In order to understand the sensitivity of our sensor to human respiration in an enclosed room, we initially monitored the change in CO2 concentration when one adult human resides in a simulated OR at ambient pressure and without ventilation. Two sensors were placed near opposite walls in the room in order to understand whether or not large spatiotemporal fluctuations in CO2 concentration develop. The CO2 level as a function of time for a representative trial is shown in Fig. 2 (sensor 2 data shown; see supporting information S1 for sensor 1 and subsequent trials data). Prior to entry, the background level of CO2 in the room was relatively constant, fluctuating around 525 ppm. After the individual entered the room and assumed a resting position seated in a chair, the CO2 level increased by approximately 200 ppm over a period of 1 hour. After 1 hour at rest, the individual stood up and walked in a circular path around the OR for 1 hour, reversing direction every 5 minutes. The increased activity level produced a faster rate of CO2 accumulation in the enclosed room, increasing by 350 ppm after 1 hour. After 1 hour of pacing the OR, the individual re-assumed the initial resting position. The CO2 level continued to rise, but at a reduced rate compared to the first and second hours in the OR, increasing by 156 ppm after 1 hour and bringing the total concentration to approximately 1259 ppm, which is approximately 2.4 times the initial background level. The individual then left the room, closing the door after exiting. The CO2 level remained relatively constant while uninhabited. After monitoring the CO2 level in the empty room for 1 hour, the door was propped open. The CO2 level rapidly reduced as the door remained open. Replication of the experiment multiple times showed consistency in the total change and rate of change in CO2 concentration for both sensors (See S1-S5).
The average change in CO2 concentration and average slope for each of the three hours spent in the room are shown in the bar graph on the right in Fig. 2. The rate of CO2 accumulation consistently increases when the individual transitions from a resting state to moderate activity, and consistently decreases when the individual re-assumes a resting state. The average slopes for the initial resting phase (At Rest I), the active phase (Pacing) and the second resting phase (At Rest II) are 3.8 ± 0.3 ppm/min., 6.4 ± 0.5 ppm/min. and 2.4 ± 0.1 ppm/min, respectively. The average change in CO2 for the corresponding phases are 190 ± 10 ppm, 390 ± 30 ppm and 150 ± 7 ppm, respectively. The results show that a single individual can considerably increase the concentration of CO2 in a room and, by extension, affect the quality of the air. Importantly, an individual’s contribution to the CO2 level in a room can be reliably detected with a portable NDIR sensor and the rate of increase corresponds to the occupant’s activity level. Higher activity increases metabolic rate which in turn increases CO2 production. A recent study has similarly concluded that measured CO generation rates are positively associated with physical activity levels.[18] In all trials the CO2 level did not exceed 1400 ppm, which is below the Occupational Safety and Health Administration (OSHA) permissible exposure limit of 5000 ppm (averaged over 8 hours), however, the need for awareness is apparent. As noted above, a growing body of cross-disciplinary literature in fields that include cognitive psychology, indoor air quality and neuroscience suggests that increasing CO2 concentrations can impair cognitive function. For example, Allen et al.[25] reported declines in “cognitive function scores” when indoor CO2 levels were increased from approximately 500 ppm to approximately 950 ppm, and greater declines when the CO2 level was raised to approximately 1400 ppm. Activities that involve increasing amounts of time and/or number of people in an enclosed space with limited ventilation could produce CO2 levels above permissible limits or exceed levels associated with compromised cognitive performance.
In order to understand if metabolic CO2 production can be detected in an OR operating at standard conditions, we repeated the experiment under standard OR airflow conditions. The room ventilation operated at 20 air changes per hour (ACH) with a positive room pressure of 0.03 in. H2O. Figure 3 shows the CO2 concentration as a function of time for an individual in the enclosed OR. When at rest, it is very difficult to detect occupancy due to the air flow conditions. When the individual begins to walk in the room as described above, the CO2 level increases with time. After one hour of walking the maximum change detected was approximately 73 ppm. Similar results were obtained with the additional sensor in the room (see S6). This result shows that while the air circulation system and positive pressure have a beneficial effect on the air quality, it does not completely eliminate the respiratory influence on the air composition. The metabolic CO2 production of a single person can be detected even with standard OR air flow conditions, indicating that the exhalation products of one person can accumulate at a rate that is higher than its displacement by the ventilation system.
In order to understand if the CO2 level increases with the number of individuals in an OR, we monitored the CO2 level when occupied by a pre-selected number of individuals for a period of 20 minutes under normal OR air flow conditions. Figure 4 shows a graph of change in CO2 level as a function of time for different quantities of occupants. The presence of a single individual pacing the room produces a clear rise in CO2 concentration. After repeating the experiment three times the average increase for sensors 1 and 2 show accumulations of approximately 54 ± 7 ppm and 59 ± 9 ppm, respectively. After exiting and ensuring that the door was closed, the detected CO2 concentration briefly increases before showing a long-term attenuation to the baseline level as shown by the representative trace in Fig. 4. As the number of occupants in the room increases, the CO2 level increases. When four occupants pace the room for 20 minutes, the CO2 level increases by 290 ppm. When three and two occupants pace the room for 20 minutes, the respective CO2 concentrations are 181 and 145 ppm. A graph on the right in Fig. 4 displays the change in CO2 concentration as a function of the number of occupants that were in the room. Under the conditions employed, the CO2 rises in a linear fashion. These results show that as the number of people in a room increases, the CO2 concentration increases, even under air purifying ventilation standards that are encountered in an OR. Note that the levels of CO2 obtained will vary with individuals, activity levels and ventilation rates, however, our data shows a clear trend indicating that higher CO2 levels are detected when the number of occupants in a room increases. A difference of one person is enough to detect a change in CO2 concentration.
In order to minimize surgical site infections, it is important to maintain clean air in an OR. Individuals introduce a number of viable and non-viable impurities to the air in an OR through various mechanisms including shedding, displacing particulate matter at rest on a surface and breathing, the latter of which produces CO2 [34–37]. We have demonstrated that changes in CO2 concentration provide a biomarker of human presence that can be detected even under air flow conditions of 20 ACH and positive pressure. Tracking CO2 level in an OR can help manage human traffic and facilitate the epidemiology of surgical site infections. Although well-managed ORs are ventilated and in many cases under positive pressure, our results show that the presence of one breathing individual creates detectable changes in the composition of the air in a properly ventilated OR. While at relatively low concentrations CO2 itself is benign, it provides a proxy for more harmful substances that are emitted via expiration, including the highly disruptive SARS-CoV-2 which causes COVID-19 [23]. Additionally, a number of health effects associated with rising CO2 levels in an enclosed space include drowsiness, loss of concentration, nausea, headaches and more severe consequences associated with oxygen deprivation as higher concentrations are reached [38]. Gaining an awareness and understanding of the effect of occupancy time and population on the CO2 concentration in an OR will facilitate public health risk management and spur the development of new technologies for using CO2 to track the occupancy and respiratory history of an OR during surgery. In combination with good ventilation, particle filtration, PPE and UV disinfection, CO2 monitoring can be employed in the toolbox of techniques used to manage, assess and prevent hospital acquired infections.