Using respirators to prevent SARS-CoV-2 transmission: evidence review
Adam Nulty provides an evidence-based literature review on the use of personal protective equipment (PPE) within the COVID-19 pandemic and beyond.
In epidemics of highly infectious diseases, such as Ebola, SARS and now the SARS-CoV-2 coronavirus, those working in the dental setting reportedly stand at greater risk than the general population. This is due to their close contact with patients. Also, their potentially contaminated bodily fluids (Office for National Statistics, 2020).
To be effective and also practical, PPE (such as masks and respirators) must prevent respiration of the bacterial or viral particulate. It should also be comfortable. The use of respirators requires compliance to be effective (Nichol et al, 2008).
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) may make work more difficult and uncomfortable. This is thought to eventually lead to an increased, rather than decreased, risk of infection. As they increase the burden on the clinician or worker to perform (Roberge, 2008).
It is therefore important to provide understanding and clarity of the scientific reasoning for current PPE protocols. The standard operating procedures of dental practices have incorporated these measures.
In the UK, the Control of Substances Hazardous to Health Regulations 2002 covers exposure to hazardous chemicals. It also covers exposure to 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’ (HSE, 2004).
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:
- CE marked to the European PPE Directive
- Selected, used and maintained by properly trained people
- Correctly maintained, examined and tested
- Correctly stored.
The British Standard EN 149:2001 covers disposable filtering facepiece (FFP) respirators. FFP respirators are classified according to the level of protection afforded as assessed by specified laboratory tests, with FFP3 offering the most protection (HSE, 2008).
The European PPE Regulation Guidelines cover respiratory protective equipment (RPE). This directive excludes surgical masks and they are not certified for use as RPE in the UK. Surgical masks can be certified compliant with the Medical Devices Directive and be ‘CE’ marked. However, the placing of a ‘CE’ mark on a surgical mask does not denote the ability to provide respiratory protection under the PPE Directive (EU; 2018; BSI; 2019).
Surgical masks do provide a degree of protection against droplets and splashing. The British Standard covering surgical masks (BS EN 14683:2005) states that: ‘The surgical masks intended to be used in operating theatres and health care settings with similar requirements are designed to protect the working environment and not the wearer. When the primary intention is to protect the wearer from infection, the use of respiratory protective devices should be considered’.
Requirements by British-Adopted European Standard BS EN 149:2001 and NIOSHH standards are such that respirator masks have the following protection:
- N95/FFP2 – maximum permitted inward leakage: 8%. Minimum filter efficiency: 94%
- N99/FFP3 – maximum permitted inward leakage: 2%. Minimum filter efficiency: 99%
It may seem sensible in the time of a pandemic to use the maximum filter efficiency type respirator mask. N99/FFP3 masks are inherently uncomfortable. They can become intolerable in long periods of wear (Locatelli, LaVela and Gosch, 2014). Many clinicians found this, as the pandemic has progressed.
Respirator masks require the use of a fit test to confirm efficiency and breathing ability. However, healthcare workers may find difficulty in achieving suitable generic sizing to sufficiently fit. Studies of respirator mask fit in workforces have shown large differences in how well the respirator mask fits. In one study, only 36-62% of staff passed the fit test required to confirm effectiveness to N99/FFP3 standard due to variance in facial shape (Foereland, Robertsen and Hegseth, 2019).
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 (Cherrie et al, 2006).
The evidence base for face mask usage
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 versus FFP3) may not lead to more contamination, but may have greater user satisfaction (Verbeek et al, 2019).
Another recent Cochrane review on the reopening of dental services from 16 international sources discusses 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’ (CoDER Working Group, 2020).
It is therefore clear that dental professionals working in practice are to wear appropriate levels of PPE dependent on the type of care provided. This is even more paramount as case numbers have increased in the SARS-CoV-2 pandemic.
Face masks make up one component of barrier protective equipment, the others being eyewear, gloves, face shields and protective outerwear.
The use of standard fluid-resistant surgical masks (FRSM), or FFP2 (N95) masks or indeed FFP3 (N99) masks and their relative effectiveness is widely debated.
The current standard operating procedure from NHSE advises the use of FFP3 (N99) masks in all Urgent Dental Care Centres. This was understandable given the available information about SARS-CoV-2 at the time.
Face masks were introduced to minimise post-surgical infection in patients from exhaled microbes shed by the surgical team (Hogan and Samaranayake, 1990).
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 (Musher, 2003).
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, ie through airborne droplet transmission (Yu, Li and Wong, 2004).
Therefore for standard, non-aerosol-generating procedures (non-AGPs), standard three-ply surgical masks have been shown to be as effective as respirator masks (Greenhalgh and Chin, 2020).
Furthermore, for non-AGPs, there is no evidence that respirator masks add value over standard masks. Especially when both are used with the recommended wider PPE and risk reducing measures as outlined in this guidance (Long et al, 2020). However, this evidence may be used to provide confidence to both staff and patients. Ultimately, underlining the benefits of wearing masks in social or waiting room areas.
A recent meta-analysis of N95 respirators versus surgical masks in the prevention of infection transmission of influenza showed that while 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 (Smith et al, 2016). However, N95 respirators have been associated with less filter penetration. Also, with face-seal leakage compared with surgical masks (Radonovich et al, 2019).
Overall, there is a paucity of data and studies comparing surgical masks and N95/FFP2 or N99/FFP3 respirator masks. Highlighting the 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 (Seto, Tsang and Yung, 2003) and SARS (Teleman et al, 2004).
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 30 times greater when compared to surgical masks (Wen, Yu and Yang, 2013; Lee, Grinshpun and Reponen, 2008.)
Liverman and colleagues (2009) developed a report and made conclusions and recommendations on behalf of the Institute of Medicine (IOM) Committee on Respiratory Protection for Healthcare Workers in the Workplace against the Novel H1N1 Influenza A.
They describe a cluster randomised clinical trial that was conducted to compare the clinical efficacy of standard surgical masks versus N95/FFP2 masks with and without fit testing, versus control in influenza transmission in 1,936 healthcare workers in China. The N95/FFP2 masks were found to have statistically significant efficacy, while surgical masks showed no efficacy. Overall conclusions were difficult to draw as the authors stated that they did not have the full details of the study.
Compliance and face mask usage
In comparing standard surgical masks with the efficiency of N95/FFP2 or N99/FFP3 masks, it is also important to reflect on user compliance. It is feasible that the improved efficiency of an N95/FFP2 or N99/FFP3 respirator over a standard surgical mask may be reduced. Only if compliance is poor due to comfort in long clinical sessions.
The very small gains offered by N99/FFP3 over N95/FFP2 are negated by the increased risks introduced through poor wearer comfort. Due to the tighter fitting respirator that is more difficult to breathe through (Rebmann, Carrico and Wang, 2013). 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 reusable. However, it is important that these reusable masks have a clear and medically certified method for cleaning/sterilisation.
The SARS-CoV-2 virus has been shown to have the ability to remain active on the respiratory surface for up to three days on plastic (Van Doremalen, Bushmaker and Morris, 2020). Hence their repeated use should be used in combination with a medically certified method of sterilisation.
It is also clearly important to understand whether there is any increased risk of transmission from an AGP. 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.
Yamada and colleagues did a study (2011) to understand the aerial dispersal of blood-containing aerosols in various dental procedures. It was clear that without high volume suction there is a dissemination of the blood within the operating area. It can be concluded that the relevance of the findings with regards to respiratory pathogens is likely to be insignificant.
However, a study by Rautemaa and colleagues (2006) 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.
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 spatter-reduction effectiveness. Also, aerosol containment of eight dry-field isolation techniques. This study makes it clear that spatter and aerosols appear reduced in the region of 99.7% when high volume evacuation (HVE) devices (Ravenel et al, 2020).
N95/FFP2 masks come in a variety of shapes and sizes. Fit testing is required to determine if a particular size or model provides an acceptable fit for the individual. The benefits of N95/FFP2 respirator masks used in aerosol generating procedures will be negated if the mask does not form a tight seal around the face and nose. Therefore fit testing is required.
A fit test is conducted by a trained fit tester to verify that the respirator device performs as intended. It should be both comfortable and effective. Fit tests are either qualitative or quantitative.
A pass/fail test method using a hood over the head of the staff member. The test uses the staff member’s sense of taste or smell to detect a test substance introduced into the surrounding air space. This can include:
- Isoamyl acetate, which smells like bananas
- Saccharin, which leaves a sweet taste in the mouth
- Bitrex, which leaves a bitter taste in the mouth
- Irritant smoke, which causes coughing.
Qualitative fit testing does not detect the amount of leakage. The pass/fail result is solely dependent on the detection of the test substance.
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. This allows connection to the machine through a hose.
Self-seal test and facial hair
A staff member should conduct a user seal check using the N95/FFP2 face mask upon donning. This will confirm positive and negative pressure seal.
During positive pressure seal check: the staff member exhales gently while 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 while maintaining pressure blocking the paths of air escape.
In order to achieve the best levels of protection, it is strongly advised that employees are clean shaven in the area of the seal before they begin their shift or take a face fit test (HSE, 2014).
In principle, surgical masks, 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 (HSE, 2008; World Health Organization, 2020), or 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 (World Health Organization, 2020).
When considering the above literature and current evidence base, the conclusion is that a basic minimum standard of protection is surgical face masks. AGPs should involve the use of N95 (FFP2) masks. This will further protect the respiratory tract if no HVE device is used. If a HVE device is used there should be no requirement to the use of N95(FFP2) or N99(FFP3) respiratory masks.
There are many challenges that a dental practice will need to overcome to minimise risk of transmission of the virus. Especially as the virus may be in general population for the long term.
The majority of 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, while 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 summary, from the evidence available, it is apparent that in the lab setting N95/FFP2 masks are more efficient. But in the clinical setting such a difference is not seen as clearly.
Faced with the emergence of the COVID-19, it is logical to use FFP2/N95 respirator masks in AGPs. 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 N95/FFP2 or N99/FFP3. Balanced with the extra risk of compliance in wearing a standard fluid-resistant surgical mask in the long term. As well as the risk of , cost and comfort.
This review does not include a discussion of the extra clinical waste produced with current PPE protocols. This is worthy of further discussion. As the environment is an important factor that must be considered in the long-term.
The author acknowledges that there are many issues and risk factors. A practice will undoubtedly examine these in their decision-making process. Depending on the country, this will aid in forming clinical operating procedures. As the pandemic progresses, more clinical and research data will becomes available worldwide. We may find that the universal precautions undertaken, appear quite sufficient. Also, the clinician’s 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 practise in a timely and efficient manner. Above all, in the best interests of patients and staff.
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