A Review of Evidence: Using Respirators to prevent Sars-Cov-2 Transmission

This literature review has been compiled to form an evidenced-based review on the standards for Dental Practices in their choice and use of personal protective equipment (PPE) within the COVID-19 Pandemic and beyond: it is prepared on the basis of the current best available evidence. The review encompasses risk management strategies for both Dental Personnel and Patients in the application and use of Face Masks & Respirators. In summation, from the evidence available, it is apparent that in the lab setting N95/FFP2 masks are superior in their efficiency but in the clinical setting such a difference is not seen as clearly. As such the minimum standard of care should be that of a standard surgical mask. Faced with the emergence of the virulent disease that is Covid-19, it is logical to use FFP2/N95 respirator masks in aerosol generating procedures where they offer greater resistance to fluid penetration and a better face seal when adequately fit tested as a gold standard. But if a dry field isolation technique involving high volume evacuation is used, there is no clear benefit of respirator masks (N95/FFP2 or N99/FFP3) when balanced with the extra risk of compliance, cost and comfort in wearing a standard fluid-resistant surgical mask.


Introduction
In epidemics of highly infectious diseases, such as Ebola, SARS and now the Sars-COV-2 Coronavirus pandemic, dental professionals and healthcare workers in the dental setting are reported to be probably at greater risk than the general population, due to their close contact with patients and their potentially contaminated bodily fluids. 1 Figure 1. Respiratory droplets released through talking, coughing, or sneezing The be effective and also practical, PPE such as masks and respirators must prevent respiration of the bacterial or viral particulate but should also be comfortable. The use of respirators requires compliance to be effective. 2 When comfort is lacking, overprotection can introduce problems. For example, in proposing the use of masks of higher rating in some scenarios, these higher rated masks such as N99/FFP3 masks may make work more difficult and uncomfortable and eventually lead to an increased, rather than decreased, risk of infection as they increase the burden on the clinician or worker to perform. 3 The Control of Substances Hazardous to Health Regulations 2002 (COSHH) in the United Kingdom, covers not only exposure to hazardous chemicals but also biological agents. The regulations assert that 'every employer shall ensure that the exposure of his employees to substances hazardous to health is either prevented or, where this is not reasonably practicable, adequately controlled 4

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Where PPE is employed as a control measure then it should be 'adequate' for the anticipated exposure levels and 'suitable' for the task, for the environment and for the wearer. In addition the PPE must be: Another common reason for breach of the PPE barrier is sweating and poor breathing ability causing the worker to wipe their face or adjust the PPE visor or mask. 10 A Cochrane Systematic Review of Personal protective equipment for preventing highly infectious diseases due to exposure to contaminated body fluids in healthcare staff found very low quality evidence that more breathable types of PPE (eg. FFP2 vs FFP3) may not lead to more contamination, but may have greater user satisfaction. 11 Another recent Cochrane review on the re-opening of dental services from 16 international sources discuss the most common approach to the types of Masks and respirators used. "Most sources recommend filtering facepiece class 2 (FFP2, equivalent to N95) masks for non-COVID-19 cases undergoing AGPs and all suspected or confirmed COVID-19 cases undergoing any procedure." 12 It is therefore clear that dental professionals working in practice are to wear appropriate levels of PPE dependent on the type of care provided. Face masks were introduced to minimise post surgical infection in patients from exhaled microbes shed by the surgical team. 13 As healthcare has evolved, so has the realisation that the face mask protects the healthcare worker in equal respect. The transmission of airborne infection depends on the virulence and particulate viral load respirated. 14 In the case of SARS-Cov-2 coronavirus, the viral transmission is believed to be of the same route as that of coronavirus-induced pneumonia leading to SARS i.e. through airborne droplet transmission. 15 Therefore for standard, non-aerosol generating procedures (non-AGPs), standard 3-ply surgical masks have been shown to be as effective as respirator masks. 16 Furthermore, for non-AGPs, there is no evidence that respirator masks add value over standard masks when both are used with the recommended wider PPE and risk reducing measures as outlined in this guidance. 17 In a recent meta-analysis of N95 respirators vs Surgical masks in the prevention of infection transmission of influenza, showed that whilst in the laboratory setting the N95 mask may have the benefit of smaller particle filtration, there is insufficient data to definitively determine superiority in clinical settings. 18 However, N95 respirators have been associated with less filter penetration and face-seal leakage compared with surgical masks. 19 Overall there is a paucity of data and studies comparing surgical masks and N95/FFP2 or N99/FFP3 respirator masks in mitigating the risk of transmission of the SARS-Cov-2 virus from the patient to the clinical team. However there is data that compares these masks in their protection against the influenza virus 20 and SARS 21 .

The Evidence Base for Face Mask Usage
When the overall performance of surgical masks, N95 masks and N99 masks are compared in their performance against viral aerosols, studies have demonstrated that the protection factor of N95 was thirty times greater when compared to surgical masks. 22   The very small gains offered by N99/FFP3 over N95/FFP2 are negated by the increased risks introduced through poor wearer comfort (touching mask to adjust, breathing difficulties, creation of fomites, skin damage and face sores) due to the tighter fitting respirator that is more difficult to breath through. 25 The greater comfort of the N95/FFP2 respirator therefore makes it the mask of choice for dental AGPs. Some N95/FFP2 masks may be commercially available and certified as re-usable. However, it is important that these re-usable masks have a clear and medically certified method for sterilisation.
The Sars-COV-2 virus has been shown to have the ability to remain active on the respiratory surface for upto three days on plastic 26 and hence their repeated use can only be recommended when used in combination with a medically certified method of cleaning and sterilisation.
It is also clearly important to understand whether there is any increased risk of transmission from an aerosol generated procedure, if extra precautions such as high volume suction devices are employed ie whether there is any overall benefit to the use of N95/FFP2 respirator masks in these AGP procedures. If aerosols can be actively reduced, then the risk of transmission would therefore reduce in turn.
In a study by Yamada et al 27 to understand the aerial dispersal of blood-containing aerosols in various dental procedures, the study was clear that without high volume suction there is a dissemination of the blood within the operating area but it can be concluded that the relevance of the findings with regards to respirator pathogens is likely to be insignificant.

However, a study by Rautemaa et al does provide evidence that procedures involving high-speed
and ultrasonic dental instruments can cause significantly greater contamination than non-agp procedures and also provides weak evidence of these procedures generating small inhalable aerosols. 28 There is a lack of studies on methods of reducing these aerosols -such as high volume suction -and their relation to transmission risk with respiratory pathogens. We need to consider this to enable a fair and evidence based assessment of the need to employ greater protection during AGP procedures. A paper from the MUSC research team in the United States has studied an evaluation of the -Quantitative A qualitative test uses a machine to measure the actual leakage of the respirator mask. The mask type used requires a probe attachment to be fitted or pierced into the mask to allow connection to the machine through a hose.

Self Seal-Test
A user seal check should be conducted by the staff member using the N95/FFP2 face mask upon donning, to confirm positive and negative pressure seal.
-During positive pressure seal check, the staff member exhales gently whilst blocking the paths of air to escape and exit the facepiece. On slight pressurisation, there should be no leakage.
-During a negative pressure seal check, the staff member inhales sharply whilst again maintaining pressure blocking the paths of air escape

Facial Hair
In order to achieve the best levels of protection, we strongly advise that employees are clean shaven in the area of the seal before they begin their shift or take a face fit test. 30

Overall Guidelines for Mask & Respirator Usage
In principle, surgical masks that are worn correctly should provide adequate protection against large droplets, splashes and contact transmission. They may also reduce to some degree any residual aerosol risk, although this level of protection might not sufficiently reduce the likelihood of transmission via this route. Consequently they should not be used in situations where close exposure to infectious aerosols is likely, 31,32 when better fitting respirator type 2 (N95/FFP2) masks are available.
Importantly, masks and respirators should not be considered as isolated interventions. Other protection includes hand hygiene, aprons or gowns, goggles or face shields, and gloves.

Conclusion
There are many challenges that a Dental Practice and individual Dental Practice staff members will need to overcome to minimise risk of transmission of the Sars-COV-2 virus.
The vast majority of Dental Practice owners will want to maintain treatment provision to that which was routinely provided before the escalation and lockdown that occurred with the COVID-19 pandemic. All Dental Professionals will be eager to serve their patients to the best of their ability, in patients' best interests, whilst using current evidence based guidance to create a Standard Operating Protocol for their clinic. Respiratory protection is one part of a systematic multi-pronged infection prevention and control strategy.
In summation, from the evidence available, it is apparent that in the lab setting N95/FFP2 masks are superior in their efficiency but in the clinical setting such a difference is not seen as clearly. The author acknowledges that there are many issues and risk factors which a practice will undoubtedly examine in their decision making process to form clinical operating procedures. As the pandemic progresses and more clinical and research data becomes available worldwide, we may find that the universal precautions that we have always undertaken, which include surgical masks, face shields and high volume suction, will be sufficient and the clinicians choice of risk reduction through the use of surgical mask or respirator must then be reviewed accordingly.
Through the adoption of the protocols outlined in this literature review and other evidence based guidance issued in their respective country, these dental practices will be able to both minimise risk and also practice in a timely and efficient manner and above all, in the best interests of patients and staff.

Conflict of interest statement
There is no conflict of interest to declare.
The author has no financial interests or connections, direct or indirect, that might compromise the perception of the authors as impartial. There is no financial interest that includes commercial or other sources of funding for the author or associated department(s) or organization(s), personal relationships, or direct academic competition.

Author Contributions
Author: Adam Nulty Contributed to conception, design, data acquisition and interpretation, drafted and critically revised the manuscript All authors gave their final approval and agree to be accountable for all aspects of the work.