [2].

Envelopes < 5–10 microns in diameter are not called droplets, but airborne particles or infectious droplet nuclei. Airborne particles will remain suspended in the environment for a period of time depending on a number of factors including air circulation, humidity and atmospheric pressure [2].

Aerosolisation, such as that produced during aerosol‐generating procedures, produces airborne particles as well as both small and large droplets [2, 3]. Aerosol‐generating procedures are thought to increase the risk of infection to healthcare providers [4].

Tracheal intubation is one of the highest risk aerosol‐generating procedures performed due to direct exposure to the airway and potential patient coughing during induction [4, 5] and is of particular concern to frontline anaesthetic, emergency and intensive care teams.

This has created a race to manufacture aerosol containment devices including improvised protection strategies for use during tracheal intubation. There has been significant promotion of these devices on social media, resulting in rapid proliferation and discussion globally [5-10].

Whereas these innovations aim to reduce aerosol dispersal, many have not been tested and are presented as viable options with only short reports and correspondence being cited in peer review literature. Currently, the use of aerosol containment devices is not recommended by any international PPE guideline [11-15].

The consequences of promotion of such untested devices include either a false sense of security using these devices, or paradoxical increase in healthcare workers exposure [16, 17].

Some devices claim to protect against both large and small droplets as well as airborne particles in small simulation experiments. Using a simulated cough producing fluorescent droplets, Canelli et al. [16] compared the contamination of a laryngoscopist with and without an aerosol box.

They concluded that the device reduced the macroscopic contamination of both the laryngoscopist and their immediate surroundings. However, their simulation did not test for air turbulence and flow direction, nor were they able to view small droplets or airborne particles contained in aerosols.

They noted that the aerosol box restricted hand movement and would require specialised training. This could result in increasing the risk of a difficult or failed intubation in what is already a difficult procedure. Chahal et al. (unpublished observations, designed an aerosol containment enclosure constructed from silicone sheets with an internal negative pressure environment using standard wall suction.

They demonstrated containment of vapour smoke, saccharin and nebulised fluorescein within the enclosure. However, this did not demonstrate any quantifiable reduction in either particle dispersal or subsequent risk of infection. Begley et al. [17] performed a simulated crossover study assessing the effect of two aerosol boxes on tracheal intubation performance and found the safety of the laryngoscopist and patient could be compromised by an increased time to intubation and potential damage to PPE.

To guide our institutional protocols for the airway management of patients with suspected or confirmed COVID‐19, we sought to test whether different aerosol containment devices confer any protective advantage to the laryngoscopist specifically with respect to airborne particle dispersal. We also aimed to examine the pattern of airborne particle dispersal in the room upon removal of these devices.

Our primary research question was how aerosol containment devices (aerosol boxes and plastic drapes) placed over a patient during tracheal intubation compared with no intervention with respect to exposure of the laryngoscopist to airborne particles?

Our secondary research question was to measure the size and distribution of the particles for each device and how effective they are, or not, at reducing airborne particle dispersion over a 5‐min time period, and at 60 s post‐removal of the aerosol containment device.

The race to generate sustainable equipment to protect healthcare workers during tracheal intubation procedures in patients with suspected or proven COVID‐19, particularly in settings where PPE supply is limited, has flooded the scientific community and social media with a variety of novel devices meant to contain potentially infectious aerosols produced by patients.

Evidence for the safety and efficacy of these devices is lacking.

Many international organisations have released consistent recommendations in the appropriate use of PPE based on the modes of viral transmission. None of these recommendations include these novel devices [11-15].

The dispersal of droplets and airborne particles from the patient depends on the aerosol (size, flow rates, turbulence, physiochemical properties) and patient (position, lung function) characteristics [18-20].

It is not clear in the manufacturing specifications of these novel devices that variables such as these have been fully considered in addition to the variability of employing these devices.

The use of such devices adds to the complexity of an already complex procedure (tracheal intubation following local COVID‐19 protocols including airborne PPE use) with the potential to compromise the safety of both the laryngoscopist and the patient [21].

Multiple methods of producing in situ aerosols in order to assess potential healthcare provider exposure are described in the literature. Techniques include the use of tracer particles of nebulised liquid droplets in a cloud or solid particles in smoke [1, 22-24].

These droplets are detected using optical particle detection techniques, such as particle counters and electron microscopy [22]. We selected an established and reproducible aerosol dispersion method that would maximise aerosol generation across multiple particle sizes.

The number of airborne particles produced via nebulisation of saline may far outnumber that produced during coughing and following paralysis of the patient for tracheal intubation.

We selected nebulisation to generate large amounts of airborne particles to better discriminate the protective benefit, if any, of various aerosol containment devices compared with one another and to that of no device use.

Despite small numbers of volunteers, our study demonstrates a statistically significant difference in the quantity of airborne particles between the various devices described, with only a fully sealed box on suction demonstrating airborne particle quantity similar to the ICU room at baseline.

We were surprised to find airborne particle contamination of the laryngoscopist increased substantially using the aerosol box compared with all other devices and to no device use. Spikes of airborne particles were clearly seen on the time series graph, coinciding with patient coughing.

We hypothesise that these represent particles escaping from the arm access holes in the aerosol box as a result of the Bernoulli principle. This was demonstrated in a recent simulation study by Dalli et al. [24] where photographic images showed that substantial amounts of air moved out of aerosol boxes into the operating theatre during coughing.

These data may be extrapolated to assess the utility of aerosol containment devices for tracheal extubation, where coughing is far more likely. However, this was not the objective of this study and further trials would need to be conducted.

Equally concerning is the increased exposure of the laryngoscopist at 300 s to 1.0, 2.5 and 5.0 micron particles when using the aerosol box, compared with not using any aerosol containment device (Table 1).

When compared with all other proposed aerosol containment devices and to no device use, exposure of the laryngoscopist to 5.0 micron particles was significantly greater with the aerosol box (Fig. 4e). The sealed box with suction appears to maintain airborne particle count at baseline but only with the use of ongoing suction.

We hypothesise that the efficacy of suction depends on the negative pressure generated, relative to the volume of the sealed box, the potential leak of the sealed box, and the length of time it takes for the air within the sealed box to become saturated with aerosol.

Whereas the sealed box was effective at maintaining environmental airborne particle counts, its design renders tracheal intubation mechanically impossible. It does, however, demonstrate the degree of enclosure necessary to eliminate particle contamination.

Wall suction has been proposed in many experimental setups [6-8] over high volume extraction as it is present in most intubating areas and is readily available. It should be noted that the flow of most wall suction is intentionally limited to reduce the risk of trauma to patients [25, 26], is not connected to any particle filter and was never intended to be used for the purposes of removal of airborne particles.

We acknowledge limitations with this small prospective in‐situ simulation study. There was significant variability in the airborne particle in the aerosol box, no intervention group and vertical sheet setups compared with the other methods.

This suggests that the level of laryngoscopist exposure is unpredictable and likely influenced by the aerosol containment device setup and the volume, negative pressure generated from suction and frequency of patient coughing.

To better assess variability, an increased number of subjects would be required, yet such variability suggests an inability to fully predict airborne particle contamination based on device alone.

This experiment also did not assess the particle count for individuals at the side of the bed or elsewhere in the room. Given airborne particles are known to spread over distances greater than 1.5 m [1, 2, 23], this is especially pertinent for the laryngoscopist’s assistant, who is often standing very close to the patient’s head during tracheal intubation and extubation.

In conclusion, this study demonstrates that devices such as the aerosol box confer minimal to no benefit in containing aerosols during an aerosol‐generating procedure and may increase rather than decrease airborne particle exposure.

A sealed box with suction appears to decrease airborne particle exposure of the laryngoscopist, although whether it hinders assistance or execution of tracheal intubation remains a point of study.

Further large‐scale studies are needed to examine aerosol containment devices in this context, as well as others, such as tracheal extubation. The use of any aerosol containment device has been eliminated from our intubation protocols until their safety can be properly established.


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More information: The U.S. Centers for Disease Control and Prevention has more on COVID-19.



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