The goal of this study was to evaluate the viability of aerosolized particles containing surrogate virus produced during a simulated orthodontic deband procedure without water. We also evaluated the role of a face shield in reducing the concentration of aerosols in the breathing zone of the operator. We observed that a face shield was not effective in reducing the particle concentration or live MS2 virus concentration in the breathing zone of the clinician during debanding procedures. These observations suggest that dental care providers may be at risk for inhalation exposure to viruses during debanding procedures from patients actively shedding virus in oral secretions. Furthermore, wearing a face shield provided no reduction in viral concentrations in the breathing zone of the dental care provider. The results of this experiment have implications for the health and safety of dental care professionals as the profession addresses risk to providers during the ongoing COVID-19 pandemic and other emerging or endemic viral illness.
Bacteriophage MS2 was used in this study as a surrogate virus for SARS-CoV-2 and has also been used as a surrogate for noroviruses, influenza and coronaviruses. 16,20,21 MS2 is considered an ideal virus surrogate when evaluating a virus’s ability to replicate, translate, and infect due to its small size, ease of growth, and simple composition. Additionally, MS2 is easily purified, durable, and harmless to humans which makes it an ideal candidate for acting as a respiratory virus surrogate. 17
A bioaerosol sampler is needed in order to capture the aerosolized MS2 produced during the simulated debanding procedure. The SKC BioSampler is a liquid impingement sampler and was used in this study for bioaerosol collection. Liquid impingers work by directing airflow and impinging particles of a specified size in a liquid media. 22 Multiple studies have used the SKC BioSampler in order to study aerosolized viruses and has also been used to sample aerosolized MS2 in a simulated vomiting study. 20,23−25 However, the SKC BioSamplers collection efficiency of submicrometer and ultrafine particles, which typically contain virus, has been shown to be < 10%. 26,27 Another disadvantage to liquid impinger samplers is that prolonged sampling time can cause evaporation and loss of liquid media, resulting in cell damage which could influence the viability of the virus. 22 This limitation was not of major concern for our study as our sampling times for each trial were less than 10 minutes.
When sampling for viruses using a liquid impingement sampler, the use of a liquid sampling media is required. During sampling, the liquid media helps to maintain the viability of the target virus while also allowing immediate sample analysis and virus enumeration. Buffers that have been successfully used in sampling for viruses include PBS, distilled water, Hanks Balanced Salt Solution (HBSS), peptone broth and transport media nutrient broth. 22,25 Phosphate buffered saline was used in our study and has been used previously to detect aerosolized virus while using impingement samplers. 26 Another study using the SKC BioSampler compared PBS and HBSS and found that PBS maintained virus viability at a significantly higher rate than HBSS. 22
In order to better understand the clinician’s risk when exposed to aerosolized virus, the use of plaque assays is the most common method in determining virus viability and is also considered to be the gold standard. 28 Plaque assays consist of a monolayer of cells that are infected by a viral solution. The infection is allowed to spread to bacterial cells that are immobilized in a layer of agar and these cells eventually undergo cell lysis. Everywhere cell lysis occurs, a hole in the monolayer of cells, also called a plaque, is created. Each plaque is believed to represent one viral particle from the viral solution, but it is possible cells in that area of the plaque were infected simultaneously by more than one viral particle. 19,22
While it is well known face shields are effective at protecting against splatter produced during dental procedures, data are limited on what effect face shields have in protecting clinicians against smaller aerosols. One study evaluated the effectiveness of face shields against smaller aerosols, approximately 0–7 mm, during a simulated cough and found that face shields were only effective in blocking 2% of these particles. 29 Another study aimed to measure the concentration of aerosolized particles in the breathing zone of the operator while using various facial protection and suction devices during a simulated dental procedure. Experimental results demonstrated that, with or without the face shield, there was not a significant reduction of aerosolized particles in the breathing zone of the operator. 30 The results of our study were similar and found there was no significant difference in particle concentration or virus concentration near the oral cavity and in the breathing zone of the operator while using a face shield.
As the pandemic continues, these experimental results suggest that using only a face shield will not prevent exposure to virus aerosols so additional protection efforts are needed for infection control strategies. Additional information is needed about exposure concentrations to SARS-CoV-2 resulting in COVID-19 to fully understand the level of risk clinicians and patients experience during dental procedures. Although the infectious dose for SARS-CoV-2 in humans is currently undefined, it is assumed to be very low since the virus is highly contagious and transmits rapidly. A previous study evaluating the infectious dose of SARS-CoV found doses corresponding to 10% and 50% illness were 43 PFU and 280 PFU, respectively. 31 Studies have been performed aiming to determine the infectious dose of SARS-CoV-2 in monkeys but applying these data to humans is likely to be inaccurate as the dose was highly variable between species. These studies examined various inoculation methods including intraocular, aerosol, intrathecal, intragastric, and intranasal. Infection via aerosols was consistently found to have a significantly lower infectious dose and was often associated with more severe disease as smaller aerosols can travel to the lower respiratory tract. The results showed the infectious dose ranged from 2,000 PFU in African Green Monkeys to 28,000 PFU in Rhesus Macaques. 32 Another study used sputum data from hospitalized COVID-19 patients and combined this with computational fluid mechanics to estimate an infectious dose between 100–300 viral particles for SARS-CoV-2. 33 Our results found that the average PFU/m3 near the oral cavity and in the breathing zone of the operator were 8.46x106 and 1.15x106, respectively. Comparing our results with the data from the previously mentioned studies suggests there is enough aerosolized virus produced during a deband procedure without water to cause infection if proper equipment and infection control protocols are not in place.
Our experiment was not without limitations. For example, the room ventilation was not characterized prior to the experiment which we believe this may be of little effect since the experiment was performed in a teaching clinic room. The teaching clinic is not negative pressure meaning the results of the virus and particle concentrations are what is occurring and not changed by the room conditions. The clinic, where orthodontic procedures occur, at the University of Iowa, is not negative pressure either. Foot traffic was also limited in the teaching clinic to limit the possibility of changing airflow. We also did not adjust for background particle concentration, but the effects of this are limited by performing a paired t-test.
To limit the potential for contamination across trials, the mouth of the simulated patient was cleaned between trials with an antiseptic rinse. In addition, no more than two trials were performed in one day to give time for any aerosolized MS2 to settle between trials. During the analysis, the plaque assay was run in triplicates to reduce the impact of random error. The SKC Biosamplers were autoclaved (sterilized) prior to each use. Plaque assays were also performed on the stock MS2, left in the laboratory, and the experimental MS2, brought and used at the clinic, to ensure that there was no contamination, and that the concentration remained the same.
The experimental setup remained unchanged throughout the study. However, there may be slight differences in the orientation of the provider in the setup across trials. To limit this source of variability, the setup was not moved between trials so that the provider was required to remain in the same location for each trial. However, the provider’s torso, arms, and hand placement many have varied slightly across experiments trials. We believe this error had little effect on the experimental results as the samplers were always oriented to be sampling behind the face shield of the provider.
We made our research study as generalizable as possible by conducting the daband with no water, high volume suction, and remove the excess composite with a carbide bur like done in most ortho clinics. There is a small percentage of clinics that use external suction units in addition to the high-volume suction, but this is not standard.
The use of N95 respirators and extraoral suction units have become common practice in dental and orthodontic clinics. While our study and others alike have suggested that face shields do not provide increased protection to aerosolized particles, N95 masks have been shown to be 95% effective in filtering particles of 0.3 µm. 34 During our simulated deband procedure, our optical particle counters sampled for particles with a size of 0.3 µm and found an average of 2.16x107 and 2.12x107 particles/m3 near the oral cavity and in the breathing zone of the operator were produced, respectively. Remington et al evaluated the use of extraoral suction units during an aerosol generating procedure and found, when used in conjunction with high speed suction, it was effective in reducing the amount of aerosolized particles that reached the operator. 30 With the development of new variants and the continued spread of SARS-CoV-2, it is likely exposure to this virus is inevitable, and it is important we remain diligent in our infection control protocols.