Article
‘Life, is of course a misnomer, since viruses, lacking the ability to eat or respire, are officially dead, which is in itself intriguing, showing as it does that the habit of predation can be taken up by clusters of molecules that are in no way alive.’
Any guest editorial on COVID-19, an opinion piece by another title, has to be predicated on the notion of ‘incompleteness’. In this case, I have elected to update my previous essay on aerosols and dentistry, as our knowledge base has expanded but remains a jigsaw puzzle, pieces missing, perhaps never to be found. It is also the case that, since the date I wrote this piece, things may well have evolved further in this rapidly changing crisis.1
We are at a crossroads for the whole future of dentistry as a scientifically-based healing profession, balanced, as much of it is, with the requirement for the business of dentistry to thrive. On one hand, our absolute requirement to ‘first, do no harm’ has to somehow remain steadfast, as we are compelled to make life-defining decisions regarding our businesses, our financial security and preserving any joie de vivre we may have had for our profession before this ghastly virus slinked its loathsome way into our lungs.2,3
Aerosol generating procedures – a false flag operation
Aerosol science is an enormous subject, both in scope, breadth and depth, with many prestigious peer-reviewed journals presenting on an incredible array of fields, genres and minutiae, linked, somehow, by the word ‘aerosol’. It is human nature to start going down the proverbial rabbit hole; one fascinating article leading to another, yet another followed by still another.
I have had to remain disciplined here, but let us define an aerosol, and then take a look at dental aerosols, and the procedures, or otherwise, where these happen.
An aerosol is a suspension of fine solid or liquid droplets in air or some other gas. Aerosols can be natural or anthropogenic.
Dental aerosols are not one thing, they are at least three groups.
When we hear the term Aerosol Generating Procedure (AGP) used in relation to dentistry during the current time of crisis, many people immediately think of the high-speed drill, the ultrasonic tip of the scaler, or the 3 in1 syringe with both buttons depressed. What we need to do, however, is break this down a lot further, to start to get a handle on real, perceived and also irrational, fear-based risks.
I am not sure that it helps the situation that the World Health Organization definition of aerosol generating procedures, which does not include dentistry, is at odds with the UK one, which does include certain procedures. One of the tasks that I feel we should pursue, as a profession, is to clarify where our dental specific scientific guidance is coming from. When we are seeing other countries re-establishing wide provision of practice-based dentistry, with minimal changes to pre-COVID-19 PPE, I think it is vital that we know who is providing our scientific advice, so that we can perform due diligence upon it and inform the narrative.2,3
I propose to use three groupings in my attempt to provide clarification:
Type 1 – Respiratory aerosols
Breathing produces aerosol, in other words, a complex continuum of respiratory secretion droplets from the large – more than 20 µm in diameter, through small droplets – 5–20 µm in diameter, to the aerosolized droplets – less than 5 µm in diameter. As diameter, and thus mass, decreases, the relative influence of gravity also decreases, as the influence of air diffusion increases. Large droplets drop to earth quickly, smaller ones, more slowly, tiny ones – well they may not drop for a very long time.6
Unfortunately, this is a gross simplification, so bear with me as I ramp up the complexity. Dependent on the temperature, humidity and air-flow speed of the atmosphere we breathe into, droplets in all three groups can desiccate at variable rates to become dried bodies called droplet nuclei. The smaller droplet nuclei become less affected by gravity and more by the diffusion of the air. Thus, they can float in the air as aerosol.7,8
Accordingly, we can have a combination of dried droplet nuclei and tiny respiratory droplets within the aerosolized component of our respiration. In addition, we have droplets that drop over a range of distances from our mouths and noses, dependent on their size and speed.9
We produce expiratory aerosols during tidal breathing through both our mouths and also our noses. As we speak, shout and sing, the quantity of these increases. Interestingly, lower frequency (bass tone) sounds produce more, as do increased decibels.10,11
Once we factor in ballistic droplet events, such as coughing or sneezing, a recent study suggests that droplets can be measured 8 metres away from the subject immediately post-sneeze, and the aerosol disperses widely using air currents to follow a complex dispersal path that even the latest computer modelling struggles to depict.12
Both us, our patients and everybody within our dental surgery are producing this Type 1 aerosol, all the time. If we were not, we would be dead!
Type 2 – Clean Water Anthropogenic aerosols
Type 2 is what some of us may have envisaged when we think of AGPs. The plume of spray from our well-adjusted, high-speed handpiece, the mist surrounding the tip of our ultrasonic scaler or the haze coming from our 3 in 1 when we push both buttons down. For many years, we have used antimicrobials to keep our waterlines free of microbial biofilms, and so it is fair to say that the combination of droplets and aerosol emitted by these devices is made up of clean water. You could argue that residual biofilm may be present, as well as lubricating oil, particularly if the equipment has been excessively lubricated. Not ideal if trying to prepare a tooth for an adhesive restoration!13,14
During the COVID-19 crisis, much discussion has centred upon adding viricidal agents to the water supply of our air turbines, ultrasonic scalers and 3 in 1s. Obviously, a lack of human toxicity, coupled with an absence of detrimental effects on the physical integrity of the equipment, is vital, but just as important is that any purported chemical addition actually has a viricidal effect on SARS-CoV-2, when it enters the mouth. It would also be useful if the effect could remain for the entire clinical procedure being undertaken.
Type 3 – ‘Mixed’ aerosols
This is my tentative definition of the complex result as our clean water combination of high velocity water flow, individual multi-sized droplets and aerosol impacts on the structures of the oral cavity. The pools of saliva with its measurable SARS-CoV-2 virion load.15 The moist, complex surface of the tongue, the hard surfaces of the teeth and, on occasion, the hard-walled sides of the cavity being prepared. We then add the various other fluids also present within the mouth, respiratory secretions from post-nasal drip or via the oropharynx, blood and gingival crevicular fluid (GCF) too.16 Further to this complex mix, we must then remember the likelihood of various viable and denatured viral particles, bacterial endotoxin, pulverized plaque biofilm, enamel and dentine particulates, restorative materials all mixing with that clean water, or antimicrobial/water mix, coming from our dental equipment.
This extremely variable set of circumstances creates various outcomes:
I advised that it was very complex. We should now consider the ways to reduce the volume, distribution, constituents and potential pathogenicity of these various droplet/aerosol categories.
Let's talk about aerosol mitigation
Before I go into more detail on High Volume Aspiration (HVA), I will briefly touch on other fairly simple measures that we can take to affect the specifics of the aerosols we generate.
Pre-operative rinses can introduce a viricidal agent into the oral commissure, but the limited studies available suggest that, although three commonly used chemicals; Hydrogen Peroxide 1.5%, Povidone Iodine 1% or Hypochlorous Acid 0.05%, are all suitably viricidal, their microbial substantivity is poor.18
Perhaps a pre-operative nasal spray could be used instead/as well? Povidone Iodine has been looked at for this purpose.19
For many years, it has been clear that rubber dam use is associated with a large reduction in what I termed Type 3 mixed aerosols. Obviously, this is procedure specific but, in my view, the adoption of rubber dam as a requirement during appropriate dental procedures has many benefits, both related to the current crisis but also from the viewpoint of improved restorative outcomes and, perhaps, reducing perceived patient resistance to it, as rubber dam becomes ‘the standard’ in our post-COVID world.20,21
High Volume Aspiration (HVA) and adjuncts
When we achieve that state of ‘flow’ time receding into the background as dentist/DCP and dental nurse subconsciously establish mutual behavioural pattern recognition and optimization so that true four-handed dentistry can occur. The tissues expertly retracted by judicious use of the aspirator, as the operating field remains free of excess moisture and conditions are created for the best possible outcome.22 I think we all instinctively know two things. When the aerosol/aspiration interface is poorly controlled, it makes dental artistry more like a Chaos Painting by Marc Quinn, than the spare, architectural mastery of an Edward Hopper.23,24 Secondly, even when the dental team are working together in perfect harmony, it is rare that our face shields don't reveal our inability to control the droplet and aerosol miasma entirely.25
Many studies use high-speed video and/or UV visible dyes to help quantify this imperfect system of dental aspiration: videos of the halo of mist billowing vertically above the laminar flow of aerosol entering the aspirator, the mist then dispersing widely, including directly into the facial zone of the operator.26
No scientific study has compared violent expiratory events such as coughing, sneezing or choking (what can be termed, ballistic droplet events), with mechanically created dental aerosols. Many studies of bioaerosol use standardized solutions containing the microbes or virions, aerosolized by nebulizers and kept optimally dispersed for extended periods in technical apparatus, such as Goldberg Drums. This standardization is required for statistically relevant results, but these laboratory-based experiments bear little relation to our dental surgery-based experiences.27
That being said, we instinctively know that high volume aspiration removes the majority of droplets and aerosol. Once we have ensured that our aspiration system is working as well as it can be, let's look at methods to increase that majority.
As complex aerosol, droplet and particulate mixtures enter the aspirator, the science of gaseous flow resistance is important. It seems intuitive to me that widening the orifice of the aspirator tip will improve the clearance of our generated aerosol. This obviously depends on maintenance of the pressure differential between the aspirator motor-created vacuum and the opening of the tip.
Now, I don't want to blow all our minds with a treatise on flow resistance differentials, gaseous conductance, Knudsen and orifice-related choked flow, but I do want us all to reflect that, as the aerosol/droplet mix goes down the aspirator tube, the flow resistance is partly friction between the gas/aerosol particles, but also friction between the gas and the pipe walls. That is why a smooth, clean bore is so important.28
Several companies have attempted to address this, by providing either ready-made or 3D printer-ready adaptors for our existing HVA systems. These have far larger orifices, with a gradual reduction with bore volume as you progress towards the tubing, to optimize gas flow. No formal scientific studies exist at the current time, looking to quantify the percentage improvement in aerosol clearance achieved by these devices.29
What about External Oral Suction Apparatus, of which many systems are commercially available? Often presented as a free-standing wheeled unit with a universally hinged rigid tube and large orifice, these can be positioned in front of the patient's mouth, hopefully still allowing adequate access for the dental team. Many of these machines contain HEPA filters, UV-light sources and plasma filtration to provide a safe air exhaust. The noise, reduction in access and cost have prevented these systems becoming mainstream prior to the current crisis.
Regarding expensive pieces of new kit for our surgeries; floor-standing, wall- and ceiling-mounted air purifier systems have become popular, with anecdotal evidence that some corporate dental companies are buying these in bulk for their dental surgery estates. There is no evidence that floor-standing units provide any reduction in infective risk for SARS-CoV-2, and for the wall- and ceiling-mounted units, research showing their use within hospital-based laminar flow or vertical ventilation system operating theatres confounds any positive effect they may have on any airborne measure of infectivity.30,31 In my opinion, buying expensive, unproven bits of kit such as these as a gesture to show your staff and patients that you are thinking about their comfort and safety is a relatively noble gesture. Where I get concerned is where a spurious safety differential is used to attract new patients. We are scientists, not snake oil sellers.
Do we have evidence that dental aerosols transmit SARS-CoV-2?
No, we don't. In fact, there is no evidence of aerosolized SARS-CoV-2 as the primary infective vector from any study worldwide. This includes, but is not limited to, aerosols of dental origin.
Now, that doesn't mean that SARS-CoV-2 hasn't been detected from collected aerosols. In vitro studies have not only shown the virus within aerosol (nebulizer-sourced aerosol, suspended via Goldberg Drum aerosol stabilizing apparatus).32,33
A preprint study from the University of Nebraska medical Centre suggested airborne transmission of viral RNA as one putative possible mode of spread to explain viral shedding into the environment, alongside droplets, fomites, toileting (faecal spread) and direct patient-to-patient contact spread.34
In addition to this study, a study in Nature, discussed the aerodynamic analysis of a proxy measure of the virus (viral RNA), within two Wuhan hospitals. This was undetectable or very low within patient wards but was higher inside patient toilets and a soiled PPE doffing area.35
What these studies have not done is either grow live virus from collected virions from aerosol or shown infective potential.
Perhaps it is worth looking at super-spreader events?
I am sure many of you are aware of the term ‘super-spreader’. These individuals who, for only partially elucidated reasons, seem to infect a greater number of people with any specific pathogen than the mean. Some reports suggest that 80% of the transmission potential for a given pathogen is from about 20% of the given population. The super-spreaders are in this group.36,37 What about Super-Spreader Events (SSEs)? Are these caused by super-spreaders? Well, that isn't clear. An SSE is a geographical and chronological occasion where a notable cluster of infections occur that are far in excess of the mean background infectivity events (secondary attack rate) for that pathogen. A high specificity and frequency of infections originating from one place at one time.38
Specifically, for SARS-CoV-2, the documented SSEs have common features; reduced social distancing, lots of close face-to-face speaking, raised voices due to background noise, intimacy such as kissing and sharing alcoholic beverages/cigarettes, social events such as birthday parties and funerals and all being indoors.
What is really interesting though are the events not associated with SSEs; going to the cinema or a concert, travelling by bus, train or aeroplane, working in an open cubicle office. Lots of people in close proximity but not talking in raised voices with reduced social distancing and/or the social lubricant of alcohol perhaps?
For me, this adds weight to the primary infectivity vectors of ballistic droplets and direct contact, whilst suggesting airborne spread is not causing SSEs.
However, are we up close and personal with our patients? We are, but we use well tested PPE, our surgical masks and face shields preventing droplet contamination of the wet areas of our faces. When we speak with our patients, we use the hush socially accepted norms of the professional clinical environment. Not one SSE has been reported from a dental surgery worldwide.39,40
So, what about returning to Dentistry?
The dental-specific scientific evidence base that has been used to advise Public Health England (PHE) and downstreamed to the Office of the Chief Dental Officer (OCDO), has been anonymous in source and has lacked transparency in content. Despite members of the Scientific Advisory Group for Emergencies (SAGE), and its relevant subcommittees, New and Emerging Virus Threats Advisory Group (NERVTAG), Scientific Pandemic Influenza Group on Modelling (SPI-M) and Independent Scientific Pandemic Influenza Group on Behaviours (SPI-B), electing, in the vast majority to be named, the source of dental specific advice remains unknown.
This creates two specific serious issues:
Conclusions
As a profession, we risk being led down a path towards new working practices that will fundamentally change the nature of our working lives. Surely it is vital that we are able to ensure that any measures that are taken are scientifically valid, evidence-based and can withstand the scrutiny of the ones affected – us?
It is clear from the lack of evidence of excess dental team SARS-CoV-2 infection rate or mortality worldwide that our well-tested standards of cross infection control, PPE and universal precautions, did a good job of protecting us all.
Careful pre-screening of our patients, for COVID-19 infection/pre-exposure status, co-morbidities and risk factors is prudent and sensible.
A false distinction between AGPs and non-AGPs must not be allowed to define our practice of Dentistry going forwards. Our being alive is an AGP.
COVID-19 is likely to remain with us in the long term. It is facile to imagine that it will just suddenly go away. Thus, we must remain coolly rigorous, objective and dispassionate in our analysis of the risks and benefits of any direction we take.
As bringers of health, we must continue to engage with the science and challenge the passive acceptance of poor, arbitrary or confused guidance, wherever it may come.
We are all responsible for our futures. The time to stand united is here.