Ventilation Assessment by Carbon Dioxide Levels in Dental Treatment Rooms

Abstract

It is important for dental care professionals to reliably assess carbon dioxide (CO₂) levels and ventilation rates in their offices in the era of frequent infectious disease pandemics. This study was to evaluate CO₂ levels in dental operatories and determine the accuracy of using CO₂ levels to assess ventilation rate in dental clinics. Mechanical ventilation rate in air change per hour (ACH_VENT) was measured with an air velocity sensor and airflow balancing hood. CO₂ levels were measured in these rooms to analyze factors that contributed to CO₂ accumulation. Ventilation rates were estimated using natural steady-state CO₂ levels during dental treatments and experimental CO₂ concentration decays by dry ice or mixing baking soda and vinegar. We compared the differences and assessed the correlations between ACH_VENT and ventilation rates estimated by the steady-state CO₂ model with low (0.3 L/min, ACH_SS30) or high (0.46 L/min, ACH_SS46) CO₂ generation rates, by CO₂ decay constants using dry ice (ACH_DI) or baking soda (ACH_BV), and by time needed to remove 63% of excess CO₂ generated by dry ice (ACH_DI63%) or baking soda (ACH_BV63%). We found that ACH_VENT varied from 3.9 to 35.0 in dental operatories. CO₂ accumulation occurred in rooms with low ventilation (ACH_VENT ≤6) and overcrowding but not in those with higher ventilation. ACH_SS30 and ACH_SS46 correlated well with ACH_VENT (r = 0.83, P = 0.003), but ACH_SS30 was more accurate for rooms with low ACH_VENT. Ventilation rates could be reliably estimated using CO₂ released from dry ice or baking soda. ACH_VENT was highly correlated with ACH_DI (r = 0.99), ACH_BV (r = 0.98), ACH_DI63% (r = 0.98), and ACH_BV63% (r = 0.98). There were no statistically significant differences between ACH_VENT and ACH_DI63% or ACH_BV63%. We conclude that ventilation rates could be conveniently and accurately assessed by observing the changes in CO₂ levels after a simple mixing of household baking soda and vinegar in dental settings.

Carbon dioxide (CO₂) level is an important indicator of ventilation in occupied indoor environments. CO₂ is a by-product of human metabolism and exists in high levels in exhaled air. Atmospheric CO₂ level is approximately 400 parts per million (ppm) in outdoor environments, but CO₂ in human exhaled air reaches on average 40,000 ppm in concentration (Issarow et al. 2015). CO₂ level is therefore often used as a proxy for indoor air quality as well as a risk marker for transmission of airborne diseases since it is inert, its indoor emission source (human) is known, and its measurement is inexpensive and accurate (Batterman 2017). Increased CO₂ level is often associated with poor ventilation and overcrowding. Accumulation of CO₂ occurs concurrently with accumulation of respiratory pathogens in a room where an infected person is present but not wearing a mask, which may increase the risk of disease transmission. High levels of CO₂ have long been associated with the transmission of infectious respiratory diseases such as tuberculosis, influenza, and rhinovirus infections (Myatt et al. 2004; Richardson et al. 2014; Wood et al. 2014). Indoor CO₂ levels have therefore been widely used to model the risks of airborne infectious disease transmission (Rudnick and Milton 2003; Issarow et al. 2015; Harrichandra et al. 2020), including that of coronavirus infection transmission in dental offices (Zemouri et al. 2020).

Clinical spaces with good ventilation should have CO₂ levels close to that of outside air at approximately 400 ppm (ASHRAE 2007; Issarow et al. 2015). Higher CO₂ levels indicate poor ventilation, accumulation of exhaled air, and increase in the fraction of “rebreathed air” in the indoor environment, which is proven to be a risk factor for infectious disease transmissions (Rudnick and Milton 2003; Richardson et al. 2014; Wood et al. 2014; Issarow et al. 2015). CO₂ levels have been used to estimate ventilation rates in dental offices (Godwin et al. 2003; Helmis et al. 2007; Helmis et al. 2008). Ventilation rate was found to be 1.12 air change per hour (ACH) in a typical dental clinic in the United States (Godwin et al. 2003) and was on average 5 ACH in a dental school clinic in Greece where the doors and windows were opened for cross-ventilation (Helmis et al. 2007). Both studies used natural buildup of CO₂ levels in dental clinics to estimate the ventilation rate through mathematic models, but none actually verified the ventilation estimates using standard methodologies such as high-precision airflow sensors (ASHRAE 2017). It is not known if these estimates were accurate as both methods require mathematical modeling based on several assumptions related to CO₂ generation, buildup, and dispersion over a relatively lengthy period of time, which may result in erroneous estimates if any of the assumed conditions are not met (Batterman 2017). A simpler and more reliable method is needed if CO₂ level is to be used by dental care professionals to assess the ventilation rate of their treatment rooms.

The purpose of the present study was 2-fold: 1) to evaluate CO₂ level and its associated factors in dental operatories and 2) to determine the accuracy of various methods using CO₂ levels to assess ventilation rate in dental clinics. Our aim was to find a practical tool that will enable dental care professionals to conveniently and accurately monitor CO₂ levels and assess the ventilation rates in order to devise a pragmatic and effective strategy for ventilation improvement in their work environment.

Methods

Study Settings

We conducted the CO₂ concentration and ventilation rate assessments in 10 closed treatment rooms ranging from 667 to 1,221 cubic feet (ft³) in size in a multifloor building in an academic dental institute. Mechanical ventilation of the rooms was provided by 3 air handlers that drew 60% outside air to the ventilation system.

Determining Room Airflow and Mechanical Ventilation Rates

Mechanical ventilation in air change per hour (ACH_VENT) was measured with an air velocity sensor integrated in an airflow balancing hood (ADM-850L Airdata Multimeter with CFM-850L FlowHood; Shortridge Instruments) as described elsewhere (Ren et al. 2021).

Assessing CO₂ Levels during Dental Treatment Procedures

We measured CO₂ levels in 2 dental treatment rooms when dental procedures were performed. The 2 rooms represented 2 extremes in ventilation rates, with one at 3.9 air change per hour and the other at 35. The number of persons in the rooms was recorded in real time when a person was entering and leaving the room. CO₂ levels were measured at a 1-min interval using a consumer-grade CO₂ sensor (Aranet4, range 0–9,999 ppm, accuracy ±50 ppm; SAF Tehnika).

24-h Continuous Monitoring CO₂ in Dental Treatment Rooms

To further explore the dynamics of CO₂ levels in dental treatment rooms throughout the day and assess accuracies of the steady-state models of CO₂ for ventilation assessments, we continuously measured the CO₂ levels in 10 dental treatment rooms for 24 h and recorded the procedures performed and number of persons in the room.

Assessing Ventilation Rate by Natural CO₂ Level Modeling in Dental Treatment Rooms

We used the steady-state model described by Batterman (2017) to calculate the air change rate of treatment rooms and compared the outcomes with that of measured mechanical ventilation.

The steady-state air change rate (ACH_SS) is calculated as follows (Batterman 2017):

$$AC{H}_{SS}=6\times {10}^{4}nGp/\left[V\left(C_{SS}-C_R\right)\right],$$ (1)

where n = number of persons in the room, G_P = average CO₂ generation rate, V = volume of the room in cubic meters (m³), C_SS = steady-state indoor CO₂ level in ppm, and C_R = CO₂ level in outdoor air in ppm.

The CO₂ generation rate G_P is affected by many factors and may vary by human activity, physical size, sex, and race (Qi et al. 2014; Persily and de Jonge 2017). G_P of 0.46 L/min or 0.30 L/min was used in previous studies to represent CO₂ generation (Godwin et al. 2003; Batterman 2017). As CO₂ generation rates and activity levels by dental care providers and their patients are unknown and may not be constant, we used both values to calculate the air change rate ACH_SS and assess the correlations between ACH_SS and ACH_VENT at 2 G_P levels.

Assessing Ventilation Rates by CO₂ Decays Using Dry Ice

Ventilation rates by CO₂ clearance using dry ice (ACH_DI) were determined as described by Batterman (2017) using CO₂ concentration decays:

$$AC{H}_{DI}=1/\Delta tln\left[\left(C_1-C_R\right)/\left(C_0-C_R\right)\right],$$ (2)

where Δt = period between measurements, C₀ and C₁ = CO₂ levels measured at the beginning and the end of the decay period (ppm), and C_R = CO₂ level in outdoor air (ppm).

Assessing Ventilation Rates by CO₂ Decays Using Baking Soda

Considering that dental care professionals in private practices may not have ready access to dry ice, we developed a method to rapidly generate CO₂ in dental offices using baking soda (Arm & Hammer Pure; Church & Dwight) and vinegar (Heinz all-natural distilled white vinegar) (see Appendix for detailed protocol). Mixing baking soda (NaHCO₃) with vinegar containing 5% acetic acid (CH₃COOH) will generate CO₂ as follows:

$$NaHC{O}_3+C{H}_{3}COOH\to C{H}_{3}COONa+H_{2}O+C{O}_2.$$ (3)

Ventilation rates by CO₂ clearance using baking soda and vinegar (ACHBV) could then be calculated using Equation 2 as above.

Estimating Ventilation Rate by Time to 63% Removal of Excess CO₂

Based on a commonly used formula for rate of purging airborne contaminants, 1 complete air change will replace 63% of airborne contaminants in the room with outdoor air (Nardell et al. 1991; Fernstrom and Goldblatt 2013; Jimenez 2020). Ventilation rate can therefore be simply calculated using the time needed to reach a 63% reduction of excess CO₂ from its peak level:

$$AC{H}_{T63\%}=60/\left(t_2-t_1\right),with{t}_1=0,$$ (4)

where t₁ = initial time point with indoor CO₂ at peak level, and t₂ = time point (min) when excess CO₂ is reduced by 63%. Indoor CO₂ at peak level (C_S) is the sum of outdoor CO₂ (C_R) and excess CO₂ (C_E) generated by dry ice or baking soda. As CO₂ measurement starts at peak level, t₁ is therefore always 0. Time needed to remove 63% C_E, or t₂, is the time point when indoor CO₂ level is at C_63%E = C_S – 63% C_E, where C_E = C_S – C_R.

Statistical Analysis

We performed multiple regression analysis using CO₂ levels as the dependent variable and number of persons in the room, ventilation rate, room size, and outdoor CO₂ level as independent variables. We analyzed the dynamics of CO₂ levels during dental treatment procedures using descriptive analysis and compared the steady-state CO₂ levels between rooms with poor and good ventilation using t tests. Mechanical ventilation rate was compared with air change rates calculated with different methods based on CO₂ levels to assess the correlation (Pearson’s r) and differences (paired t test) between the 2 methods of ventilation assessments.

Results

Mechanical Ventilation Rate of the Dental Treatment Rooms

The volumetric sizes, airflow rates, and ventilation rates of the rooms are presented in Table 1. The rooms are on average 882 ft³ in volume (range 667–1,221 ft³). Air change rate by ventilation varied from 3.9 to 35.0 with a mean of 13.2 ± 10.6 per hour.

Table 1.

Volumetric Sizes and Mechanical Ventilation Rates of Dental Treatment Rooms.

Room No.	Volume, ft3	SAF, ft3/min	EAF, ft3/min	ACHS	ACHE	ACHVENT	Floor
002	815	82	27	6.0	2.0	6.0	0
003	787	69	27	5.3	2.1	5.3	0
008	1,221	149	103	7.3	5.1	7.3	2
012	1,015	152	64	9.0	3.8	9.0	2
019	686	59	400	5.2	35.0	35.0	1
021	861	75	51	5.3	3.6	5.3	1
022	833	55	46	3.9	3.3	3.9	1
031	962	337	210	21.0	13.1	21.0	2
032	667	289	220	26.0	19.8	26.0	2
033	970	211	220	13.1	13.6	13.6	2
Mean	882	148	137	10.2	10.1	13.2	N/A
SD	166	101	122	7.6	10.6	10.6	N/A

ACH_E, air change per hour based on exhaust airflow rate; ACH_S, air change per hour based on supply airflow rate; ACH_VENT, air change per hour based on mechanical ventilation; EAF, exhaust airflow rate in cubic feet per minute; N/A, not applicable; SAF, supply airflow rate in cubic feet per minute.

CO₂ Levels during Dental Treatment Procedures

As shown in Figure 1, CO₂ levels were significantly higher in the room with low ventilation (less than 4 air change per hour) and reached nearly 1,600 ppm when 6 persons were in the room. The increased number of persons was related to teaching activities involving dental implant surgery where additional graduate students were allowed to observe the procedures. Comparing the 2 rooms with the same number of persons for the same restorative procedures, CO₂ levels reached 1,100 ppm at the peak in the room with 3.9 air change per hour but stayed below 700 ppm in the room with 35 air change per hour (P < 0.0001). CO₂ accumulation appeared to be associated with crowding and low ventilation rate.

Figure 1.

Carbon dioxide (CO₂) levels during dental treatment procedures in operatories with low and high mechanical ventilation. (A) Significant CO₂ accumulation occurred in a room with low ventilation (air change per hour [ACH] = 3.9) and multiple persons in the room during clinical teaching activities for dental implant surgery. CO₂ level is associated with ventilation rate in rooms with the same number of persons. Peak CO₂ level reached 1,100 ppm in the room with 3.9 ACH (B) but stayed under 700 ppm in the rooms with 35 ACH (C).

Continuous CO₂ Monitoring in Dental Treatment Rooms

We continuously monitored the CO₂ levels for 24 h in the 10 treatment rooms. The dental procedures included exams, hygiene, extractions, restoratives, endodontics, dental implant surgery, and periodontal surgery. Number of persons in the rooms varied from 2 to 6, with more people in the room during dental implant surgeries. The CO₂ levels in early morning (5:00–7:00 a.m.) were at a level of 421 ± 10 ppm, similar to outdoor levels (413 ± 15 ppm) (Fig. 2). The steady-state CO₂ level (C_SS) during dental procedures, which is the mean concentration of CO₂ at the plateau level when the number of persons in the room stays unchanged for at least 5 min, ranged from 543 ppm to 1,374 ppm (786 ± 207 ppm) (Appendix Table 1). Multiple regression analysis showed that CO₂ levels were significantly correlated to the number of persons in the room (β = 90.2, P = 0.006), ventilation rate (β = 11.0, P = 0.001), and volumetric size of the room (β = −0.50, P = 0.049) but not to outdoor CO₂ levels (β = 4.15, P = 0.160).

Figure 2.

The 24-h continuous measurements of carbon dioxide (CO₂) levels in 10 dental treatment rooms with various ventilation rates. CO₂ accumulation occurred in rooms with lower ventilation rates (air change per hour [ACH] ≤6). CO₂ levels stayed under 800 ppm in rooms with a higher ventilation rate and lower number of persons. CO₂ level in nonworking hours is close to that of outdoors at 400 ppm in all rooms.

As shown in Figure 2, CO₂ levels in rooms with more than 6 air change per hour rarely reached 800 ppm. In rooms with less than 6 air change per hour, however, the CO₂ levels were consistently greater than 800 ppm and approached 1,400 ppm in a room with 3.9 air change per hour when number of persons in the room increased.

Ventilation Rates by Steady-State CO₂ Level Modeling

Ventilation rates with CO₂ generation at 0.30 L/min (ACH_SS30) and 0.46 L/min (ACH_SS46) were calculated using equation (1) for 2 dental procedures in each room (Appendix Table 1). Both ACH_SS30 and ACH_SS46 were similarly correlated with ACH_VENT (r = 0.83, P = 0.003). ACH_SS30 approximated closely to mechanical ventilation rates in rooms with less than 6 air change per hour (mean difference = −0.6, paired t = −1.24, P = 0.304) but significantly underestimated those in rooms with greater than 6 air change per hour (mean difference = −8.7, paired t = −2.59, P = 0.049). The opposite is true for ACH_SS46; it significantly overestimated the ventilation rates in rooms with 6 air change per hour or less (mean difference = 3.7, paired t = 6.78, P = 0.007) but approximated closer to those in rooms with greater than 6 air change per hour (mean difference = −3.5, pairedt = −1.14, P = 0.307) (Table 2).

Table 2.

Comparisons between Mechanical Ventilation Rates and Ventilation Rates Estimated from Natural CO₂ Levels and CO₂ Released by Dry Ice or Baking Soda.

Room No.	ACHVENT	ACHSS30	ACHSS46	ACHDI	ACHBV	ACHDI63	ACHBV63
002	6.0	5.4	8.2	5.6	6.1	5.6	6.1
003	5.3	6.1	9.5	4.8	4.7	4.9	4.9
008	7.3	4.9	7.5	5.2	6.9	6.7	6.8
012	9.0	6.1	9.3	9.2	9.2	9.2	9.4
019	35.0	11.0	16.8	28.5	27.0	30.8	27.3
021	5.3	5.9	9.0	4.4	4.8	4.4	5.3
022	3.9	5.6	8.6	3.6	4.8	4.1	4.6
031	21.0	11.2	17.2	17.6	16.3	17.2	16.7
031	26.0	16.3	24.9	19.0	22.5	17.3	22.2
033	13.6	10.2	15.5	14.3	15.8	14.3	16.2
Mean	13.2	12.6	8.3	11.2	11.8	11.5	11.9
SD	10.6	5.7	3.7	8.4	8.1	8.2	8.1

ACH_BV, ventilation rate estimate by carbon dioxide (CO₂) decay constants using baking soda and vinegar; ACH_BV63, ventilation rate estimate by time needed to remove 63% excess CO₂ by baking soda and vinegar; ACH_DI, ventilation rate estimate by CO₂ decay constants using dry ice; ACH_DI63, ventilation rate estimate by time needed to remove 63% excess CO₂ by dry ice; ACH_SS30, ventilation estimate by steady-state CO₂ level with CO₂ generation at 0.3 L/min per person; ACH_SS46, ventilation estimate by steady-state CO₂ level with CO₂ generation at 0.46 L/min per person; ACH_VENT, mechanical ventilation rate.

As ventilation rate is likely below 6 air change per hour in private dental practices in small freestanding buildings in the United States (Godwin et al. 2003), CO₂ generation rate (G_P) of 0.30 L/min is more appropriate for ventilation estimates using equation (1). We list the corresponding CO₂ levels and ventilation rates in Appendix Table 2. Dental care professionals may use the CO₂ levels measured in their treatment rooms to roughly estimate the ventilation rate using this table.

Ventilation Rates by CO₂ Decays Using Dry Ice or Baking Soda

Results of ventilation estimates by CO₂ decay using dry ice or baking soda are shown in Table 2. CO₂ decay curves demonstrate that CO₂ levels decreased faster over time in rooms with high air change rate (Fig. 3A, B). ACH_DI values ranged from 3.6 to 28.5 (11.2 ± 8.4) and were highly correlated with the mechanical ventilation rate (r = 0.99, P < 0.0001) (Fig. 3C). ACH_DI was slightly lower than the mechanical ventilation rate (mean difference = 2.0, paired t = 2.32, P = 0.046). Similarly, ACH_BV ranged from 4.7 to 27.0 (11.8 ± 8.1) and also correlated highly with mechanical ventilation rate (r = 0.98, P < 0.0001) (Fig. 3D). There was no statistically significant difference between ACH_BV and the mechanical ventilation rate (mean difference = 1.4, paired t = 1.48, P = 0.174) or between ACH_BV and ACH_DI (mean difference = 0.59, paired t = 1.26, P = 0.239).

Figure 3.

Correlations between ventilation rate and carbon dioxide (CO₂) clearance in dental treatment rooms. (A) CO₂ decay constants by dry ice and (B) CO₂ decay constants by baking soda and vinegar. (C, D) Correlations between mechanical ventilation rates measure by airflow (ACH_VENT) and ventilation rates measured by CO₂ decay using dry ice (ACH_DI) and baking soda and vinegar (ACH_BV) in dental treatment rooms. Rooms with high mechanical ventilation rates showed a rapid decrease of CO₂ concentrations over time (A, B). Both ACH_DI and ACH_BV are linearly correlated with ACH_VENT (C, D). (E, F) Correlations between ventilation rate by airflow (ACH_VENT) and ventilation rates by time needed to reach 63% removal of excess CO₂ generated using (E) dry ice (ACH_DI63) and (F) baking soda and vinegar (ACH_BV63).

Ventilation Rates by Time to 63% Removal of Excess CO₂

Ventilation rates calculated by time needed to remove 63% of excess CO₂ generated by dry ice or baking soda are presented in Table 2. ACH_DI63 values ranged from 4.1 to 30.8 (11.5 ± 8.6) and were highly correlated with the mechanical ventilation rate (r = 0.98, P < 0.0001) (Fig. 3E). There was no statistically significant difference between ACH_DI63 and mechanical ventilation rate (mean difference = 1.8, paired t = 1.93, P = 0.086).

Similarly, ACH_BV63 ranged from 4.6 to 27.3 (11.9 ± 8.1) and correlated highly with the mechanical ventilation rate (r = 0.98, P < 0.0001) (Fig. 3F). There was no statistically significant difference between ACH_BV63 and mechanical ventilation rate (mean difference = 1.3, paired t = 1.34, P = 0.213) or between ACH_BV63 and ACH_DI63 (mean difference = 0.50, paired t = 0.76, P = 0.467).

In Appendix Table 4, a Microsoft Excel template is provided that will allow dental care professionals to calculate the ventilation rate of their offices by inputting the values of peak CO₂ level (C_S), outdoor CO₂ level (C_R), and time (min) needed to reach 63% removal of excess CO₂ generated by baking soda (Jimenez 2020).

Discussion

Our findings affirmed that dental operatories with low ventilation rates and overcrowding facilitate CO₂ accumulation. Ventilation can be measured by assessing natural or experimental buildup of CO₂ levels in dental treatment rooms using a consumer-grade CO₂ sensor. Ventilation rates in air change per hour could be accurately assessed by observing CO₂ levels after a simple mixing of household baking soda and vinegar in dental settings. Time needed to remove 63% of excess CO₂ generated by baking soda could be used to accurately calculate the ventilation rates with the help of a basic calculator.

Our findings show that CO₂ level may consistently stay above 800 ppm in rooms with ventilation rates below 6 ACH, especially when 3 or more persons (including the patient who is not wearing a mask) are in the room during dental treatments. We observed that CO₂ level stayed above 1,000 ppm and approached 1,600 ppm when 3 to 6 persons were in a room with 3.9 ACH in clinical teaching scenarios. High levels of CO₂ indicate high concentrations of respiratory aerosols in the room. It is possible that these aerosols contain pathogens if the patient is not wearing a mask and is infected but asymptomatic or presymptomatic. Effective mitigation measures will be required in these rooms to improve air quality even without the ongoing infectious disease pandemics. Overcrowding should be avoided in rooms with poor ventilation. In dental operatories with ventilation rates higher than 10 ACH, the CO₂ levels stayed consistently below 700 ppm in most cases with 3 persons in the room. Our data demonstrated a clear dependency of CO₂ levels on number of persons in the room and the ventilation rate. CO₂ level is a proxy for indoor air quality as it represents the fraction of rebreathed air, or the proportion of inhaled air that was exhaled by others in the same indoor environment. Although numerous epidemiological studies indicate CO₂ begins to have negative health effects at 700 ppm and respiratory symptoms may occur when indoor CO₂ is above 1,000 ppm (Azuma et al. 2018), our main concern is the concurrent accumulation of respiratory aerosols that may contain infectious disease pathogens. Numerous studies have shown that exhaled air from infected patients contains respiratory disease pathogens, including rhinovirus, influenza virus, and Mycobacterium tuberculosis (Fabian et al. 2009; Issarow et al. 2015; Lindsley et al. 2016; Yip et al. 2019). Patients with early stages of coronavirus disease 2019 (COVID-19) may release millions of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral copies per hour in exhaled air (Qian et al. 2020). In a recent study that modeled factors associated with the spread of respiratory infectious disease in dental offices, CO₂ levels were found to play the most important role in the risk of disease transmission. CO₂ levels at 774 ppm were considered low risk, but those at or above 1,135 ppm may increase the risk of disease transmission in dental offices (Zemouri et al. 2020).

Ventilation rates varied greatly between rooms on the same air handling system due to variations in their sizes and locations and the addition of separate air exhaust fans in some rooms designated for nitrous oxide conscious sedation (Ren et al. 2021). Accurate measurements of ventilation rate in dental settings are important for risk assessment and for risk mitigation planning in an era of frequent infectious disease pandemics. Mechanical ventilation rate is usually assessed by quantifying the amount of outdoor air flowing into and out of an indoor space using highly sophisticated instruments operated by trained professionals (ASHRAE 2017). Technical barriers may have contributed to the scarcity of information regarding ventilation in dental settings. Besides direct airflow measurements, ventilation rate could be estimated using CO₂ as a tracer gas. CO₂ in an indoor space could be built up to a significantly higher level than in outdoor air, either through natural generation by the occupants or through experimental release of the gas (Cheng and Li 2014; Stuart et al. 2015; Batterman 2017). Analysis of steady-state CO₂ levels or the rate of CO₂ concentration decays, which are directly dependent on the outdoor airflow rate from the ventilation system, will allow an estimate of the ventilation rate of the indoor space. We found that modeling CO₂ levels using equation (1) correlated reasonably well with mechanical ventilation but may either under- or overestimate the ventilation rate based on different assumptions of human CO₂ generation rates. In comparison, CO₂ concentration decay method relied on actual CO₂ levels measured at the beginning and the end of a decay period and provided accurate assessments and better approximation to mechanical ventilation rates. CO₂ concentrations in dental operatories could be built up to a level of about 1,500 to 2,500 ppm in 2 min using either dry ice or baking soda. CO₂ decays could then be monitored using a CO₂ sensor that logs data in 1-min intervals. Many affordable consumer-grade CO₂ sensors are readily available and suitable for the purpose of observing CO₂ level changes over a period of time. The CO₂ sensor used in the present study was purchased online for $159 and appeared to be a reliable tool for measuring CO₂ levels in dental settings.

We recommend that mitigation measures be taken for dental operatories that have ventilation rates below 15 ACH, which is required for procedure rooms in outpatient health care facilities by Centers for Disease Control and Prevention guidelines (Chinn et al. 2003). While in theory the most effective measure for air quality improvement in dental offices is to increase outdoor airflow rate through the ventilation system or through natural ventilation by opening doors and windows, such measures are severely limited by the weather or climate conditions. An effective alternative is to improve air filtration using upgraded filters in the ventilation system and portable air cleaners (PACs) equipped with high-efficiency particulate air (HEPA) filters (Ren et al. 2021).

In summary, this study showed that CO₂ level in dental treatment rooms could be measured with a simple consumer-grade CO₂ sensor and that ventilation rate could be determined by either natural or experimental buildup of CO₂ levels in dental settings. Assessing CO₂ levels will allow dental care professionals to conveniently and accurately calculate the ventilation rates in their offices and help them to devise an effective strategy for ventilation improvement.

Author Contributions

Q. Huang, contributed to conception, design, data acquisition, and analysis, critically revised the manuscript; T. Marzouk, R. Cirligeanu, contributed to data acquisition, critically revised the manuscript; H. Malmstrom, E. Eliav, contributed to conception, design, and data interpretation, critically revised the manuscript; Y.-F. Ren, contributed to conception, design, data acquisition, analysis, or interpretation, drafted the manuscript. All authors gave final approval and agree to be accountable for all aspects of the work.