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ORIGINAL ARTICLE
Year : 2021  |  Volume : 19  |  Issue : 4  |  Page : 230-235

Airborne particle control methods in dental clinics: A low-cost technique of assessment


1 Department of Dental and Oral Surgery, CMC, Vellore, Tamil Nadu, India
2 Department of Infectious Diseases, CMC, Vellore, Tamil Nadu, India
3 Department of Clinical Microbiology, Hospital Infection Control Committee Officer, CMC, Vellore, Tamil Nadu, India
4 Department of Engineering, CMC, Vellore, Tamil Nadu, India

Date of Submission17-Jun-2021
Date of Decision20-Jul-2021
Date of Acceptance31-Aug-2021
Date of Web Publication07-Dec-2021

Correspondence Address:
Dr. Rabin Chacko
Professor, Dental Unit 1, Telephone No.S 0416 -2283641
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/cmi.cmi_64_21

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  Abstract 


Background and Objectives: The COVID-19 pandemic has highlighted the risk of airborne transmission of infections in health-care facilities such as dental clinics. In this experimental study, methods to control airborne particles in a simulated dental clinic setting were measured and compared using a low cost and convenient technique. Materials and Methods: Particles representing inhalable airborne particles were generated using smoke from incense sticks and their concentration measured by handheld particle sensors whereas using different engineering controls for the particle removal in dental clinic equivalent settings. Measurements were made at short (<3 ft) and intermediate (between 3 and 6 ft) distance from the source. The particle filtration through surgical masks and N95 masks was also studied. Results: Natural ventilation, by keeping windows open, can reduce intermediate range particles (removal of 4.7% of ambient particles/min). However, in closed facilities without natural ventilation, particle removal by air purifier combined with overhead fan or with high volume evacuators was found most suitable for intermediate range particles (25.9%/min) and for short range particles (27.6%/min), respectively. N95 masks were found to filter out 99.5% of the generated PM 2.5 particles. Conclusions: Potentially inhalable airborne particles can persist in the air of a dental clinic. The use of N95 masks and environmental controls is essential for the dental team's safety. The choice of an engineering control is governed by multiple factors explained in the study. Smoke particles generated by incense sticks and measurement by handheld particle sensors are low-cost methods to estimate the effectiveness of airborne particle controls.

Keywords: Airborne particles, dental clinic, engineering controls


How to cite this article:
Chacko R, Rupali P, Murugesan M, Premchander M. Airborne particle control methods in dental clinics: A low-cost technique of assessment. Curr Med Issues 2021;19:230-5

How to cite this URL:
Chacko R, Rupali P, Murugesan M, Premchander M. Airborne particle control methods in dental clinics: A low-cost technique of assessment. Curr Med Issues [serial online] 2021 [cited 2022 Jan 27];19:230-5. Available from: https://www.cmijournal.org/text.asp?2021/19/4/230/331836




  Introduction Top


In the present state of the COVID-19 pandemic, ventilation of healthcare facilities is a key preventive measure suggested by Centers for Disease Control (CDC) and studies to prevent transmission of “;severe acute respiratory syndrome-coronavirus-2” (SARS-CoV-2).[1],[2] Transmission of respiratory pathogens is inversely proportional to the rate of ventilation of the facility/area as conceived in the Wells-Riley equation.[3],[4]

The dental clinic environment is a location identified as potentially a high-risk area for the transmission of respiratory pathogens because of aerosols released during dental procedures which are potentially pathogenic.[5] In a dental clinic, the dentist and assistant are directly exposed to bioaerosols which are released from the patient's nose and mouth during procedures or regular speech,[2] since they are within 1–3 feet from the patient's mouth. These naturally released aerosols may be mixed with exogenous aerosols introduced under high pressure to cool the instruments used during tooth cutting or scaling. Larger droplets tend to fall to the surfaces around the patient within about 10 min of release and in the area of 1 m around the patient, if the ambient humidity is high. However, droplets may dry up before falling and become droplet nuclei,[5] also called particles or aerosols, <5 microns in size which continue to circulate in a closed confined space for several hours unless removed by suitable methods. These particles are capable of being inhaled by the dental staff and seeded directly into the respiratory alveoli because of the small size of the particles.[2]

The removal of circulating aerosols in the indoor environment is recommended to reduce the risk of transmission of COVID-19 infection.[6] Environmental control of indoor particles is possible only through ventilation and filtration methods. Large clinical spaces and natural ventilation provide the dilution of potential harmful particles.[3],[4] Exhaust fans and high-volume evacuators (HVE) provide room-level evacuation of particles, whereas overhead fans can disperse particles,[1] thus reducing the particle concentration. Negative pressure environments have been employed for the prevention of cross infection of respiratory pathogens.[7] However, they are expensive and difficult to maintain effectively in small, high-risk hospital spaces, especially in resource-constrained environments. Escombe et al. recommended the use of natural ventilation and other less expensive engineering controls in such situations.[4] Considering the technical challenges involved, CDC has not insisted on the use of negative pressure for dental clinics and has instead recommended the use of portable filtration units.[5]

High-Efficiency Particulate Air (HEPA) filters can remove 99.7% of particles of the size of 0.3 μ.[8] Peak concentrations of SARS-CoV-2 containing aerosols have been found to be in diameter ranges of 0.25–1 μ in the submicron region and larger than 2.5 μ in the supermicron region.[6] Portable air purification units called air purifiers (AP) containing HEPA filters suck air into the units, filter off the airborne particles, and release clean air controlling the indoor particle concentration.


  Materials and Methods Top


Study design

This was a pilot experimental simulation study attempting to determine the various low-cost engineering methodologies that could potentially prevent air-borne transmission in a health-care setting for dental procedures during the COVID-19 pandemic. The study did not involve any human or animal subjects.

Study settings

The concentration of aerosols, released from incense burn, was studied in a simulated environment of a dental clinic in a tertiary care hospital without the presence of patients. The various methods of aerosol control that were evaluated included (1) natural ventilation through open door and window, (2) portable AP, (3) HVE, (4) exhaust ventilation, and (5) Overhead fan.

The controls were compared by releasing particles at a steady rate in a room of approximately 2400 cubic feet (170 sq. ft floor space and 14 feet high ceiling). This size of room corresponded to a single chair dental clinic in our hospital. The single window for natural ventilation was 16 sq. ft corresponding to about 10% of the floor area of the room. The standard size door was located on the wall opposite the window.

Materials

We used a carbon dioxide (CO2) monitor and a low-cost particulate matter (PM) measurement sensor (Kaiterra Laser Egg).[9],[10] The equipment was precalibrated and used exclusively for the study.

Initially, effectiveness of engineering controls was assessed by measuring the change in CO2 concentration in ppm using a CO2 monitor to assess the air change rate. The gas was released from a cylinder in the room and the change in concentration (concentration decay) was measured after running the engineering control.[4]

This method of assessment of the engineering controls was replaced by the measurement of the rate of removal of small particles by the same controls expressed as particle removal per minute (PRM).

Smoke particles generated from incense sticks, each of a standard size purchased from a single manufacturer, were used to simulate particles produced in a dental clinic. The source of particles simulated the head and mouth of the patient and was fixed on a table corresponding to the height of a dental chair when in function. The position of the source remained the same throughout the experiments.

The particle sensor, using an optical particle counter, measured PM 2.5 and PM 10 (which refer to the concentration of particles <2.5 and <10 micron diameter) with a range of 0–999 μg/cubic meter.

The exhaust fan and AP used in the study had airflow volumes of 225 cubic feet/minute (CFM) and 200 CFM, respectively.

Measurement

The incense stick was burnt in an open top container and the released smoke particles were measured from the air every minute over a period of 15 min, whereas the specific engineering control was functional, at different distances from the source. The average change in particle concentration per minute (particle removal rate) was documented for each of the controls.

The location of the particle counter in relation to the particle source was varied for each of the controls to assess the relative particle removal capacity at short range with direct exposure (<3 feet in the direct line of particle release) and for the intermediate range outside the direct line of exposure of the particles (between 3 and 6 feet from the source).

N95 masks were compared with surgical masks. Each mask was securely taped around the mouth of a container (with a lighted incense stick inside used as the source) ensuring an air-tight seal. The air immediately above the mask was tested with the particle counter to detect the release of particles from the source through the mask (similar to the method used by Schultheis et al.).[11] These results were compared with the particle count measured directly over the source, without a mask.

Variables

We studied variables that could potentially add/modify the results and affect the level of protection for the staff in a dental clinic, for example, distance from the source corresponding to the head of the patient, direction of flow of particles, and the use of masks.

Indoor particle concentration being dependent on the outdoor particle concentration fluctuates with the weather (wind movements, temperature, and humidity) and pollution levels.[12] This variable was taken into account whereas comparing the various controls by calculating the rate of the removal of particles as a percentage of the ambient particle concentration of the indoor air at the time of the study, i.e., PRM as a percentage of the ambient particle concentration in the room at the time of testing particle removal per minute as percentage (PRMP).

Ethics committee review

The authors ratify that this study required Institutional Review Board/Ethics Committee review, and hence prior approval was obtained (IRB Min. No. 12965 dated 24-06-2020).


  Results Top


Face masks tested for particle control

N95 masks were found to allow about 0.5% of PM 2.5 particles through the mask giving a filtration efficiency of 99.5%.

Surgical masks filtered out water droplets but allowed about 5% of PM 2.5 particle aerosols to filter through for a filtration efficiency of 95%. Measurements taken of the particle concentration at short range from the source without an intervening mask were used for comparison of the relative efficacy of the masks [Figure 1].
Figure 1: Particle concentration within 1 foot of source with intervening filtration by N95 mask, surgical mask and without mask.

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Environmental controls tested with carbon dioxide concentration

In the initial phase of the study, the air changes per hour (ACH) were checked using a CO2 monitor to measure the concentration decay of a short burst of CO2 released in a dental clinic from a CO2 cylinder which was tested with different ventilation methods. This was followed by a similar release of smoke particles in the same clinic measured by a particle meter, and a significant difference was found in the rate of removal of gas molecules versus PM. Exhaust ventilation cleared CO2 molecules within 15 min, but it took up to 41 min to remove smoke particles [Figure 2] in a room with the same ACH.
Figure 2: Difference between clearance of carbon dioxide gas and particles by the same control and air change rate.

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Environmental controls tested with particle concentration measurement

When using particle producing sources such as smoke from incense burn the following observations were made:

Intermediate range particles

In the single dental chair clinic setting, all of the methods of clearance can help in preventing particle build up as compared to no airborne particle control in the closed room without ventilation when the particles tend to accumulate even after stopping the source. The particle counter was kept at a distance of 5 feet from the source and measurements were made at this distance for all the methods of ventilation control [Table 1].
Table 1: Comparison of efficacy of different controls for clearance of Intermediate range particles calculated as particle removal per minute as percentage and relative ranking

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An enclosed room without ventilation or filtration facility will result in the accumulation of particles, i.e., negative removal of particles. The rate of removal was found to be 13.1%/min. Natural ventilation using open window reduced the particle concentration at the rate of 4.7%/min and AP in an enclosed room was the single most efficient method of reducing the particle count (9.6%). The combination of AP with overhead fan had the best result for intermediate range particle removal (25.9%).

Particle concentration was less behind the source than in the line of particle flow from the source toward the control which could be an AP or exhaust fan [Figure 3].
Figure 3: Difference between the concentration of particles in the line of particle flow from the source to control and outside the line of flow.

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Short range particles

Particle counts are maximum immediately above the particle generating source and in the path of particle flow (PRMP of - 474%/min). When measuring the concentration of particles per minute at <3 feet from the source, the particle count is not affected significantly by the use of open windows (natural ventilation) alone. However, some other methods of particle removal were fairly successful in reducing the particle concentration. The best result in controlling the particle level was found in the use of a combination of AP and HVE (27.6%/min). HVE also functions well with natural ventilation (7%/min) [Figure 4].
Figure 4: Comparison of efficacy of different controls for clearance of short range particles.

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[Table 2] demonstrates the relative efficacy of different environmental controls as compared to the closed room with no environmental control and they are ranked according to their particle removal capacity.
Table 2: Comparison of different combinations of controls in relation to the distance from source

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  Discussion Top


In the initial analysis, we found that the results of the CO2 concentration measurements for calculation of the air change rate do not correspond to the rate of clearance of particles as measured by the particle counter [Figure 2]. This is because particles, unlike gas molecules have inertia with different movement and removal mechanisms including gravitational settling.[13] Homogenous mixing of the room air is also less with particles than with gas molecules. Air change rates are calculated on the premise that the air is homogenous and uniformly mixed.[14] Since viruses are expected to be airborne within PM,[15] the study thereafter was largely restricted to the measurement of particles (aerosols) released from the various sources and their rate of removal in the dental clinic setting. The different methods of aerosol control were compared using the changes in particulate concentration rather than the ACH of the room.

There were no significant changes in the ambient particle levels during the use of dental equipment. No changes were observed with the use of ultrasonic scaling equipment. Since the particles released from dental equipment could not be used as a standard for measuring the relative efficacy of different environmental controls, smoke from incense sticks was used as a surrogate for small PM capable of entering the lung alveoli as their average particle sizes are in the respirable range.[16],[17] These particle sizes also match the respirable range of aerosols produced during speech and normal exhalation and might be expected to be released from the nose and mouth of the patient seated in the dental chair.[18],[19] The size of the incense sticks and its composition was standardized by the use of the same brand for all the tests.

Through the experiments, it was found that masks are useful in filtering out PM but N95 masks are superior to surgical masks in dealing with small particles of 2.5 micron size [Figure 1].[20] Apart from its filtration efficiency, the fit of the N95 mask to the face of the user makes it preferable to surgical masks when used by staff in a dental clinic.[21] In this study, we used taped surgical masks sealed to the mouth of the container but taped surgical masks are rarely used in clinical practice. The performance of unsealed masks is inferior to the taped masks.[22]

In closed rooms with limited or no ventilation, the particulate counts remain high during the period of particle generation, as seen in this study. We found that ventilation significantly reduces the intermediate/long range particle concentration through natural ventilation using windows, cross ventilation using open door opposite the window, and with the use of exhaust. Adequate size of clinical spaces ensuring safe distancing and effective ventilation is recommended to reduce the risk of airborne infections.[3],[4],[10]

The risk of exposure of the dental health-care worker (DHCW) to aerosols from the patient's airway is high because of the close distance of the staff (<3 feet) to the patient's nose and mouth while examining or treating the patient. This area includes the breathing zone of the staff and patient.[23] Since the patient's airways are the source of potentially infected airborne particles, the concentration and distribution of short range particles are of significance. This distance from the source is also of relevance to droplets emanating from the mouth and from dental procedures.

As expected, the concentration of airborne particles was found to be higher when close to the source and in the line of flow of the particles toward the clearing device, for example, exhaust or AP [Figure 3]. Intermediate and long range particles (more than 3 and 6 feet from the source, respectively) put the circulating dental nurse, other staff, and waiting patients at risk.

The study shows that a HVE is useful in reducing the concentration of PM close to the source [Figure 3].[24] This occurs by evacuating the particles into the suction system which should include a HEPA filter to filter out virus particles and prevent their recirculation. HVEs are commercially available and have been a feature of dental clinics for several years. Their use is recommended by CDC.[25] An overhead (ceiling) fan also can reduce the small particle count by the dispersion of particles away from the area of concentration around the source, which in the dental clinic is the breathing zone of the patient and DHCWs.[1],[26]

APs, also called portable air cleaning units, are fitted with HEPA filters and have a type of exhaust fan system which draws room air through multiple filters to remove the PM and return cleaned air back into the room. Results of the study show high efficacy for clearance of intermediate range airborne particles using AP.[27] The effectiveness of this type of equipment is related to its filtration capacity, airflow volume expressed as CFM, and the size of the room, in which it is being used. The unit should not be placed behind the DHCW because of the direction of flow of particles from the patient's mouth to the unit [Figure 3] and therefore the foot end of the dental chair would be a preferable location.[18],[20] The overall airborne particle clearance effectiveness is increased when the AP is combined with the use of HVE or overhead fan [Table 2].

Although the cost of the sensor used in this study is a small fraction of that of advanced particle measurement systems such as the laser sheet diffraction technique, the accuracy and versatility of handheld sensors have been validated in the previous studies.[9],[10] Indian incense (agarbatti) sticks for particle generation have also been successfully used for the measurement of airborne particles in dental clinics.[27]

It is evident from this study that the technique employed in this study for the generation and measurement of airborne particles can be effectively used to measure particle clearance in dental clinics and similar health-care facilities to compare and choose the optimum method of aerosol control, especially in resource-constrained regions.

The study did not evaluate the particles generated during actual use in dental clinics with patients and the comparison of masks was done measuring 2.5 μ particles and not in the submicron range (<1 μ). These were limitations of this pilot study. However, enough information is obtained to guide decision-making on the choice of aerosol controls for dental clinics and to enable further research in these aspects.


  Conclusions Top


In a dental clinic setting with the high aerosol generation, potentially inhalable airborne particles tend to accumulate if there are no engineering controls. The concentration of airborne particles varies with distance from the source and the direction of flow of particles. When considering the safety of the DHCW, personal protective equipment such as N95 masks and environmental controls play a pivotal role. The choice of specific engineering control must be determined according to the size of the clinic, availability of natural ventilation, and the need for air conditioning. It is also evident that smoke from incense sticks and handheld particle sensors are low-cost mechanisms to test the effectiveness of environmental controls of air exchange in a health-care setting.

Acknowledgments

Faculty of the dental department for their suggestions and Mr. Prakash (Department Secretary) for his valuable support during the purchase of equipment and data collection.

Research quality and ethics statement

All authors of this manuscript declare that this scientific study is in compliance with the standard reporting guidelines set forth by the EQUATOR Network. The authors ratify that this study required Institutional Review Board/Ethics Committee review, and hence prior approval was obtained (IRB Min. No. 12965 dated 24-06-2020). We also declare that we did not plagiarize the contents of this manuscript and have performed a Plagiarism Check.

Financial support and sponsorship

CMC Research grant for purchase of equipment and materials.

Conflicts of interest

There are no conflicts of interest.



 
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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
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