Effects of face masks on oxygen saturation at graded exercise intensities

Eligibility criteria

This study was approved by the Midwestern University (MWU) Institutional Review Board (IRB, AZ#1422). The inclusion criteria for this study included adults between 21 and 75 years of age with no underlying heart or respiratory illness. Participants who had knowledge of an existing medical condition that can hinder their ability to perform the study, such as hypertension, diabetes mellitus types I and II, and respiratory and cardiovascular illnesses, were excluded. The exclusion criteria were presented as a list to each individual participant. Data were collected from November 2020 to May 2021. Our study participants consisted of both males and females from MWU, including faculty and students. The health assessment was performed on a self-assessment basis. All participants provided written informed consent by signing paper consent forms that were collected by the study observers. In addition, participants filled out MWU’s COVID-19 health questionnaire upon enrollment into the study. The participant’s age, height, and BMI were collected prior to the start of the activity. HR and SpO₂ via pulse oximeter were collected after every completion of each activity level. All experiments were performed on university campus grounds abiding by university and CDC guidelines on social distancing.

Study design

A cross-sectional study was performed where physiological parameters were measured while the participants participated in three activity levels (duration of 10 min each) in a randomly assigned order: rest (sitting), walking, and exercise (stair climbing), with a minimum of 30 min of rest between each exercise. Participants served as their own controls. Participants conducted every activity level wearing every face mask condition separately: no mask, surgical mask (Morestep, disposable medical mask, model 175 × 95 mm), and N95 respirator (3M 1860). Thus, a total of nine activities were completed by each participant (Figure 1). In the walking condition, participants walked at an average pace of 110 steps/minute, and in the stair-climbing condition, participants climbed stairs at an average pace of 122 steps/minute.

Figure 1:

Representative diagram of study activities. Participants performed each activity (rest, walk, stair climb) under every mask condition (no mask, surgical mask, N95). Each activity was performed for 10 min. For rest, the subject remained seated. For walk, the subject walked a designated course. For stair climbing, the subject climbed a designated three-flight stair set.

Physical assessment and pulse oximetry

To quantify physical activity difficulty, the Borg 15-point scale of perceived exertion was utilized. This scale is a participative analysis of the perceived intensity experienced by the participants. The scale ranges from 6 to 20 (Figure 2). The device utilized to measure SpO₂ was the McKesson Fingertip Pulse Oximeter (MFR#16-93651, McKesson, Richmond, VA), which provides rapid, non-invasive assessment of the participant’s SpO₂ and pulse rate. Per the manufacturer’s website, the McKesson Pulse Oximeter has the following sensitivities: measures pulse rates between 30 and 235 bpm with an accuracy of ±2%, measures SpO₂ levels ranging from 70 to 99% with an accuracy of ±3%, and requires no calibration. Pulse oximeter SpO₂ measurements were taken immediately at completion of each timed activity.

Figure 2:

The Borg 15-point scale was utilized to evaluate the participants’ perceived exertion after each study activity.

The scale ranges from 6 to 20, reflecting a range between very light exertion to very hard.

Study and patient characteristics

Forty participants who met the eligibility criteria successfully completed all nine study activities. In total, there were 20 females (mean age, 29.10 ± 10.67 years; range, 22–61 years) and 20 males (mean age, 28.35 ± 8.61 years; range, 24–64 years; Figure 3A). The mean age was found to be comparable between male and female cohorts (p=0.8081). However, the mean BMI was significantly higher in males (27.79 ± 4.26 kg/m²; range, 22.3–37.0 kg/m²) compared to females (24.01 ± 3.02 kg/m² [range, 18.5–29.8 kg/m²], p=0.0027; Figure 3B). Table 1 contains mean Borg, HR, and SpO₂ values under all mask and exercise conditions, and Table 2 contains results from the mixed-effects ANOVA model.

Figure 3:

Comparative assessment of age and BMI between male and female participants. (A) Mean age and (B) BMI of subjects. There were 20 male and 20 female subjects. BMI, body mass index.

Perceived exertion

All participants reported their perceived exertion after every activity utilizing the Borg 15-point scale. Figure 4A displays the interaction effects between the mask type and the Borg 15-point scale score. The use of a post hoc test to evaluate perceived exertion by mask condition indicated a significant increase in mean Borg 15-point scale score in N95 users (9.77 ± 0.196) in comparison to no-mask users (9.23 ± 0.196, p=0.007, Figure 4A). Table 3 shows the mean Borg 15-point scale scores by exercise and sex. There was a significant difference for the interaction effect between exercise and sex (p=0.0047). A significant increase in the mean Borg 15-point scale score was observed separately among the male and female cohorts when walking compared to rest, stair climbing compared to rest, and stair climbing compared to walking (Figure 4B). While evaluating exertion by exercise intensity between sexes, however, no significant difference was found between males and females at rest (p=0.9997), walk (p=0.1225), or stair climbing (p=0.9971). There were no interaction effects found in perceived exertion between mask type and any exercise intensity (p=0.1335), nor between mask type and sex (p=0.9761).

Figure 4:

Comparative assessment of perceived exertion.

(A) The interaction effect of mask type on perceived exertion (Borg 15-point scale score). (B) The effect of exercise on perceived exertion. A comparison of both sexes is shown. Perceived exertion was evaluated by self-reported Borg 15-point scale scores. Significant differences are represented by significance lines.

Heart rate (HR)

HR was measured utilizing a pulse oximeter after each study activity was completed. There were statistically significant differences in the mean HR for mask type overall (p=0.0044). From post hoc tests, there was a significant increase in mean HR for N95 users (110 ± 2.51 beats/min) compared to no-mask users (104 ± 2.51 beats/min, p=0.0031, Figure 5A). There was also a significant difference in HR for exercise type overall for the main fixed effect (p<0.001). When comparing HR between exercise types utilizing post hoc tests (Figure 5B), there was a significant increase in mean HR with each sequential exercise intensity, from rest (80.9 ± 2.51 beats/min), to walk (98.1 ± 2.51 beats/min), to stair climbing (142.3 ± 2.51 beats/min, p<0.0001, for all comparisons between stairs vs. rest, stairs vs. walk, and rest vs. walk). No differences were found for any interaction effects when evaluating mean HR between mask types at any exercise intensity (p=0.9208), between mask type and sex (p=0.9910), or between exercise intensity and sex (p=0.3029).

Figure 5:

Comparative assessment of HR (beats per minute).

(A) The influence of mask type on HR. (B) The influence of exercise type on HR. HR was evaluated via pulse oximeter. Significant differences are represented by significance lines. HR, heart rate.

Blood oxygen saturation (SpO₂)

Oxygen saturation was measured utilizing a pulse oximeter after every activity level. There was no difference in mean SpO₂ when evaluating mask type overall for the fixed effect (p=0.0854), or between the interaction effects of mask type at any exercise intensity (p=0.9208), or between mask type and sex (p=0.8680). The mean SpO₂ values in males and females at each exercise condition is shown in Table 2. There was a significant difference in mean SpO₂ for the interaction effect of sex and exercise intensity (p=0.0055, Figure 6, Table 4). The mean SpO₂ in females at rest was significantly higher than in their male counterparts at rest. The mean SpO₂ in females while walking was significantly higher than in males walking. There was, however, no difference in mean SpO₂ between males and females in high-intensity stair climbing. A significant difference in mean SpO₂ was also found within sexes for some comparisons in exercise intensity. Mean SpO₂ in females while walking was significantly higher than in females during stair climbing. Mean SpO₂ was significantly higher in males while walking compared to males at rest, as well as in males while stair climbing compared to males at rest.

Figure 6:

A comparative assessment of mean SpO₂ and the interaction effects between sex and exercise type. SpO₂ was recorded via pulse oximeter. Significant differences are represented by significance lines.

Perceived exertion

A common concern among the general population relating to mask wearing is the increased perception of discomfort and perceived claustrophobia when wearing a mask. According to our data, there was an increase in perceived exertion in N95 users in comparison to no-mask users. The increased perceived exertion in N95 users may be attributed to mask discomfort rather than a change in physiological parameters such as SpO₂ and HR. A previous study by Li et al. [17], evaluated the subjective experience of mask wearing during intermittent treadmill exercise, and reported that N95 users (n=11) reported the mask feeling tight, unfit, itchy, odorous, and salty, with an increased perception of fatigue in comparison to no-mask users during aerobic exercise. Additionally, masks may increase inspiratory and expiratory resistance with increased facial temperature and humidity during exercise, thereby increasing perceived respiratory effort [18]. Cloth masks, although widely utilized, were not evaluated in our study due to their diversity and lack of standardization, which hinders generalizability. However, Doherty et al. [19] (n=12) compared participant-owned two-ply cloth masks against surgical masks and no-mask controls and reported significantly greater perceived exertion in cloth mask users. The findings by Doherty et al. [19] are in contrast with a study by Shaw et al. [20] (n=14) that evaluated a three-ply cloth mask and found no increase in perceived exertion in comparison to surgical masks and no-mask controls. The discrepancies between these two studies may be attributed to differences in the cloth masks themselves, participant fitness, or the method of testing. Neither of those two studies made comparisons to the N95. Future studies conducting direct comparisons between multi-ply cloth, surgical, and N95 masks could provide valuable insight on their effects on perceived exertion.

Heart rate (HR)

Our study found a significant increase in HR with each sequential exercise intensity in all mask conditions, as expected to occur with incremental levels of exertion. An unexpected outcome of our study was a significant increase in HR for N95 users in comparison to no-mask users in the main fixed effect. The increased HR in N95 users may be due to the constrictive and claustrophobic nature of the N95 having the potential to increase the sympathetic nervous system response and its sequelae such as elevated HR [21]. The increased HR in N95 users is corroborated by Li et al. [17] and Kim et al. [22], both of which report peak HR in healthy N95 users (n=20) during aerobic exercise. On the other hand, Epstein et al. [12] reports no significant difference in HR between healthy no-mask and N95 users (n=16), whereas Kampert et al. [23] found a significant decrease in HR in healthy N95 users (n=20) in comparison to no-mask users during aerobic exercise. The evidence on the effects of N95 on HR is inconsistent and should be considered in future studies while evaluating HR between mask conditions with or without underlying heart or lung conditions.

Blood oxygen saturation (SpO₂)

In our study, mean SpO₂ was higher in our female cohort in comparison to the male cohort under the rest and walking conditions. There were no significant age differences, underlying medical conditions, or differences in SpO₂ contributed by mask type in either sex. The main differentiating characteristic was mean BMI, which was greater in males than in females. A study by Petrofsky et al. [24] evaluated SpO₂ and BMI in 81 healthy subjects undergoing graded exercise and showed an inverse relationship between BMI >30 kg/m² and SpO₂. Although mean BMI did not exceed 30 kg/m² in our participants, our findings agree with the inverse relationship between BMI and SpO₂. The relationship between BMI and SpO₂ may be attributed to breathing technique rather than oxygen delivery because a higher BMI may correlate with increased abdominal adiposity that restricts diaphragmatic movement and lung perfusion [24]. Additional mechanisms of sex-related SpO₂ differences reported in the literature highlight reproductive hormones such as progesterone, a known respiratory stimulant, which may account for higher SpO₂ in females [25]. A previous study whose primary goal was to evaluate the relationship between sex and SpO₂ (n=207) found a significant difference in mean SpO₂ between males (97.1 ± 1.7%) and females (98.1 ± 1.0%), and interpreted this finding as the result of stimulatory effects of progesterone on central and peripheral chemoreceptors, leading to increased respiratory rate [25]. The same study also evaluated data on the phase of the female menstrual cycle and found that female SpO₂ was unaffected by menstrual phase. It may be beneficial in future studies to collect data on the phase of the female menstrual cycle to corroborate the relationship between reproductive hormones and SpO₂. An additional finding in our study was that mean SpO₂ was not significantly different between males and females in the high-intensity stair-climbing phase under any mask condition. A potential reason for this finding is the equal and maximal effort of breathing by all participants at high intensity. Differences in SpO₂ in our study were contributed only by sex and BMI. SpO₂ was not changed by masks at any exercise intensity and indeed remained consistently above 95% in all conditions of our study for both sexes. The main findings of our study support that surgical masks and N95 respirators do not affect SpO₂ at rest and exercise.

Limitations

Because our study was performed during the pandemic, the experimental design was modified to meet federal, state, and university health mandates at the time. Due to the social distancing mandate, data collection was limited to the outdoor setting where strict intensity controls could not be implemented, such as utilizing a treadmill. These restrictions hindered the ability to standardize the activity level achieved by participants during exercise, which otherwise may have helped strengthen the perceived exertion findings in our study. We also limited our testing conditions to 10 min each to account for the length of the study in which participants were required to perform a total of nine 10-min exercise conditions. We chose 10 min to more closely mimic clinical testing; however, time to exhaustion may be better representative of the real world for future studies. However, time to exhaustion may have been largely different among each participant, which could not be controlled. Additionally, HR and SpO₂ data were obtained immediately postactivity, which could produce different values in comparison to HR and SpO₂ during the midst of the activity. We lacked the means to obtain said values during activity bouts in a controlled environment because our study was conducted outdoors on campus grounds. Future studies should consider implementing means to measure HR and SpO₂ during activity to obtain real-time values. Similarly, the tools we selected to obtain data were those that required the least amount of contact with participants in compliance with state/university mandates. Such tools included the pulse oximeter, for which the sensitivity is not calibrated, potentially impacting the detection of differences in SpO₂ at different exercise and mask conditions. Despite this, we chose the pulse oximeter to measure SpO₂ because it is utilized in clinical practice. Our study excluded participants with known medical conditions that could impact their ability to perform the activities of this study. However, the overall fitness level of the participants in our study may only be interpreted with the average BMI. The participants were recruited from a healthcare professions institution, which may impact the generalizability to the public. Additionally, as noted in our study, females had a higher SpO₂ than males in two out of the three activity levels, possibly because of the stimulatory effects of progesterone on respiration. We had not collected data on female menstrual stage to corroborate this interpretation, which may be beneficial in future studies.

Future studies would benefit largely from including data on additional parameters such as blood pressure, partial pressure of CO₂, respiratory rate, and work of breathing, which were not collected in our study because our study design was modified to meet state and university mandates for social distancing. Such parameters would provide insight as to whether the surgical mask or N95 mask impaired breathing and increased the work of breathing compared to the subject’s perceived difficulty in breathing. Future studies should also focus on evaluating the effects of face masks on physiological parameters to perform a comparative assessment in those with and without medical conditions. Additionally, to our knowledge, our study includes the largest sample size inclusive of males and females in this topic of research; however, future studies with even larger sample sizes could generalize the data to greater degrees.