To Prove a Negative by Prof. Ross Hobson

To Prove a Negative

That is the question? How, do you prove something is not what it is?

In dentistry this is now an extremely important question as we are being told the ‘Dental practices are COVID heaven’ (Prof Mike Lewis; TV interview); Dentistry creates aerosols and hence these aerosols are dangerous and transmit COVID. The fact that that these are now defined as AGPs (Aerosol Generating Procedures) and by having its own acronym implies it is an important and significant issue.

We are being influenced by the opinion of personalities and supply companies to apply a significant of number costly and time inefficient solutions ie no AGPs, ‘sterilisation’ of surgeries, room extraction to name a few. But where is the evidence to support these, when challenged we are directed to research that shows:-

That the upper respiratory tract is where COVID virus is shed and that (medical) AGPs ie tracheal intubation, non-invasive ventilation, tracheostomy, CPR, manual ventilation before intubation and bronchoscopy. These are the WHO listed AGPs, note that dental procedures are not included, which is at odds with the UK interpretation of what defines an AGP.

That COVID virus can be found in breath, coughs and sneezes and that the virus can survive for about 2 hours. That COVID can survive on certain hard surfaces for up to 72 hours.

This research is undertaken in LABORATORY conditions where the conditions are set up to maximise the opportunity for the virus being detected and surviving, not real-life indoor or outdoor situations.

So, on the face of it, the evidence is overwhelming, that dentistry creates aerosols and hence is dangerous to both staff and patients!
So how do we challenge this righteous stance? We have to prove a negative. However, as those of us who have tried to enter into a scientific debate of opinion with marketing companies and personalities we rapidly enter the world of pseudoscience and facts get left behind. The images below are to remind us of the evidence pyramid and its pseudoscience counter.

As an evidence-based profession, we MUST be led by the scientific facts and not opinion, particularly as many of the personalities involved have financial interests in what they are promoting.

We need to look at the ‘filtered information’ accepting that there are as yet no Randomised Control Trials on COVID transmission in dentistry and there are unlikely to be any for many months or years. So we need to look at other areas of information that we can assimilate and filter to get to a conclusion.

We know that COVID is of similar size as the Influenza virus and has extremely similar means of transmission:-

Aerosol (transmission by coughs and sneezes)

Contact/fomite (transmission where a contaminated surface is touched and carried to the face). Hence, we can use the transmission of influenza virus and other upper respiratory infections to examine the available evidence on these diseases to see if dentistry is a risk.

In the dental environment, fomites are best managed by good surface decontamination combined with hand hygiene. These are an integral part of routine dental cross-infection protocols especially as it is known that COVID, like all viruses, are killed by simple cleaning using soaps, disinfectants and alcohol sanitisers.

Looking at aerosols: Coughing and sneezing are ‘ballistic’ events and uncommon in the dental practice unless a patient has respiratory tract symptoms ie coughing/sneezing.

Dental aerosols are very different from medical aerosols in that the water in the aerosol originates from the dental instrument, is clean and can have a disinfectant added. The resulting dental aerosol is mainly water with some level of contamination from the patient’s saliva.

It has been reported that an infected COVID patient breath up to 38% of the droplets can carry the virus. When that breath/saliva contamination is diluted by dental instrument water aerosol it is reasonable to assume that percentage of contamination will be greatly reduced. In addition, dentistry uses high-speed suction to aspirate the dental aerosol, reducing it further by 80-90%.

Dental staff routinely wear FSRM (Fluid Resistant Surgical Masks) and eye protection and there is evidence that to shows that wearing an FFP3 mask only confers 0.4% greater protection than FRSM. In addition, there is little evidence to indicate that eye protection imparts greater protection from virus transmission.

The summary of the above is that dental surgery cross infection controls are effective and that pre-COVID team protection ie FRSM and eye protection is more than adequate in the reduction of exposure to both potential aerosol and non-aerosol transmission.

So moving from what occurs in the dental surgery, we should look at the incidence of disease (particularly upper respiratory tract infection) transmission between staff-patient and patient- patient in the dental environment. In short, it doesn’t happen!

Looking at the latter first, there are no recorded cases of patient-patient disease transmission occurring in the dental environment.

When examining the potential risk of dental staff we can look are a number of studies based on influenza virus. Firstly there are a number of Randomised Controlled Trials that show that mask-wearing has no influence on ‘flu being caught by health workers. Secondly, the perceived exposure/risk of dental staff to aerosol is high in self-reported studies. The image below is USA data, which has been copied across to the UK by the ONS.


The data presented above suggest that dental staff are at daily high risk of exposure to disease. However, we need to look at how the data was gathered and analysed. First, to note it is a self-reporting questionnaire with small sample size, so small that the sub-sample size itself may be too small to produce a reliable estimate. Secondly, it is based on 2 questions in a 57 question work context questionnaire:

i) How physically close to other people are you when you perform your current job?

ii) How often does your current job require that you be exposed to diseases or infection?

So, influential conclusions on dental team risk are being based on responses to 2 questions of self- reported, self-selected sample. The numbers of staff responding to the questionnaire was small and possibly so small that the data is unreliable!

For actual data, we can look at the incidence of respiratory disease in the professions (ONS data analysed by Dr Mark-Steven Howe) presented below.

So although dentists and medics are acknowledged to work in very close proximity to patients the incidence of respiratory disease is extremely low, having 3.5 times LESS respiratory disease than non-healthcare peers.

We must conclude that the perceived risk is not borne out when examining the level of upper respiratory tract infections and that the pre-COVID cross-infection procedures were effective in both protecting staff and patients. In addition, across the world where dental clinics have not been closed, or have been reopened there are no reports in infection or deaths affecting the dental team occurring within the dental practice. However, there are numerous reports of patients suffering unnecessarily due to this action by the UK Government.

The overall conclusion of the ‘filtered information’ above indicates that:

1. That the ‘pre-COVID’ dental environment is an extremely safe one for patients and staff and we should be able to continue as normal with some minor changes.

2. That we should be proud of this; promoting dentistry as both safe and effective in its cross infection control.

3. UK dental practices should not have been closed and dentists been allowed to assess the risks and provide continuing care for their patients.


Mark- Steven Howe: covid-19/
Office for National Statistics (ONS) s/whichoccupationshavethehighestpotentialexposuretothecoronaviruscovid19/2020-05-11

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