What do masks mask? A study on transdermal CO2 monitoring

Abstract

Medical professionals have complained of extreme discomfort and fatigue from continuous wearing of N95 respirators (N95) overlaid with surgical masks (SM) and face shields (FS) during COVID-19 pandemic. However, there are no reports on the effect of face coverings on transdermal CO₂ (TrCO₂) levels (a measure of blood CO₂) during moderate activity. In this study, real-time monitoring of TrCO₂, heart rate and skin surface temperature was conducted for six subjects aged 20-59 years with and without wearing personal protective equipment (PPE). We initially studied the effect of wearing PPE (N95+SM+FS) at rest. Then, the effect of moderate stepping/walking activity (120 steps per minute for 60 min) while wearing PPE was evaluated. In addition, we investigated the effect of exercising intensity with different masks. We observed a significant difference (p < 0.0001) in TrCO₂ levels between without and with PPE during moderate exercise, but not while resting. TrCO₂ levels were correlated to exercise intensity independently with masking condition and breathability of masks. For the first time, we present data showing that a properly fitting N95 worn along with SM and FS consistently leads to elevated TrCO₂ under moderate exertion, which could contribute to fatigue over long-term use.

Keywords: COVID-19; Carbon dioxide; Facemask; Personal protective equipment; Transdermal monitoring; non-dispersive infrared.

Fig. 1

Systems and experimental design. (A) Overview of sensor equipment used for different measurements. (B) Detailed view of sensor head placed on the inner side of the subject's forearm. A TrCO₂ sampler connected to the previously reported rate-based transdermal CO₂ analysis system. (C) Two types of experimental conditions were used to investigate the effect of masks on CO₂ levels in the body.

Fig. 2

Effect of PPE on TrCO₂ rate and vitals while resting. (A) A typical variation in TrCO₂ and vitals over time in static conditions observed without and with PPE- N95, SM, and FS. P1, P2, and P3 represent the data before, during, and after PPE wearing, respectively. (B) Comparison of the average TrCO₂ emission rate and HR in each of the phases. n.s.: no significant difference.

Fig. 3

Time course studies on TrCO₂ emission rate, HR, RT, RH, SST, and AT in the dynamic study at 120 spm. (A) Data from trial without PPE; (B) Data from trial while wearing PPE-face coverings, N95 overlaid with SM and FS during P2′. ΔTrCO₂ and ΔHR were defined as the difference between minimum value of 0–30 min range and average value of 80–90 min.

Fig. 4

Effect of wearing PPE face coverings on TrCO₂ levels during moderate exercise. (A–F) Time course studies on ΔTrCO₂ without PPE (▲) and while wearing PPE-N95 overlaid with SM and FS (●) in subjects 1–6. N = 3 except for subject 5. Each plot and error bar indicates mean value and SD. (G) Comparison of differential values of ΔTrCO₂ emission rate without and with PPE obtained on the same day. There are six curves represented subject 1 (●), subject 2 (▲), subject 3 (◆), subject 4 (×), subject 5 (■), and subject 6 (▼). Each plot and error bar represents mean value and SD that were calculated with experiments performed on different days. (H) Comparison of ΔTrCO₂ emission rate between without and with PPE in each of the subjects with error bars representing SD. Mean value and SD were calculated from the values at 80–90 min in each experiment shown in (A–F). n.s. represents no significant difference. (I) Comparison of ΔTrCO₂ emission rate between without and with PPE using data from all subjects (six subjects, thirty-two experiments). Boxes, white lines, error bars, and subplots (●) indicate quartile range, median, data range, and individual experimental data, respectively.

Fig. 5

Effect of PPE face coverings breathability on TrCO₂ levels. Time course of ΔTrCO₂ emission rate with various types and overlaying patterns of PPE during 140 spm of stepping exercise: with N95 overlaid with SM and FS (●), with N95 overlaid with SM (◆), with N95 (■), with SM (▼), with FS (+), with CM (×), and without PPE (▲). Each plot and error bar indicates mean and SD calculated with data from triplicate studies.