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 Table of Contents  
ORIGINAL HYPOTHESIS
Year : 2021  |  Volume : 12  |  Issue : 3  |  Page : 161-167

Potential for Spatial Laminar Airflow to Prevent Interdental-Chair Contamination in Multichair Dental Operatories


1 Department of Periodontics, College of Dentistry, University of Sulaimani, Iraq
2 Facilities Management Department, The American University of Iraq-Sulaimani
3 Department of Preventive Dentistry, College of Dentistry, University of Sulaimani, Sulaimani, Iraq

Date of Submission21-Nov-2020
Date of Decision13-Feb-2021
Date of Acceptance05-Mar-2021
Date of Web Publication2-Nov-2021

Correspondence Address:
Dler Ali Khursheed
Department of Periodontics, University of Sulaimani, Sulaymaniyah, Kurdistan Region
Iraq
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/denthyp.denthyp_47_21

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  Abstract 


Introduction: The dental clinic has for a long time been considered as a risky place for infection dissemination. Due to aerosol generating procedures, the risk of cross-transmission in dental clinics has recently risen. Open dental clinics should undergo present-time reassessment concerning infection control, in particular, to consider the implications of the SARS-CoV-2 pandemic for today’s advanced technological and medical practices. It might be necessary to make urgent and appropriate modifications to the design of air circulation systems in the dental environment to prevent microbial transmission. The Hypothesis: In order to minimise cross-transmission in multi-chair dental operatories, we have designed two model ventilation systems with 12 and 36 air change/hour capacities and with laminar airflow direction. The conditioned air directly blows into the dental treatment units, especially into the aerosol generating area, where the contamination is more concentrated. We hypothesise that these new designs could serve to isolate dental treatment units to function separately like closed dental operatories while keeping them open to each other. Evaluation of the Hypothesis: Thorough physical and biological investigations will be required to determine how these designs can be applied effectively in terms of the required spatial separation of dental treatment units in the open multi-chair dental operatories.

Keywords: aerosols, COVID19, dental clinics, infection control, laminar airflow, SARS-CoV-2


How to cite this article:
Khursheed DA, Muhsin BM, Rauf AM. Potential for Spatial Laminar Airflow to Prevent Interdental-Chair Contamination in Multichair Dental Operatories. Dent Hypotheses 2021;12:161-7

How to cite this URL:
Khursheed DA, Muhsin BM, Rauf AM. Potential for Spatial Laminar Airflow to Prevent Interdental-Chair Contamination in Multichair Dental Operatories. Dent Hypotheses [serial online] 2021 [cited 2021 Dec 7];12:161-7. Available from: http://www.dentalhypotheses.com/text.asp?2021/12/3/161/329763




  Introduction Top


The multi-chair dental operatory (MChDO) can be defined as a room that contains more than one dental chair and in which either similar dental treatments are provided, as in the dental school setting by dental students, or where different treatments are provided by such as dentists in public healthcare centres or polyclinics. These clinics are equipped with dental chairs that are spaced about 2 m apart as recommended by the Centers for Disease Control and Prevention (CDC) (https://www.cdc.gov/coronavirus/2019-ncov/hcp/dental-settings.html). The dental chairs also share the same water and air pressure line sources. They may be separated with partitions or open to each other under the same ventilation conditions. In the dental school environment, MChDOs are designed to enable academic supervisors to move around easily to oversee the work of dental students.

The water/air pressure from dental handpieces (sonics and ultrasonics, slow and high-speed handpieces) and dental polishing procedures undoubtedly produces substantial amounts of bioaerosols which are composed of water, saliva, blood, bacteria and viruses, fungi, and tooth particles.[1],[2] Owing to the closeness to each other of dental chairs in the open MChDO, during working hours the patients and dental health care workers (DHCWs) may be exposed to risk. Studies have shown that air contamination can travel for several meters from working dental chair areas, with higher concentration around the patient’s mouth,[3],[4],[5] as demonstrated by cultures of bacteria and fungus obtained from the contact eye lenses of dental operators.[6] Infection agents can be transmitted directly through bioaerosols and infected instruments and indirectly through contaminated fomites.[3],[7] On the other hand, some environmental factors may further enhance microbial survival and transmission, such as airflow, humidity, temperature, and solar radiation.[5],[8] In this kind of environment, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission is possible where there are confined spaces, prolonged exposure, and high concentrations of virus in the air.[9]

As patients have their mouth and nose open during aerosol generating procedure (AGP), they may contract infections from each other and from the growth or accumulation of bioaerosols of past treated patients in the MChDO.[10] Meanwhile, dental students’ lack of knowledge or poor compliance with infection control guidelines and lesser skill in handling dental handpieces and suction tools during dental procedures may also contribute to higher bioaerosols generation.[11],[12],[13] Furthermore, it is the case that viruses can be transmitted more easily than bacteria because they are smaller and lighter than bacteria.[5] Although reliable information about virus transmission in the dental clinic is lacking,[10],[14] coronavirus disease 19 (COVID-19) disease, which is caused by SARS-CoV-2, is considered as an airborne disease (https://www.cdc.gov/coronavirus/2019-ncov/more/scientific-brief-sars-cov-2.html) and the virus may stay airborne for hours and on fomites for days.[15],[16] That being so, provision of treatment by dental students in the MChDO may further increase the risks of this infection spreading. Modification in the ventilation system, adjustment of relative humidity (RH), and proper temperature setting may minimize viral spreading in the MChDO when combined with observance by dental students and DHCWs of certain universal preventive measures, such as wearing gowns, mask, gloves, and eyeglasses. Other preventive measures include preprocedural mouth wash, rubber dam use, high volume evacuator and saliva ejector, extraoral vacuum, air filtration, and surface decontamination.[17]


  The Hypothesis Top


According to our knowledge, there have been no previous studies on the role of central ventilation in general[14] and spatial ventilation in particular in preventing microbial transmission in the MChDO. Studies have, however, reported the presence of SARS-CoV-2 in air samples from Wuhan hospitals.[18],[19] Furthermore, saliva infected with SARS-CoV-2 may be aerosolized during dental AGPs, thereby contaminating the dental operatory atmosphere and potentially exposing patients and DHCWs to risk of infection. This threat has led the WHO and CDC to recommend and provide guidance on appropriate ventilation in dental operatories to reduce the virus load. Therefore, it may be that the safe continuation of routine dental work will depend on redesigning and modifying the current ventilation systems to prevent nosocomial virus transmissions in dental settings. Accordingly, we hypothesize that the following two proposed laminar flow ventilation systems, designed for the open MChDO, could potentially prevent microbial spread from one dental chair to another during AGPs.


  Evaluation of the hypothesis Top


Oral cavity as reservoir for severe acute respiratory syndrome coronavirus

Although saliva offers some protection against bacterial and viral infections, it can also be a risk factor for infection transmission.[20] Saliva can become contaminated with various bacteria and viruses,[21] the main sources being supra- and subgingival microorganisms and dental caries. Moreover, due to their close anatomical proximity with the oral cavity, secretions from the nose, throat, and respiratory tract can contaminate saliva with pathogenic streptococci and staphylococci, common cold and influenza viruses, herpes viruses, and also the SARS viruses.[21],[22],[23] Saliva exists at the entry to the respiratory system and SARS-CoV-2 nucleic acid has been found in the saliva of infected persons.[24] Studies have found that live coronavirus samples can be cultivated in saliva.[24],[25] A previous study also reported that in one sample live virus was still being shed into the saliva after 11 days of hospitalization. Meanwhile, periodontal pockets could provide an additional appropriate reservoir for SARS-CoV-2 in the oral cavity.[26]

Aerosol generating procedures

Aerosols and splatter are the major virus transmission concerns in dentistry because of their potential consequences for the health of patients and DHCWs.[14],[27],[28] Since the emergence of SARS‑CoV in 2003, concerns over the spread of viral infection through dental AGPs have been increasingly expressed.[22]

Many dental procedures involve aerosol and splatter formation.[10] Although both of these are produced simultaneously during dental procedures, they behave in different ways, biologically and physically. Both contaminate the local atmosphere, but the aerosols pose greater risks to respiratory system health because of their deep human body tissue penetration capability and their lighter physical weight that enables them to stay airborne for a long time.[27],[29],[30] On the other hand, splatters are usually ejected across small distances and eventually splashed onto masks, goggles, face shields, skin, and clothes of patients and operators.[31],[32]

Contaminations in multichair dental operatories

While many aerobiological studies have investigated the spread of oral microorganisms in closed dental operatory environments,[14] only a few studies have investigated this problem in open MChDOs. A recent study by Zemouri et al.[5] monitored the university treatment room and compared it with three private clinics, with the former exhibiting lesser microbial growth [<2 colony forming unit (CFU)/plate in 30-minute exposure time] at control sites, 150 cm from the patient’s mouth. They also found more aerobic/anaerobic contamination (oral origins) around patients’ mouths (50–80 cm), while microbial cultures at control sites were mostly aerobic, indicating nonoral origin of microorganisms. This result was attributed to the optimal ventilation rate at university clinics compared to the less optimal ventilation at the three private clinics. In contrast, a study by Grenier[3] found a noticeable air contamination in an inactive dental treatment site in the MChDO, within an 11 m radius of active sites. He concluded that bacteria could spread to areas where no dental activities were taking place. This finding was attributed to the air-handling system and human activity. His final recommendation was for installation of laminar unidirectional airflow systems to control microbial transmission in dental clinics. Additionally, Al Maghlouth et al.[7] highlighted the importance of having proper ventilation systems with regular maintenance checks in their study of bioaerosol contamination in the MChDO at King Saud University. They found a fivefold increase in microbial air contamination during working time and the subsequent decrease in microbial air contamination, in a range from 50% to 70%, 30 minutes after work had ended. However, in some cases the levels of contamination remained as high as during the working period. This indicates that the produced droplet nuclei could stay airborne for a long period after dental work has stopped. A recent study on a similar theme to that of Al Maghlouth et al., conducted by Jain et al.,[17] demonstrated multiple increases in microbial aerosols during and after working sessions in the MChDO. Moreover, a study by Kedjarune et al.[33] reported that microbial circulation decreased compared to the beginning of working hours. They concluded that shutting down the ventilator power overnight would have led to microbial build up in the air before the start of the next day’s treatment sessions, whereas the ventilation reduced the bioaerosol level during working hours. They also found no significant relationships between different dental procedures and the level of air contamination in MChDOs. However, the concentration of the total bacterial aerosols did not significantly differ between active and nonactive distant treatment areas. Therefore, they suggested that appropriate preventive measures should be taken to control air contamination, especially during nonworking hours. Finally, research conducted by Kimmerle et al.[34] on MChDO air contamination found that while there was no significant difference in bacterial count between the dental treatment room and the public area, exposure to risk might be greater in the treatment room than the public area due to the nature of the microorganisms, host susceptibility, and the exposure time.

It is worthy of mention that the culture media used in these aforementioned studies did not permit growth of all types of oral and waterline microorganisms, including viruses, since growth of each microbial species requires the use of special culture media.[7],[5] Furthermore, droplet nuclei may contain pathogenic bacteria and viruses, generated during dental procedures, which may travel even greater distances and remain airborne for a longer time. The majority of these studies confirmed that microbial transmission can occur from active treatment sites to distant nonactive sites. These results could be viewed as warning signals regarding the risk of microbial travel from one active treatment site to another in the MChDO. Specifically, microbial interchanges between patients and dental students’ lack of knowledge or skill in relation to the correct use of aerosol reducing tools and other infection control measures may further increase air contamination. Therefore, there is a need for appropriate installation and/or modification of (in) air ventilation systems in MChDOs to minimize these risks.

In the current article, we describe 12 and 36 air change/hour (ACH) systems, with emphasis on spatial air control and designed to minimize interdental-chair microbial transmission. Studies have demonstrated that the operation of sophisticated mechanical ventilation systems in hospitals improves air quality, particularly when laminar directional airflow and filtration systems with increased ACH are used.[35] Theoretically, a 12 ACH system requires 23 and 35 minutes to reach 99% and 99.9% efficiency, respectively, in airborne removal capacity. Therefore, the higher the ACH, the lesser the time required to clear the room of contamination (https://www.cdc.gov/infectioncontrol/guidelines/environmental/appendix/air.html).


  Dental clinic heating ventilation air conditioning design Top


Ventilation systems undoubtedly play an important role in controlling airborne microbial transmission in dental operatories.[36],[37] In order to achieve effective infection control, various factors need to be considered in the design and setting up of the ventilation system. For clinical areas, the most appropriate temperature setting is from 21°C to 24°C, while RH should be between 40% and 60%, ACH at least from 6 to 12, and the airflow should pass from clean to less clean areas. RH is an important factor in reducing transmission of droplets and some airborne viruses.[38] Setting the RH at the above level would decrease the survival rate of pathogens and prevent quick drying of disinfectants on contaminated surfaces. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers recommends 6 ACH for dental departments in hospitals and recently the CDC has raised the recommended level to at least 12 ACH in response to the SARS-CoV-2 infection (https://www.cdc.gov/coronavirus/2019-ncov/hcp/dental-settings.html).

Delivering air from the ceiling via laminar unidirectional downward movement to the operation area and then to several exhaust/return opening grills placed low on opposite walls is the most effective air movement pattern for reducing contamination in the operating room. Dual low and high exhaust grills may work slightly better than either all-low or all-high exhaust grills.[39] Meanwhile, the main priority in terms of positioning the ventilation system is its location at the center of the operational field in the operating room (https://www.ashrae.org/file%20library/technical%20resources/covid-19/si_a19_ch09healthcarefacilities.pdf). In regard to dentistry, the dental treatment unit (DTU) is the central region in the operatory room. On the other side, a turbulent air flow must be installed before opening the MChDO for dental students or dental practitioners. This will assure atmosphere cleaning throughout the MChDO room space.

In response to the need to control airborne infectious transmission, we propose the use of two ventilation systems, one to supply the MChDO with standard 12 ACH and the other to supply fully conditioned fresh air at about 36 ACH. This proposal is based on the assumption that increasing the rate of ventilation with fresh air will be relatively more effective in preventing microbial transmission in indoor spaces.[37] Since untreated outdoor air may be unsafe to patients suffering from some systemic diseases like respiratory disease, ventilation systems should provide indoor spaces with air that is virtually free of dust, dirt, odor, and chemical and radioactive pollutants.

The first design was created based on the CDC recommendation of 12 ACH. We considered a 164 m3 room containing eight open DTU. This room volume would require 6000 m3/hour of conditioned air. In order to achieve 12 ACH for this volume, we multiplied the room volume (164 m3) by the 12 ACH and this indicated that approximately 2000 m3/hour outdoor air volume would be required to enter and 2000 m3/hour of indoor air to exit simultaneously. This air would pass through an air handler unit [Figure 1]. This unit would regulate temperature, humidity, and filtration [fitted with high-efficiency particulate air (HEPA) filter] and contain an air mixing box where returned air would be mixed with fresh air before passing through the HEPA filter. In the first scenario, we hypothesize that the HEPA filters would have the capability to filter SARS-CoV-2. Therefore, about one-third (2000 m3/hour) of the exhausted air would be discarded and the remaining two-thirds (4000 m3/hour) of the exhausted air would be returned through the mixing box to be mixed with 2000 m3/hour of fresh air and save power for the cooling and heating. The exhausted air would also pass through a HEPA filter to prevent outdoor air contamination. For the second scenario, design 2, we hypothesize that the HEPA filters would not have the capability to filter SARS-CoV-2. Therefore, the system would allow 6000 m3/hour of fresh air to enter and 6000 m3/hour of exhausted air would be discarded completely without passing through a mixing box. While this process would maximize the number of air changes to approximately 36 ACH, this design would incur more power cost during operation.
Figure 1 Proposed multi-chair dental operatory heating ventilation/air conditioning design

Click here to view


Meanwhile, in each DTU in the MChDO, a laminar diffuser would be mounted, to allow air passage at a rate of 750 m3/hour, controlled by volume dampers [Figure 2]. The center of the diffuser would be located approximately at the patient’s headset position in the DTU. This design feature was intended to allow the laminar air to blow the generated aerosols down to return grills. Four return grills would be positioned at each corner of each DTU at 25 cm height. In order to prevent possible air contamination because of air turbulence, another four additional high return grills would be mounted directly above all four low return grills at 200 cm height. As an additional precaution, we placed HEPA filters at the opening of each diffuser and return grill to prevent contamination of the duct system as the power supply to the ventilation system would be shut down during nonworking hours. Finally, we would expect the produced aerosols to be directed to return grills without passing on to other dental chairs and microbial transmission to other dental patients and DHCWs would thereby be prevented.
Figure 2 Dental treatment unit ventilation system

Click here to view


Other factors that should also be considered as potentially affecting the quality of these proposed systems:
  1. Presence of anteroom to minimise air turbulence during working hours.
  2. The distance of diffusers from the patient’s mouth and flowrate of the air when it reaches the patient’s face area.
  3. Positions of the operators during dental treatment procedures may intercept the airflow direction.
  4. Positions of the dental lights may intercept the airflow direction.
  5. Whether dental treatment units are partitioned or completely open to each other.
  6. Students’ skills regarding using aerosol reduction equipment and presence/absence of dental assistant.



  Evaluation of the laminar airflow design capacity Top


This design was created to be implemented in a multi-chair dental clinic at the College of Dentistry/University of Sulaimani. The intention is to start the pilot study in summer 2021.

The following methodologies for testing these designs will be carried out:
  1. Manikins with simulated fluorescein salivation will be used to test contaminated aerosols, in different locations, produced by ultrasonic scaler and air-polishing, by photometric measurements as described by Kaufmann et al.[40]
  2. Microbiological examination (waterlines, periodontal hand and powered scaler use, tooth preparation procedures) including active and passive sampling for bacterial culturing as described by Zemouri et al.[5] The samples will be taken at 3 different time points: before treatment, during treatment, and after treatment. We will examine the microbial contaminations (CFU/plate) in different distance areas.


Stratification: In order to minimise confounding, we will test these ventilation designs with and without high-volume evacuation (HVE) and saliva ejector versus with HVE and saliva ejector and without ventilation.


  Conclusions Top


When patients are seated on dental chairs in the open MChDO during dental AGP, spatial separation is critically important for their safety. In addition, spatial ventilation is crucial to prevent the transmission of droplet nuclei from one treatment area into another.[41] Patient separation is an important factor in achieving infection control as potentially infected patients must be segregated from those who are not infected in order to impede infection transmission. Nevertheless, an airborne infection can result in large clusters of infection within a short period of time.[42] The release of microorganisms into the air as aerosols will also lead to increased microbial load and eventual contamination of all fomite surfaces in the clinic.[43] Therefore, there should also be focus on infection control measures that deter surface decontamination as splatter cannot be controlled by ventilation systems and will eventually cause deposition on the surrounding surfaces. A systematic review by Rashid et al.[44] concluded that large amounts of respiratory viruses can remain on surfaces for a few days and some pathogenic bacteria and fungus may stay viable for months if proper surface disinfection is not performed.Despite advances in eco-epidemiology, the modern world has continued to witness the emergence and reemergence of new communicable microorganisms that pose a threat to humankind.[45] These designs were created in response to the need to control the spread of highly infectious diseases such as the COVID-19 virus that has led to the shutting down of dental clinics and hospitals throughout the world. This health emergency has prompted the American Dental Association (ADA) to advise the treatment only of urgent dental problems and avoidance of all kinds of AGP.[46] In order to respond to the current situation and be ready for further possible disease outbreaks, especially in dental academic settings, we have created two designs that could allow safe provision of dental treatment in both the MChDO and single chair dental clinics. However, thorough physical and biological investigations will be required to determine how these designs can be applied effectively in terms of the required spatial separation of DTUs in the open MChDO.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Introduction
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