COVID-19 and Dentistry: Aerosol and Droplet Transmission of SARS-CoV-2, and Its Infectivity in Clinical Settings

Aerosols, droplets and disease spread

An aerosol can be defined as a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be either natural – such as fog, mist, dust, or anthropogenic – created by humans or animals when they go through normal activities, such as speaking, sneezing, or coughing, for instance. Aerosols can also be visible, like fog, but more often invisible, like dust or pollen. Irrespective of the visibility or size, they may contain live/dead microbes and hence the term bio-aerosols. It is noteworthy that aerosols can be produced indirectly (by humans) via mechanical means, such as respirators and poorly maintained air-conditioning systems. In our context, bioaerosols are the offending agents that contribute much to COVID-19 spread.

The characteristics of bio-aerosols and their spread vary considerably, depending on numerous confounding factors. There are conflicting reports in the literature on the size of the aerosols and how long they are airborne. For instance, early researchers have classified particles <100 μm in diameter as aerosols, and those >100 μm as droplets or ‘spatter/splatter’ (Figure 1). The latter usually fall on to the ground immediately after they are expelled. The former, aerosols, on the contrary, may be entrained or suspended in the air for considerable periods, depending on the humidity, airflow, and temperature of the environment into which they are expelled, such as a dental clinic operatory or a hospital ward. Similarly, the large diameter ‘droplets’, containing millions of viral particles, can contaminate surfaces in the immediate vicinity, and spread a few metres, yet again depending on the ambient conditions, such as the airflow.

It should, however, be noted that the transition or the cut-off point between the aerosols and droplets is a matter of contention. Various sources cite transition points at 5 µm, 10 µm, 20 µm or, indeed, 100 µm, as defined above. This is a key distinction, in terms of infection control, and determines the regulations appertaining to the airborne precautions and droplet precautions, as declared by US Centers of Disease Control (CDC), and various national regulatory agencies. COVID-19 is likely to be contracted through either of these routes, although the weight of combined data to date supports airborne spread as the predominant route by which healthcare workers (HCWs) acquire the infection.

As mentioned, bio-aerosols can be generated either by natural, anthropogenic or by various mechanical devices. Humans produce bio-aerosols by talking, breathing, sneezing, or coughing, depending on the infectious state of a person, and these may contain fungi, bacteria or viruses (Figure 1). On the other hand, mechanical devices, such as clinic/hospital ventilation systems, air conditioners, air rotors with coolant water spray used in dentistry, may spread bio-aerosols equally efficiently. Engineering strategies, such as the integration of high efficiency particulate arresting (HEPA) filters must be incorporated into such systems to minimize or eliminate bio-aerosol spread.

It is also important to note, in this context, that several microbial factors, such as their virulence, the number of infectious particles in the aerosols, and the pathogenicity of the offending microbes, as well as host factors, including their immune status, determine the susceptibility of acquiring an infectious agent via a bio-aerosol.

Compared to the population at large, HCWs run a higher risk of acquiring respiratory pathogens by virtue of their profession. This was clearly shown in the severe acute respiratory syndrome (SARS) epidemic of 2003, which led to numerous deaths of HCWs, and exemplified in the current COVID-19 pandemic where HCWs have disproportionately succumbed to the disease. Although thus far, there has been no recorded mortality amongst dental HCWs due to COVID-19, they are considered to be the professional group that has the highest likelihood of acquiring the disease through aerosols.

Viral infectivity and survival

The infectivity of SARS-CoV-2 during the incubation period is still being debated. Although SARS-CoV, that caused the SARS epidemic, is genetically allied to SARS-CoV-2, their variance in infectivity is profound. Such differences in the epidemiological characteristics between these ‘siblings’ could mainly be attributed first, to the relatively high SARS-CoV-2 loads in the upper respiratory tract of the infected individuals and, secondly, the higher potential of the latter to be shed and transmitted during the asymptomatic phase.

Some studies have shown that, up to 40% of people infected with COVID-19, are asymptomatic carriers of the virus and may surreptitiously act as human vectors of community infection. Moreover, SARS-CoV-2 can remain viable and infectious in aerosols for hours, and on surfaces up to days, depending on the inoculum. For instance, in one study, SARS-CoV-2 was detectable up to 3 hours in bio-aerosols, up to 4 hours on copper, up to 24 hours on cardboard and up to 2–3 days on plastic and stainless steel. These figures are rough estimates only, as there are several other confounding factors that determine the survival of viral particles on an inanimate object and aerosols, and these include (Figure 1):

In practical terms, the SARS-CoV-2 virus loses infectivity at 56°C for 15 minutes due to the denaturation of its outer protein coat, and after 5 minutes exposure to commonly used disinfectants, including 10% formaldehyde, 10% hypochlorite, 75% ethanol and 2% phenol.

Aerosol generating dental procedures (AGPs)

Many interventional procedures are known to aerosolize respiratory secretions in healthcare settings. In dentistry, viral particles may be aerosolized by the high-speed handpiece and the accompanying air jet, ultrasonic scaling, air polishing, and air/water syringes. Unless judiciously controlled, these AGPs appear to be one of many intrinsic hazards the dental profession faces, that have been brought into focus by the COVID-19 pandemic. Thus, in a very early laboratory study, Miller observed that aerosolized microbes generated by high-powered dental drills and periodontal scalers could transmit to around 200 cm distance in the operatory.¹ More alarmingly, van Doremalen and colleagues have recently noted that the SARS-CoV-2 virus could remain aerosolized for up to 3 hours under laboratory conditions.² Whilst the data provide some pointers to the viral decay in a static environment, how this could be extrapolated into the dental clinic situation is unknown, as yet.

Clearly, the foregoing have critical implications in dentistry as the threat of airborne transmission of SARS-CoV-2 in dental aerosols, as well as their deposition on various clinical surfaces, have to be appropriately addressed for safe dental practice. Indeed, Li et al, during the post-SARS era, suggested additional measures for bio-aerosol reduction, and concomitant safe dental practice fit for adaptation in the event of impending viral epidemics.³ These include manual scaling, chemo-mechanical caries removal, atraumatic restorative treatment (ART), open debridement for periodontal surgeries, rubber dam isolation, use of pre-procedure oral rinses, improved general ventilation, and good air filtration. Most of these have been adopted by various dental jurisdictions during the current pandemic.