Introduction

Most nations introduced the use of face masks in the community to contrast SARS-CoV-2 pandemic.^1,2 Italy, one of the first countries to experience an epidemic wave,³ required masks to be worn by all people older than 5 years, indoors and outdoors.⁴

Surgical masks and respirators are assumed to reduce the spread of SARS-CoV-2,² and are believed to decrease the incidence of other airborne infections.⁵ On the other hand, a prolonged mask use has been associated with higher viral loads and more severe symptoms in infected people (possibly due to the re-inhalation of viral particles trapped in the mask),⁶ with skin disorders due to pathogens contamination, a higher likelihood of frequent cough, sputum production, dyspnea, and panic attacks,^7,8 with delayed cognitive development in infants,^9,10 and with a substantial rise of inhaled carbon dioxide (CO₂), which in turn may cause other symptoms.^11,12

Few studies, however, directly assessed CO₂ in air inhaled while wearing masks in the general population.^13,14 Three of these studies had an overall sample of 12 adults and 45 children, and used measurement tools which could not avoid the interference of water vapor,^13–15 which is typically high in exhaled air,¹⁶ and is known to substantially decrease accuracy.¹⁷ One study used a cardiopulmonary stress testing device, finding a negative impact of masks on cardiac and pulmonary parameters during exercise.¹⁸ Finally, 4 further studies used capnography, the most appropriate instrument to verify changes in respiratory gases.¹⁹ Early markers of hypoventilation were observed after the 12-minute walking test among 22 children wearing FFP2 respirators,^20,21 after the 6-minute walking test among 100 adults wearing surgical masks,²² and even at rest in 30 adults and elderly with both types of masks.²³

With the present study, we aimed at expanding the evidence about the potential inhalation of excess CO₂ as a consequence of wearing surgical masks or FFP2 respirators, among adults, children, and the elderly. Therefore, we used a professional real-time capnograph, with water-removal tubing, in order to assess the inhaled air CO₂ concentration in a sample of healthy individuals wearing different types of masks.

Materials and Methods

Study design

In this observational, descriptive study, we measured the end-tidal CO₂ (ETCO₂) in all participants (a) without masks; (b) wearing a surgical mask; (c) wearing a Filtering Face-Piece grade 2 (FFP2) respirator. Given that the masks constitute an added dead space of the airways, with different volumes depending on mask size and face shape,²⁴ the concentration of CO₂ within this added dead space can be assessed measuring the ETCO₂, which indicates how much CO₂ is exhaled in the final phase of the expiration.²⁵ The evaluations of ETCO₂ were performed after 10 minutes of rest, with participants seated, silent, and breathing only through the nose. To account for potential oscillations, which were however infrequent (n = 6), a trained physician (CAM) took measurements at minutes 3, 4, and 5, and the final value used in the analyses was the average of the 3 measurements.²⁶

All masks were identical and were provided by the investigators, who monitored and eventually adjusted the fit.²⁷ The surgical mask was a 3-layer plane-shaped disposable face mask with ear loops (17.5 × 9.5 cm, conforming to UNI EN ISO 14683:2019 and AC:2019 regulations). The FFP2 was a 5-layer disposable respirator (15.0 × 10.0 cm, conforming to EN 149:2001 and A1:2009), equivalent to United States N95.

The measurement tool was a Rad-97™ capnograph with real-time side-stream gas measurement and water-removal tubing (Masimo Corp., Irvine, CA, USA).²⁸ The sampling point (nasal cannulas) was positioned outside the exhaled air stream—below the lips of each subject—to ensure that the detected ETCO₂ was that of the volume of air within the masks (Figure 1). Consistently, participants were also required to keep their mouth closed, thus ensuring reproducibility by preventing any exhaled air stream from reaching the cannulas. The capnography device measured CO₂ in mmHg, which was converted to ppm using a standardized conversion formula.²⁹

Figure 1.

Photos of the measurement method. (A) Capnograph and oximeter measurement while wearing no mask: no ventilation curves are apparent, ETCO₂ and respiratory rate are not detected, and oxygen saturation is 97%. The positioning of the nasal cannulas below the lower lip can be seen. (B) Measurement while wearing the surgical mask: ventilation curves are apparent, ETCO₂ is 31 mmHg, respiratory rate is 22 breaths per minute, and oxygen saturation is 97%. (C) Measurement while wearing the FFP2 respirator: ventilation curves are apparent, ETCO₂ is 33 mmHg, respiratory rate is 18 breaths per minute, and oxygen saturation is 96%.

The environmental CO₂ concentration (in ppm) was measured using an automatic Temtop mod. M2000C^® sensor (Elitech Technology Inc., Milpitas, CA, USA). All measurements were taken into a room that was constantly and amply ventilated with external air.

For each participant, information was also collected on blood oxygen saturation and respiratory rate (measured at the same time points as the ETCO₂), age, gender, weight and height, and smoking (current—at least one cigarette per week—or former). Blood oxygen saturation was measured through a LTD800^® digital finger pulse oximeter (Dimed Co. Ltd., Cavriglia, AR, Italy).

Results

Outcomes

The mean inhaled air CO₂ without masks was 460 ± 20 ppm. While wearing the surgical mask, the mean CO₂ was 5087 ± 1579 ppm (95% confidence interval 4828-5346 ppm), and exceeded 5000 ppm in 41.1% (33.0%-49.5%) of the measurements. While wearing the FFP2 respirator, the average CO₂ was 9653 ± 2874 ppm (9183-10 123 ppm), and 98.6% (95.2%-99.8%) of the participants showed values higher than 5000 ppm (Table 1). Among the minors, the mean CO₂ concentration when wearing surgical masks was 7091 ± 2491 ppm (6039-8144 ppm), and was considerably higher than among the adults (4835 ± 869 ppm; P < .01), or the elderly (4379 ± 978 ppm; P < .001). A similar difference by age class was observed also for the FFP2 respirators (Table 2).

Table 1.

Outcomes for the overall sample and results of the multiple linear regression predicting overall inhaled air CO₂ in ppm (N = 146).

Table 2.

Sample characteristics and outcomes by age-class.

The CO₂ concentration varied also by respiratory rate: wearing surgical masks, inhaled air CO₂ was 4670 ± 750 ppm among the individuals with respiratory rate ⩽14 breaths per minute, progressively rising to 5656 ± 2193 ppm when 18 or more breaths per minute were taken. A similar trend was observed for FFP2 respirators (Table 1).

Multivariate analyses substantially confirmed univariate results: a higher respiratory rate was significantly associated with higher inhaled air CO₂ wearing both masks. Regression coefficients for ⩾18 compared to ⩽14 breaths per minute were +570 (P < .05) and +1347 (P < .01), respectively, with surgical masks and FFP2 respirators (Table 1).

Male gender, age, overweight (BMI ⩾25 vs <25), and smoking were all associated with a significantly lower CO₂ concentration (model fit R² .45 for both mask types). Wearing surgical masks, regression coefficients were: −658 (−1069 to −247) for male gender, −197 (−304 to −89) for 10-year increases in age, −1069 (−1544 to −594) for overweight, and −818 (−1320 to −317) for smoking. Wearing FFP2 respirators, coefficients were: −1071 (−758 to −383) for male gender, −308 (−487 to −128) for 10-year increases in age, −2440 (−3235 to −1645) for overweight, and −1754 (−2593 to −914) for smoking.

Finally, both respiratory rate and blood oxygen saturation did not differ substantially without or with the masks. Also, when wearing masks, the mean ETCO₂ remained within 33 mmHg.

Discussion

In our sample of healthy individuals, at rest, after 5 minutes of surgical masks use, the mean inhaled air CO₂ approached the occupational exposure limit of 5000 ppm in adults and the elderly.³⁰ However, this threshold was largely exceeded in children wearing surgical masks and in all age classes when wearing FFP2 respirators. Notably, the CO₂ concentration significantly increased with increasing respiratory rates, reaching around 5600 ppm in those breathing at 18 or more breaths per minute with surgical masks, and the minors showed substantially higher CO₂ concentrations than adults.

High CO₂ concentrations in masks worn by individuals at rest were previously reported by an extensive review of studies published up to 2020,¹¹ and by 3 more recent studies,^13–15 which however had very small sample sizes and used instruments that could not avoid the interference of water vapor.^16,39 The available capnography studies report an increase in ETCO₂, suggesting that masks may impair ventilation to a certain degree, especially during physical activity.^20,22,23 The present study was the first to quantify the CO₂ concentration within face masks by using capnography. Indeed, the explanation of the observed high CO₂ values lies in the combination of tidal and mask volumes: even though the 500 ml tidal volume of the average adult man is predominantly filled with low environmental concentrations of CO₂,²⁵ the portion represented by the mask dead space had a CO₂ content so high that the overall inhaled air CO₂ increased substantially.⁴⁰

Concerning the risk of hypoxia, several evaluations found that blood oxygen saturation decreases when donning face masks, and even more noticeably when wearing them for long periods or during physical activity.^11,22,23,40 In contrast, the present study was performed at rest and for a short time, during which the recorded levels of CO₂ did not substantially alter blood oxygen saturation, as in similar studies.^13,14,41 Nevertheless, the exposure to inhaled air CO₂ values higher than 5000 ppm, for long periods, is considered unacceptable for the workers, and is forbidden in several countries,³⁰ because it frequently causes signs and symptoms such as headache, nausea, drowsiness, rhinitis, and reduced cognitive performance.^42,43 Also, reports have been published about the negative impact of respirators on healthcare professionals, such as headache or reduced tolerance to light workload, and recommendations to take regular breaks from mask wearing have been proposed to ensure the wellbeing and productivity of the workers.^44,45

With regard to the potential association between masks, CO₂ concentration and dyspnea, a threshold as high as 50 000 ppm has been proposed to induce dyspnea.⁴⁶ However, mechanisms other than CO₂, such as increased airway resistance, may contribute to the onset of dyspnea in mask-wearers,⁴⁷ and one recent study found that the subjects wearing cotton cloth masks were more likely to report dyspnea than subjects practicing rare mask-wearing (8% vs 1.3%).⁷ While the evaluation of the potential correlation between dyspnea and masks use was beyond the scopes of the presents study, it certainly necessitates further, dedicated research.

As regards the difference between mask types, of the above mentioned studies, one did not find differences in the CO₂ concentration between surgical and FFP2 (with valve) masks, but only one subject was analyzed.¹⁴ The other study did not include surgical masks in the evaluation.¹³ In fact, given the similar ETCO₂ between the 2 mask types, the larger dead space inside FFP2 respirators is expected to determine a sharp difference in CO₂ content between surgical and FFP2 masks.^36,37 This is consistent with 3 previous studies: 1 on patients whose ETCO₂ increased with increasing mask dead space,⁴⁸ and 2 on healthcare professionals (one of which used capnography) which observed CO₂ retention within FFP2 masks, whether with or without valve.^49,50

In relation to the respiratory rate, no previous study specifically evaluated its association with CO₂ concentration in healthy individuals at rest. However, an increase of inhaled air CO₂ was found during physical activity with masks,³⁸ and with higher respiratory rates in post-operatory ventilated patients.⁵¹ In addition, it is well known that, besides mask use, slow breathing is associated with significantly lower inhaled air CO₂ concentration.⁵²

Separate considerations are needed for smokers, whose lower CO₂ concentration in face masks appears counterintuitive. However, smoking a higher number of cigarettes has been associated with higher arterial and lower alveolar CO₂, consistent with the decreased pulmonary gas diffusion which characterizes smokers.⁵³ Thus, the diminished capacity to eliminate CO₂ by the smokers may likely explain the lower concentrations in face masks.

Finally, concerning the minors, no study so far directly compared them to adults, and, apart from one relatively old research which showed increased ETCO₂ concentrations in young children wearing gas masks,⁵⁴ the aforementioned capnography assessments of children identified no changes in physiologic parameters with surgical masks,²¹ but found increased CO₂ when walking with FFP2 masks, which is an earlier sign of alveolar hypoventilation than falling oxygen saturation and indicates respiratory distress.²⁰ In fact, minors can be expected to be at a disadvantage also in this evaluation, because their small build corresponds to a small tidal volume, which therefore provides less dilution of the excess CO₂ compared to the greater tidal volume of adults.^34,35 Nonetheless, given the limited number of the included minors, this finding inevitably requires validation.

As mentioned, OSHA allows CO₂ concentrations up to 5000 ppm for an 8-hour working day.^30,31 However, throughout the pandemic, most people living in cities not only had to wear masks at work, but also on public transportation, in supermarkets and other public places, and sometimes even outdoor. This exceeds the 8-hour limit, and warrants careful evaluation of the potential benefits and harms when considering face mask requirements.

Analogous considerations apply to children, who may add to 5 hours of morning classes further time of indoor extra-curricular activities, and also a variable time on public transportation. The US Environmental Protection Agency reports an upper limit for CO₂ concentration in schools of 700 ppm above outdoor concentrations (conventionally around 400 ppm), as recommended in the Standard 62-2001 of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers.⁵⁵ In Europe, regulation EN 13779 also refers to the outdoor concentration and classifies indoor air quality as low with >1000 ppm, essentially in agreement with the US standard.⁵⁶ Based on this regulation, further norms were developed on the maximum occupancy of non-residential buildings such as schools, and on the ventilation requirements, such as norm UNI EN ISO 16000-26, which recommends specific timings for air changes depending on room occupancy and use.⁵⁷

Indeed, given the recent evidence suggesting that, in crowded rooms, 2 air-changes per hour may lower aerosol build-up more efficiently than the best performing masks,⁵⁸ the choice of increased ventilation over mask-wearing could be taken into consideration when allowed by the environmental and epidemiological conditions, especially for the minors.

Moreover, the observed difference in inhaled air CO₂ according to mask types suggests that, in the presence of mask mandates, and when the usage is protracted and/or physical activity is required, surgical masks should be used instead of FFP2 respirators, as they reduce the possible negative effects of high CO₂ concentrations.^59,60 Further suggestions, both for the general public and for people whose job requires the protracted use of masks, may include: reducing physical exertion, taking breaks to remove the masks and reduce CO₂ build-up, staying hydrated (as dehydration worsens the symptoms due to high CO₂ concentration),⁶¹ and possibly use powered air-purifying respirators, which were described as more comfortable than FFP2 respirators.⁴⁴

Finally, if the relationship between CO₂ levels and respiratory rate is verified, the current guidelines to control SARS-CoV-2 pandemic should be reevaluated.⁵² This would be particularly important for blue-collar workers and the elderly, who are known to breathe faster,^62,63 possibly more so when wearing masks,¹¹ and showed higher baseline CO₂ concentrations.⁶⁴ Indeed, the average respiratory rate at rest has been estimated around 15 breaths per minute in healthy adults,⁶⁵ and, in the present assessment, 3 additional breaths per minute were enough to increase the mean CO₂ content over 5000 ppm when wearing surgical masks.

The chosen capnography device had water-removal tubing, and real-time monitoring, ensuring reliable and reproducible CO₂ measurements,²⁸ as confirmed by the similarity of our findings with those of other capnography assessments of individuals at rest.^20,22,23 Indeed, relative humidity ranges 42% to 91% in exhaled air,¹⁶ potentially altering CO₂ assessments,^17,19 which might explain the differences with the measurements of previous studies that used sensors for environmental CO₂.^{13 –15} Additionally, we examined the largest sample, so far, of healthy individuals of various ages, comparing both surgical masks and FFP2 respirators.^13,14

This study has also limitations that must be considered. First, although the sample size is the largest yet, it is still relatively scarce, especially for the minors. Second, the volume of the dead space within the mask could not be assessed for each participant, and therefore we could not closely inspect the possible influence of face shape and individual added dead space on the inhaled air CO₂. Third, the instrument’s precision of 1.5 mmHg (1974 ppm) widens the uncertainty around the measurements. Importantly, however, when 1974 ppm are subtracted to the mean inhaled air CO₂, the CO₂ in surgical masks decreases to about 3000 ppm, while it still exceeds the 5000 ppm threshold with FFP2 respirators.³⁰ Fourth, the experimental conditions, with participants at complete rest, in a constantly ventilated room, exclusively nose breathing, were far from those experienced by workers and students during a typical day, normally spent in rooms shared with other people or doing some degree of physical activity. Since it was observed that speech and even low level physical activity are associated with increases in CO₂ concentration, CO₂ values in real life are likely to be higher than those recorded in this study.^49,66 Fifth, the short measurement times (5 minutes for each type of mask) reflect our aim to calculate the CO₂ concentration within face masks, for which we needed the ETCO₂, a parameter which stabilizes after about 1 minute from the start of capnography. Therefore the present findings should be viewed only as preliminary evidence of the CO₂ inhaled when wearing masks, and they do not allow to evaluate potential changes in physiological parameters, nor the ensuing compensatory mechanisms adopted by the respiratory system to contrast hypercapnia.^67,68 Likewise, the exclusion of individuals with cardiac or respiratory diseases does not allow to generalize our findings to people with chronic obstructive pulmonary disease. These people may have impaired physiologic compensatory mechanisms, such as increased respiratory rate to compensate the increased ETCO₂,²⁰ and therefore it is plausible that their build-up of CO₂ within face masks may be higher.⁶⁹

Mask wearing is required in many countries throughout the working day, and during lectures in the case of students.² Therefore, capnography and pulse oximeter monitoring of a general population sample should be extended to hours of observation, and not only in conditions of rest, in order to verify whether the ETCO₂ detected within masks remains constant or has a tendency to increase with longer mask wearing and while performing habitual tasks. In addition, subjective symptoms such as headache and drowsiness should also be investigated.

As mentioned, the progressive rise in CO₂ with increasing breaths per minute, and the higher CO₂ in minors also requires validation from further studies with larger samples.

Conclusions

Shortly after wearing surgical masks, the inhaled air CO₂ approached the highest acceptable exposure threshold recommended for workers, while concerningly high concentrations were recorded in minors, and in virtually all individuals when wearing FFP2 masks. The CO₂ concentration was significantly higher among minors and the subjects with high respiratory rate. If these findings are confirmed, the current guidelines on masks use should be reevaluated.