Coronavirus disease (COVID-19) transmission through aerosols in restorative and endodontic practice: Strategies for prevention.

Rapid spread of coronavirus disease 2019 (COVID-19) in several countries of the world has created a state of public health emergency. COVID-19 is principally a respiratory disease, and the virus is present in respiratory secretions. Oral health-care professionals are susceptible to being infected with the disease since they work in close proximity to patient's face and oral cavity for long period of time. Restorative dentists and endodontists play a significant role in delivering "urgent" or "emergency" dental care to patients. Occupational Safety and Health Administration has categorized dentists performing aerosol-generating treatment procedures at "very high exposure risk" whereas the dentists not performing aerosol-generating procedures at "high exposure risk." Most of the restorative and endodontic treatment procedures involve generation of aerosols. Owing to the possibility of transmission of virus through aerosols, these procedures may transmit the disease to clinician or other patients. A comprehensive search of literature was conducted with the help of PubMed/MEDLINE and Scopus databases using a combination of terms, "COVID-19," "severe acute respiratory syndrome coronavirus 2," "aerosols," "restorative dentistry," and "endodontics." Along with universal precautions, some additional precautions need to be taken to prevent such transmission and cross-infection. This article reviews the research evidence about the role of aerosols in the transmission of COVID-19 and various measures which should be implemented during restorative and endodontic practice for the prevention of such transmission.

INTRODUCTION

Recent outbreak of coronavirus disease 2019 (COVID-19) has resulted in a public health crisis all over the world. The virus implicated in this disease “severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)” belongs to “Nidovirales” order and “Coronaviridae” family of viruses.[1] It is an RNA virus and is more likely to alter and mutate (similar to flu viruses) than that of DNA viruses. Current research has shown the diversity of the strains and genetic lineages of this virus.[2]

Primarily, COVID-19 is considered to be a respiratory disease. In addition to direct and indirect route of transmission, airborne transmission of the virus may occur in case of settings and environment where aerosol-generating procedures (AGPs) are performed such as a dental office. Currently, there is no evidence of airborne transmission of the virus through aerosols in absence of AGPs. Infected individuals who are yet to develop symptoms (called as “presymptomatic transmission”) as well as those never develop any symptoms can shed the virus and transmit the disease.[3]

Health-care professionals face a major challenge of identification, management, and prevention of transmission of COVID-19.[4] In consideration of the prevalent transmission of the virus and increasing cases of its transmission to the individuals involved in health care, oral health-care professionals are at high risk of the development of infection or acting as carriers of disease.[4] If adequate precautions are not taken in a dental office, cross-contamination may also result. The risk of transmission of the virus in a dental setting can be attributed to exposure of dentist to body fluids (like saliva and blood) during treatment, especially AGPs, and close contact with the patient (through face-to-face communication).[4] The spread may occur through droplet inhalation, direct contact with body fluids and mucosal surfaces, aerosols generated during treatment procedures or through fomites.[4]

Occupational Safety and Health Administration has classified the occupational risk into “very high, high, medium, and lower.” The dentists performing aerosol-generating treatment procedures are categorized as “very high exposure risk,” whereas the dentists not performing aerosol-generating procedures are categorized as “high exposure risk.”[5]

Restorative and endodontic treatment procedures form the majority of clinical work routinely carried out in a dental office. Although elective treatment may be postponed further during this period,[3] “urgent” or “emergency” dental care may be needed in clinical conditions like acute periapical abscess. Restorative dentists and endodontists play a major role in the management of such cases with severe pain or swelling. In view of these considerations, oral health-care professionals performing restorative and endodontic procedures are at very high risk for the development of COVID-19, if specific precautions are not taken, since most of these procedures involve generation of aerosols in a comparatively closed atmosphere.[3]

Although there is explosion of information available on online resources and social media, there is limited research evidence about various aspects of this disease. This leads to further confusion and anxiety in the minds of oral health-care professionals.[4] Hence, this article focuses on the possible role of aerosols in the transmission of COVID-19 in restorative and endodontic practice and the various preventive measures which need to be taken while performing these procedures.

AEROSOLS IN RESTORATIVE AND ENDODONTIC PRACTICE

In general, aerosols are particles (solid or liquid) suspended in the air, produced by human beings, animals, instruments, or machines. The aerosols containing any type of microorganism are called as “bio-aerosols.”[6] Aerosols may be in the form of “droplet nuclei” or “fine aerosols” with particle diameter <5 μm and “droplets” with particle diameter >5 μm.[78] The droplet nuclei can readily penetrate and reach down to the alveolar spaces, thus, resulting in the transmission of disease. Droplet nuclei can remain suspended in the air for hours and are able to travel over short to long distance before entering respiratory tract or falling on a surface. Droplets with diameter >20 μm usually follow ballistic trajectory, i.e., they mostly fall on the ground due to gravity. They suspend in air for relatively shorter period of time and are less likely to reach the lower respiratory tract.[9] The characteristics of aerosols are influenced by environmental factors such as humidity, temperature, and air flow.[6]

Most of the treatment procedures in Restorative Dentistry and Endodontics involve the use of high-speed air-turbine handpiece, including tooth preparation to receive direct or indirect restorative material, finishing and polishing of restorations, removal of restorative material, and endodontic access opening.[1011] These handpieces use compressed air to bring about rotation of the turbine at a high speed, along with a jet of running water.[12] The water provides cleansing effect, improves visibility, and most importantly, acts as a coolant, thereby preventing excessive build-up of heat produced from friction between bur and tooth structure or restorative material which can damage the dental pulp in case of vital teeth.[13] The rotary action removes the tooth substance/material/debris from the operative area in oral cavity, which in combination with the water coolant and oral fluids like saliva or blood leads to generation of large amounts aerosols which are expelled out at high speed.[1415] In addition, air abrasion and lasers commonly used for tooth preparation may also generate aerosols.[1617]

Aerosols in a dental setting were described and studied in details by Micik et al.[15] They designated the particles with diameter more than 50 μm as “splatter” and those with diameter <50 μm as “aerosol.” The aerosols and splatter may contain tooth substance particles, restorative material, lubricating oil, abrasive powder, oral fluids, and microorganisms.[16] During the procedures involving the use of high-speed air turbine handpieces for tooth preparation, 99% of the aerosols generated are in the range of “droplet nuclei,” i.e., diameter <5 μm, which have high potential for causing infection.[17] Colony-forming units (CFUs) of aerosols generated during endodontic procedures are greater than those during restorative procedures and also in case of longer treatment time. During endodontic procedures, the aerosols which are generated may reach up to 6 feet.[18]

TRANSMISSION OF SEVERE ACUTE RESPIRATORY SYNDROME CORONAVIRUS 2 THROUGH AEROSOLS

Dental aerosols are commonly contaminated with microorganisms from oral and respiratory tract secretions or dental unit waterlines water.[16] Aerosols containing saliva and nasopharyngeal secretions may transmit bacteria such as Mycobacterium tuberculosis or viruses such as influenza, ebola, herpes and coronaviruses such as SARS-CoV and Middle-East Respiratory Syndrome coronavirus.[9] Aerosols containing blood may transmit viruses such as hepatitis B, hepatitis C, and human immunodeficiency virus.[13]

Primary research work has shown that SARS-CoV-2 is present in the respiratory tract.[19] The saliva of the infected individuals also contains live viruses.[20] Heinzerling et al. reported a case in which COVID-19 was transmitted to health-care personnel from an infected hospitalized patient, who was subjected to AGPs during the period of hospitalization. This report indicated the possibility of transmission of SARS-CoV-2 through aerosols.[21] Another similar case report suggested the possible role of aerosols in transmission of the virus.[22]

The fact was highlighted in the results of experimental aerobiology studies by van Doremalen et al.[23] and Fears et al.[24] van Doremalen et al. found that SARS-CoV-2 remained viable in aerosols for at least 3 h (which was the duration of experiment). Infectious virus titer dropped from 103.5 to 102.7 50% tissue-culture infectious dose 50 per liter of air. The median half-life of SARS-CoV-2 in aerosol was about 1.1–1.2 h.[23] A major limitation of these studies was that the methods do not represent the actual clinical treatment procedures. More studies are needed to confirm whether COVID-19 is transmitted through aerosols, although the available case reports and studies suggest more likelihood of this possibility.[2526]

PREVENTION STRATEGIES

In general, the clinician should adopt various practices like COVID-19 screening and triaging of the patients, telemedicine, organization and duty rotation of dental office/hospital staff into small teams, restriction of movement and access to dental office/hospital, use of extraoral radiographs for diagnosis, strict infection control, and surface disinfection protocol.[27]

In addition, the clinicians should take precautions to avoid possible transmission of COVID-19 while performing restorative and endodontic treatment procedures that generate aerosols. In general, it may be recommended to avoid the aerosol-generating procedures. However, if the clinician chooses to perform such procedure, some specific prevention strategies should be undertaken. Post-COVID-19 restorative and endodontic practice will also necessitate the implementation of these preventive measures. These strategies may be classified based on the rationale behind it and different methods may be employed for the same [Table 1]. They are as follows:

Table 1

Strategies for prevention of coronavirus disease 2019 transmission through aerosols in restorative and endodontic practice

Prevention strategy	Rationale	Method
Personal protection	Barriers will protect the operator from aerosols being inhaled or deposited on the body surface	Personal protective equipment: Protective eyewear, masks/respirators, gloves, caps, face shields, and protective outwear
Reduction in viral load of aerosols	Antimicrobial mouthrinse will reduce the number of viruses in saliva and thus, in the aerosols generated	Preprocedural mouth rinse with antimicrobial agent
Reduction in backflow of handpiece	Antiretraction systems will prevent the backflow of infected saliva/body fluids into handpiece and dental unit water line, which could lead to recirculation of infected material	Antiretraction systems in high-speed air turbine handpieces or dental units
Reduction in aerosol generation and release into dental office environment	Amount of aerosols will be reduced if the intraoral operative site is isolated from saliva or the aerosols are suctioned immediately before being released into dental office environment	Use of electric handpiece
		Rubber dam isolation
		High-volume evacuator
		Extraoral suction
		Special devices (e.g., polycarbonate shield mounted to dental operating microscope)
Rapid elimination/filter of aerosols from the dental office environment	The infected aerosols escaped from high-volume evacuators or other methods and released into dental office environment may remain suspended in air and get transported along air. Such contaminated air should be eliminated rapidly	Negative-pressure treatment rooms/AIIRs
		Adequate natural ventilation
		Exhaust fans
		HEPA filters
Disinfection of air	Ultra violet irradiation will kill the viruses in the aerosols present in the dental office environment	UVGI system
Modification of treatment plan or procedures	Modifications of routine treatment plan or procedures will reduce the duration of contact with patient or will avoid/reduce the aerosols	Management of endodontic emergencies with palliative approach or long-term calcium hydroxide medicament
		Postponement of elective endodontic surgeries
		Restoration repair instead of replacement
		Minimal intervention approach: Chemomechanical caries removal, adhesive or bioactive restorative materials, self-etch adhesives, bulk-fill composites

AIIRs=Airborne infection isolation rooms, HEPA=High-efficiency particulate air, UVGI=Ultraviolet germicidal irradiation

Personal protective equipment

While performing intraoral restorative and endodontic treatment procedures generating aerosols, the splatter and aerosols usually radiate toward the face of the operator. The exposure of mucous membranes in eyes, nose, and oral cavity to aerosols containing SARS-CoV-2 may occur if these areas are not protected with a barrier.[28] Protective eyewear/goggles with solid side shields, full-face face shield, and respirator should be used by the clinician to protect the face. The protective eyewear and face shield should be disinfected adequately before the next use.[29] In addition, since the aerosols may get deposited on the clothes or exposed body surfaces of the operator, it is mandatory to wear a cap, long-sleeved gown, or protective clothing and gloves. Waterproof aprons should be used in case the gowns are not fluid resistant. The World Health Organization and Centers for Disease Control and Prevention (CDC) recommend the use of personal protective equipment for health-care professionals performing AGPs.[2930]

The diameter of SARS-CoV-2 is about 0.1 μm,[31] whereas most of the aerosols produced in a dental setting are <5 μm in diameter. Routinely used medical/surgical masks can provide up to 80% filtration rate against aerosols. However, the gaps owing to improper fit may reduce their effectiveness by 60%.[32] A study by health and safety executive compared effectiveness of different types of masks against live influenza bioaerosols.[33] They reported that medical/surgical masks provided 6-fold reduction, whereas well-fitted respirators provided up to 100-fold reduction.[33] N95 and FF3 respirators filter 95% and 99% of the particles with diameter up to 0.3 μm, respectively.[34] Although meta-analyses in systematic reviews do not suggest statistically significant reduction in risk of acute respiratory infections or laboratory-confirmed influenza/viral infections,[3536] WHO and CDC recommend the use of a particulate respirator offering protective effect comparable with that of the US National Institute for Occupational Safety and Health-certified N95, N99, European Union standard filtering facepiece (FFP) 2, FFP3, or equivalent for AGPs.[35362930].

Preprocedural mouth rinse

The saliva and nasopharyngeal secretions of the infected individuals contain live viruses, which are expelled out with splatter and aerosols.[20] If the number of viruses in these body fluids is reduced, the possibility of infection is comparatively decreased. In general, preprocedural rinse with an antimicrobial solution results in decrease in the number of oral microorganisms. Thereby, it also leads to reduction in microbial counts of aerosols.[37] A systematic review on effectiveness of preoperative mouthrinse in microbial reduction in aerosol reported that preoperative mouthrinse with chlorhexidine, essential oils, and cetylpyridinium chloride resulted in mean reduction of 68.4% CFUs.[38] Chlorhexidine may not be effective in the elimination of SARS-CoV-2.[39] Oral decontamination with 1%, 1.5%, or 3% hydrogen peroxide[40] and 1%, 1.5%, or 3% povidone iodine mouthrinse has demonstrated virucidal activity against SARS-CoV-2.[4142] CDC also recommends the preprocedural mouthrinse with an antimicrobial product.[30] Thus, the patient should be asked to gargle and rinse the mouth with hydrogen peroxide or povidone-iodine mouthrinse for 60 s. The process may be repeated after every 30 min.

Use of antiretraction handpieces

The high-speed air turbine handpiece commonly used during restorative and endodontic treatment procedures may lead to aspiration of infected oral fluids into internal chambers when it is stopped.[43] In addition, dental unit waterlines may also get contaminated from these fluids. The fluids containing microorganisms may be expelled out as aerosols during subsequent use of handpiece and may lead to cross-infection.[43] This phenomenon may be avoided by using antiretraction systems installed either within the handpiece (mechanical valve which blocks the exit of air and gets activated the turbine stops rotating) or dental unit. Such handpieces have demonstrated a significant reduction in aspiration of microorganisms into handpiece or dental unit.[44] The use of high-speed air turbine handpieces without antiretraction systems should be avoided.

Use of electric micromotor handpieces

Electric motor/speed increasing handpieces should be preferred over air turbine ones for tooth preparation and access opening as there is remarkable reduction in aerosols. They have shown to be as effective as air turbine handpieces for tooth preparation.[45]

Rubber dam isolation

Rubber dam isolates the intraoral operative site from saliva and other oral or nasopharyngeal fluids, which significantly decreases the amount of aerosols and microorganisms.[464748] In case of properly applied rubber dam, the only source of infected aerosols is the tooth being treated.[16] Leakage, if present, should be blocked using appropriate material or the leaking sheet should be replaced with a new one to prevent microbial contamination.[49] Rubber dam should be applied in every case during restorative and endodontic treatment. However, certain procedures where rubber dam cannot be used should be avoided, like very deep subgingival tooth preparation/restoration and endodontic surgery.

High-volume evacuation

High-volume evacuation system (with a large bore of 8 millimeters or greater), which is able to remove large volume of air (up to 100 cubic feet/min), suctions out the oral fluids and aerosols being released from oral cavity. It has proven to result in a significant reduction of splatter, aerosols up to 90%, as well as microorganisms.[50] The suction tip of the evacuator should be held close to the operative site for better efficiency.[51] Along with high-volume evacuation, extraoral suction systems may also be used to further reduce the amount of aerosols released.[52] Low-volume evacuators or saliva ejectors should not be used since they cannot suction large air volumes owing to smaller bore size and the backflow may occur, which increases the possibility of cross-infection.[5354]

Special devices

Special devices are being innovated to reduce the transmission of the virus. An example of such device is the one by Russell C, where polycarbonate shield mounted to dental operating microscope with an attached high-volume evacuation, was reported for use during endodontic treatment.[55]

Negative-pressure treatment rooms/airborne infection isolation rooms

Ideally, the treatment procedures leading to generation of aerosols should be carried out in negative pressure rooms or airborne infection isolation rooms.[7] A negative pressure room is one in which amount of mechanically exhausted air exceeds the amount of the mechanically supplied air. The air shall flow from cleaner areas (like corridors) into these rooms with potentially contaminated air. This shall prevent the spread of infected aerosols through the air.[5657] As per CDC guidelines, the pressure differential between the exhausted and supplied air should be at least 0.01-inch water gauge or 2.5 Pascals (Pa). The CDC recommends a ventilation rate of 6–12 air changes per hour (ACH) for effective ventilation (12 ACH for new construction).[58] However, most of the dental offices today do not operate in negative pressure rooms.

High-efficiency particulate air filters

In a dental office with natural ventilation, it should be ensured that the contaminated air gets exhausted directly outside the office, away from the portal entry of outdoor air or people. A ventilation rate of 160 l/s per patient (L/s/patient) is recommended by the WHO.[59] In case of inadequate natural or mechanical ventilation, exhaust fans or high-efficiency particulate air (HEPA) filters should be installed.

If the air cannot be exhausted out adequately, it should be subjected to HEPA filter before re-circulation. HEPA filters are capable of capturing 99.97% of airborne particles with a diameter ≥0.3 μm.[60] They can effectively reduce the infected aerosols in a single space.[616263] They can be ceiling-mounted or portable in nature. Re-circulation of all of the air in the office is desirable for effective filtration, with minimum 2 ACH.[64] HEPA filters should be positioned behind the operator, i.e., the operator should not be positioned between HEPA filter and the patient's oral cavity.[30]

Ultraviolet germicidal irradiation

Air disinfection with upper-room ultraviolet germicidal irradiation (UVGI) can be used an adjunct to other methods.[30] It is based on UV-C light of wavelength 253.7 nm. It has the ability to inactivate viruses in the aerosols. UVGI is effective at low ACH and with increase in ACH, its efficacy drops down, since its ability to kill the microorganism depends on the exposure time.[65]

Modification of treatment plan or procedures

Routine treatment plan or procedures should be modified to reduce the possibility of transmission of COVID-19. Examples of such modifications in restorative and endodontics treatment plan or procedures include as follows:

Management of endodontic emergencies

Following 2 approaches may be used for the management of endodontic emergencies:

Palliative approach – pharmacological management (analgesics/antibiotics) or nonaerosol generating procedures (such as incision and drainage, excavation of caries using hand instruments)

Initiation of endodontic treatment – in few cases, partial or complete removal of pulp may be needed. In such cases where the root canal treatment has been initiated, calcium hydroxide intracanal medicament should be placed. The obturation may be postponed.

A recent study reported a success rate of 83% with palliative approach and 77% with long-term calcium hydroxide placement.[66]

Postponement of elective endodontic surgeries

Elective endodontic surgeries should be postponed and the patients managed using palliative approach until then.

Restoration repair instead of replacement

Defective restorations should be carefully assessed for possible repair using an adhesive restorative material and replacement be avoided since it involves aerosols generation and increases the treatment time.[67]

Minimal intervention approach

In general, minimal intervention approach should be followed to reduce the need for tooth preparation and treatment time. The use of chemomechanical caries removal, adhesive, or bioactive restorative materials should be preferred. To reduce the treatment time, self-etch adhesives and bulk-fill composites may be used. In addition, with self-etch adhesives, as rinsing is not required, aerosols generation is avoided.[67]

CONCLUSION

COVID-19 has posed many challenges for oral health-care professionals in view of risk of transmission of virus and possible cross-infection. SARS-CoV-2 is present in the saliva and nasopharyngeal secretions of infected individuals. The clinician should be aware of the risk of infection and undertake adequate measures to prevent it like personal protection, preprocedural mouthrinse with antimicrobial agent, and using antiretraction or electric handpiece. Rubber dam isolation and high-volume evacuation lead to a significant reduction in the generation of aerosols. The ventilation of the dental office should be adequate, otherwise, exhaust fans or HEPA filters should be installed. Treatment plan and procedures should be modified to reduce the possibility of virus transmission. Combined use of all suitable methods of prevention will protect the operator as well as patients from COVID-19.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.