The null hypothesis was rejected, in that significant differences were found between the results of the amount of aerosol particle count with and without a High Volume Extraction device.
The purpose of the present study was to measure the particle count during dental aerosol procedures and compare the results to when a High Volume Extraction device is used. The reason why this is of particular importance in current dental practice is the risk of transmission of the SARS-CoV-2 virus through aerosol particulate.
SARS-CoV-2 Viral Diameter and relevance to particulate sizing.
Zhu et al have discussed "Electron micrographs of negative-stained 2019-nCoV particles were generally spherical with some pleomorphism. Diameter varied from about 60 to 140 nm." Using this diameter and assuming the virus is a sphere we can therefore assume that as the virus is approximately a thousandth of the aerosol particulate in this study, any PM1, PM2.5 or PM10 sized aerosol particulate produced may carry the virus and therefore potentially transmit the disease if inhaled.
PM1 sized particulate generated;
In the present study, the PM1 sized particulate generated with all procedures appears to remain within the range sampled within the control measurements with no procedure taking place. However, outside of the 3in1 procedure, there was a statistically significant reduction in particle count when an external high volume extraction device was used.
PM2.5 sized particulate generated;
There was a clear increase in PM2.5 particulate generated during the dental procedures. This statistically significant increase was approximately double the normal background sample measurements.
With the use of an external high volume extraction HVE device, the samples taken during the five dental procedures were statistically significantly reduced. There was a slight increase in the measured levels towards the end of the air turbine procedure but this was statistically insignificant (11 µg/m3 with HVE versus 24 µg/m3 without HVE).
PM10 sized particulate generated;
The PM10 sized particulate generated in each of the procedures followed a similar, yet amplified, pattern to the PM2.5 sized particulate generated.
The maximum levels of particulate generated were approximately three fold the background levels of PM10.
With the use of an external high volume extraction HVE device, the samples taken during the five dental procedures were statistically significantly reduced.
We can therefore interpret these results as meaning none, or very little PM1 particulate is generated during 3in1 dental procedures as the difference was statistically insignificant.
There is a statistically significant increase of between two and three times the background µg/m3 levels of PM2.5 and PM10 sized particulate compared to the results recorded during dental procedures without an external HVE used.
One limitation in the interpretation of these results is the biological relevance with regards to the SARS-CoV-2 virus. This virus is new and relative infectivity and mechanisms of transmission are currently under investigation.
In the SARS outbreak in 2003, Kan et al. studied the relationship between particulate levels with mortality. The study showed that PM with aerodynamic diameter 10 m (PM10) were positively associated with the SARS mortality.
It has also been reported that aerosols associated with highly virulent pathogens like SARS could travel more than two metres. Whilst not of dental origin, Feng et al carried out an ecologic analysis that found a positive relationship between PM2.5 particulate count and viral transmission in Beijing .
Small aerosols have more potential to be inhaled deeply into the lung, which can potentially cause infection in the alveolar tissues of the lower respiratory tract.
Recent research work has been focusing on providing a better understanding on aerosol and droplet transmission, which has provided evidence that aerosols may play a major role in transmitting the SARS-CoV-2 virus.[41,42]
Duguid et al. studied the number of droplets and aerosols generated sizes 1–100 µm and found that coughing and sneezing produced from a few to a few hundred droplets and aerosols per cubic metre.
However, controversy remains among many researchers as to the modes of transmission via droplet or aerosol and the quantifiable risk associated with levels of either.
In the present study aerosol particulate was recorded at statistically significantly increased levels during dental procedures without an external high volume extraction device versus with the device. These increased levels were around two dozen µm per cubic metre.
Minimizing aerosols and splatter
Recent studies have shown that the SARS-CoV-2 virus can be transmitted via particulate matter . Historically, previous studies have shown that there is a positive correlation between PM2.5 and larger particulate sizes, and transmission of virus such as influenza. The Centres for Disease Control and Prevention (CDC) carried out a study which showed that the Sars-CoV-2 virus was able to remain viable for upto 72 hours on some surfaces(32)
One could argue that the risk of transmission in the dental setting via aerosols is minor as the aerosol droplets may be entirely from the waterline. However when we look at a study on splatter and aerosol generated from an ultrasonic scaler without any coolant water used in vitro, there was still a substantial number of aerosol and splatter formed from small amounts of liquid placed at the operating site to simulate blood and saliva.  We therefore have to look at ways to mitigate this risk during the current sars-cov-2 crisis.
Harrel et al wrote that “no single approach or device can minimize the risk of infection to dental personnel and other patients completely. A single step will reduce the risk of infection by a certain amount, another step added to the first step will reduce the remaining risk, until such time as the risk is minimal.” This is a sensible approach to provide layered protection for risk mitigation.
Harrel also discusses that in the reduction of dental aerosols, the first layer of defense is personal protection barriers such as masks, gloves, visors, safety goggles and hair nets. The second layer of defense is the routine use of an antiseptic preprocedural rinse with a mouthwash such as peroxyl/povidone iodine or chlorhexadine. The third layer of defense is the regular use of a high volume evacuator (HVE) either by an assistant or attached to the instrument being used. An additional layer of defense could also be the employment of a tool to scale back aerosol contamination that escapes the operating area, such as a HEPA filter. These extra layers of defence are either commonly found or easily implemented in most dental practices.
It has also been recommended that dental practices install negative pressure airflow to prevent airborne transmission through aerosols. Some, including Harrel et al, have suggested that using a 0.2% Chlorhexidine or Listerine mouthrinse pre-operatively could be of benefit as they have been shown to reduce the oral bacterial load in aerosols, but there are no high quality peer reviewed studies on the virucidal activity of Hydrogen Peroxide. Iodine has been hypothesided to be of greater value for this purpose than chlorhexidine 
The correctly placed high volume vacuum suction and evacuator near the handpiece and the mouth can reduce 90% of the output of aerosol.[24,53] During conservative practices, use of the rubber dam barrier is also thought to reduce the risk significantly upto 98.5 %  The results for the present study confirm these figures whilst also benefitting the surgeon in that the extra oral High volume extraction unit does not require an assistant to maintain position.
To prevent the risk of transmission, especially during the sars-cov-2 pandemic, high risk personal protective equipment has been advised worldwide to varying degrees. In the United Kingdom there has also been an advisory in place for a fallow time after aerosol generating procedures. Both the protective equipment and fallow period are a large departure from the clinical norm and could impact the sustainability and running of dental clinics. Reducing the need to depart from the norm could improve patient access through the reduced wait times post procedure and improve comfort for the operator.