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Introduction The emergence of the COVID-19 pandemic has led to widespread implementation of policies mandating mask wearing, especially in indoor public spaces. The WHO has similarly recommended wearing face masks on a daily basis, which has led to mask wearing in situations not previously considered. Along with this, there has been an increase in the use of masks with high filtering performance, such as an N95-type, as well as several concerns on the physiological effects brought about by wearing masks [1–3]. We present the results from real-time measurement of transdermal CO2 (TrCO2) during exercising and resting conditions, while wearing masks. Materials and Methods Our nondispersive infrared-based TrCO2 measurement system [4], [5] automatically repeats a two-stage measurement every 90 sec: N2 purging for 30 sec and recirculation for 60 sec (Fig.1a). During the recirculation, TrCO2 is continuously released via the skin into the N2 carrier gas. Then, CO2 concentration in the recirculated N2 increases according to various parameters, depending on the blood CO2 concentration. This study was approved by the Institutional Review Board, University of Maryland, Baltimore County (Number: 89). Informed consent was obtained from all subjects before conducting the experiments. Study participants without history of ailments or symptoms for any diseases were recruited. All subjects were asked to consume a meal at least 2 hours prior to the experiments. Information about gender, age, BMI, smoking history, exercising habit, and content of the last meal was obtained through a questionnaire. A transdermal gas sampler and thermometer (Elitech) were attached to the inside of the subject’s forearm, and a heart rate (HR) sensor (Polar) was fitted around the chest. Room temperature and humidity were additional parameters monitored as part of this study. In the first experiment, TrCO2 was measured while resting with the masks on. A baseline was obtained by measuring TrCO2 while resting without masks before and after the trial. In the second experiment, subjects were asked to perform a stepping task in time with a metronome, while TrCO2 was measured with and without masks. The baseline was obtained in similar manner, as in the first experiment. Results and Discussions Two sets of experiments per day were performed for three days per subject in a randomized order. There was no difference in TrCO2 levels with or without masks during resting, even when the study participants were wearing an N95 mask overlaid with a surgical mask, and a face shield. The differences between the masked and non-masked state in TrCO2 sampling during exercise was observed for one subject among three study participants. Fig. 1b represents an overview of the exercising experiment. The subject was stepping at a rate of 140 bpm, once without masks, and then while wearing the N95 mask. Increase of TrCO2 levels during exercise from baseline (ΔTrCO2) was 0.63 ± 0.08 without masks, and 1.10 ± 0.06 ppm/sec with the N95 mask. The limitations of this study include a small sample size, and difficulty in equalizing individual differences in exercise intensity and capacity. The results are intriguing given that there is at least one young and healthy subject who showed a 1.7-fold increase in TrCO2 emission levels while wearing a mask, correlating to the CO2 levels in the body, in comparison with the other two subjects. As we do not have statistically significant results in this case, further studies are currently underway to better understand the initial findings. Ultimately, it is desirable to monitor a statistically significant population with a wearable TrCO2 sensor to understand the effect of wearing masks both while resting and exercising, excluding the physiological impact that comes along with it. Conclusions Real-time monitoring of TrCO2 levels is possible using our rate-based TrCO2 measurement system. There is no effect of wearing masks on TrCO2 levels while resting. Although two subjects did not show any significant difference with and without wearing masks during exercise, one study participant was an exception as their TrCO2 emission rate was clearly affected by wearing masks during exercise. We would like to increase the number of study participants and introduce diversity with normalized exercise protocol to obtain statistical evidence in a further study using this developed system. References [1] B. Chandrasekaran and S. Fernandes, Med. Hypotheses 144 (2020) 110002. [2] T. Greenhalgh, et al., Med. Hypotheses 144 (2020) 110255. [3] B. Chandrasekaran and S. Fernandes, Med. Hypotheses 144 (2020) 110302. [4] M. Chatterjee, et al., Physiol. Meas. 36 (2015) 883–894. [5] X. Ge, et al., Med. Eng. Phys. 56 (2018) 36–41. Figure 1