Letters
RESEARCH LETTER

Experimental Assessment of Carbon Dioxide
Content in Inhaled Air With or Without Face Masks in
Healthy Children: A Randomized Clinical Trial
Many governments have made nose and mouth covering or face
masks compulsory for schoolchildren. The evidence base for
this is weak.1,2 The question whether nose and mouth covering increases carbon dioxide in inhaled air is crucial. A largescale survey3 in Germany of
adverse effects in parents and
Supplemental content
children using data of 25 930
children has shown that 68% of the participating children had
problems when wearing nose and mouth coverings.
The normal content of carbon dioxide in the open is about
0.04% by volume (ie, 400 ppm). A level of 0.2% by volume or
2000 ppm is the limit for closed rooms according to the German Federal Environmental Office, and everything beyond this
level is unacceptable.4
Methods | We measured carbon dioxide content in inhaled air
with and without 2 types of nose and mouth coverings in a wellcontrolled, counterbalanced, short-term experimental study
in volunteer children in good health (details are in the eMethods
in Supplement 1). The study was conducted according to the
Declaration of Helsinki and submitted to the ethics committee of the University Witten/Herdecke. All children gave written informed consent, and parents also gave written informed consent for children younger than 16 years. A 3-minute
continuous measurement was taken for baseline carbon dioxide levels without a face mask. A 9-minute measurement for
each type of mask was allowed: 3 minutes for measuring the
carbon dioxide content in joint inhaled and exhaled air, 3 minutes for measuring the carbon dioxide content during inhala-

tion, and 3 minutes for measuring the carbon dioxide content
during exhalation. The carbon dioxide content of ambient air
was always kept well under 0.1% by volume through multiple
ventilations. The sequence of masks was randomized, and randomization was blinded and stratified by age of children. We
analyzed data using a linear model for repeated measurements with P < .05 as the significance threshold. The measurement protocol (trial protocol in Supplement 2) is available online.5 Data were collected on April 9 and 10, 2021, and
analyzed using Statistica version 13.3 (TIBCO).
Results | The mean (SD) age of the children was 10.7 (2.6) years
(range, 6-17 years), and there were 20 girls and 25 boys. Measurement results are presented in the Table. We checked potential associations with outcome. Only age was associated with
carbon dioxide content in inhaled air (y = 1.9867 – 0.0555 × x;
r = –0.39; P = .008; Figure). Hence, we added age as a continuous covariate to the model. This revealed an association
(partial η2 = 0.43; P < .001). Contrasts showed that this was attributable to the difference between the baseline value and the
values of both masks jointly. Contrasts between the 2 types of
masks were not significant. We measured means (SDs) between 13 120 (384) and 13 910 (374) ppm of carbon dioxide in
inhaled air under surgical and filtering facepiece 2 (FFP2)
masks, which is higher than what is already deemed unacceptable by the German Federal Environmental Office by a factor of 6. This was a value reached after 3 minutes of measurement. Children under normal conditions in schools wear such
masks for a mean of 270 (interquartile range, 120-390)
minutes.3 The Figure shows that the value of the child with
the lowest carbon dioxide level was 3-fold greater than the limit
of 0.2 % by volume.4 The youngest children had the highest
values, with one 7-year-old child’s carbon dioxide level measured at 25 000 ppm.

Table. Carbon Dioxide Values Under Various Conditions

Measurement

Participants,
No.

Carbon dioxide, % by volume
Mean (SD) [95% CI]

Range

Baseline
Pretest

45

0.268 (0.108) [0.235-0.300]

0.100-0.628

Posttesta

39

0.281 (0.105) [0.247-0.316]

0.100-0.525

Main outcome
Inhaled air with surgical mask

45

1.312 (0.384) [1.197-1.427]

0.577-2.554

Inhaled air with FFP2 mask

45

1.391 (0.374) [1.279-1.504]

0.600-2.475

45

2.650 (0.486) [2.504-2.796]

1.33-3.41

Additional outcome
Joint exhaled and inhaled air with
surgical mask
Exhaled air with surgical mask

44

3.847 (0.678) [3.641-4.053]

1.783-4.754

Joint inhaled and exhaled air with FFP2
mask

45

2.677 (0.386) [2.561-2.793]

1.660-3.418

Exhaled air with FFP2

45

3.846 (0.547) [3.682-4.011]

2.592-5.24

NA

0.074 (0.003) [0.073-0.075]

0.067-0.083

Carbon dioxide content in ambient air

Abbreviations: FFP, filtering
facepiece; NA, not applicable.
a

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Posttest scores were missing in 6
children because they stopped the
measurement after wearing the
masks.

(Reprinted) JAMA Pediatrics Published online June 30, 2021

© 2021 American Medical Association. All rights reserved.

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Letters

(Prentice); General Practice, Gernsbach, Germany (Diemer); Traindl Consult,
Vienna, Austria (Traindl); Psychotherapeutic Practice for Children and Youth,
Müllheim, Germany (Kappes); tpi consult GmbH, Bollschweil, Germany
(Hockertz).

Figure. Scatterplot of Carbon Dioxide Content in Inhaled Air Under
Filtering Facepiece Mask by Age

CO2 content of inhaled air, % volume

2.6

Accepted for Publication: June 7, 2021.

y = 1.9867 – 0.0555 × x
r = –0.39; P = .008

2.4

Published Online: June 30, 2021. doi:10.1001/jamapediatrics.2021.2659

2.2

Corresponding Author: Harald Walach, PhD, Poznan University of the Medical
Sciences, Pediatric Clinic, ul. Szpitalna 27/33, PL-60-572 Poznań, Poland (harald.
[email protected]).

2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
6

8

10

12

14

16

18

Age, y

Linear regression line with locally weighted scatterplot smoothing.

Discussion | The limitations of the study were its short-term nature in a laboratory-like setting and the fact that children were
not occupied during measurements and might have been apprehensive. Most of the complaints reported by children3 can
be understood as consequences of elevated carbon dioxide levels in inhaled air. This is because of the dead-space volume of
the masks, which collects exhaled carbon dioxide quickly after a short time. This carbon dioxide mixes with fresh air and
elevates the carbon dioxide content of inhaled air under the
mask, and this was more pronounced in this study for younger
children.
This leads in turn to impairments attributable to hypercapnia. A recent review6 concluded that there was ample evidence for adverse effects of wearing such masks. We suggest
that decision-makers weigh the hard evidence produced by
these experimental measurements accordingly, which suggest that children should not be forced to wear face masks.
Harald Walach, PhD
Ronald Weikl, MD
Juliane Prentice, BA
Andreas Diemer, PhD, MD
Helmut Traindl, PhD
Anna Kappes, MA
Stefan Hockertz, PhD

Conflict of Interest Disclosures: None reported.
Funding/Support: Mediziner und Wissenschaftler für Gesundheit, Freiheit und
Demokratie eV, a public charity, has organized this study and covered only
essential expenses, such as travel.
Role of the Funder/Sponsor: The funder had no role in the design and conduct
of the study; collection, management, analysis, and interpretation of the data;
preparation, review, or approval of the manuscript; and decision to submit the
manuscript for publication.
Data Sharing Statement: See Supplement 3.
1. Xiao J, Shiu EYC, Gao H, et al. Nonpharmaceutical measures for pandemic
influenza in nonhealthcare settings —personal protective and environmental
measures. Emerg Infect Dis. 2020;26(5):967-975. doi:10.3201/eid2605.190994
2. Matuschek C, Moll F, Fangerau H, et al. Face masks: benefits and risks during
the COVID-19 crisis. Eur J Med Res. 2020;25(1):32. doi:10.1186/s40001-02000430-5
3. Schwarz S, Jenetzky E, Krafft H, Maurer T, Martin D. Corona children studies
“Co-Ki”: first results of a Germany-wide registry on mouth and nose covering
(mask) in children. Published 2021. Accessed June 15, 2021. https://www.
researchsquare.com/article/rs-124394/v1
4. Mitteilungen der Ad-hoc-Arbeitsgruppe Innenraumrichtwerte der
Innenraumlufthygiene-Kommission des Umweltbundesamtes und der Obersten
Landesgesundheitsbehörden. [Health evaluation of carbon dioxide in indoor
air]. Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz. 2008;
51(11):1358-1369. doi:10.1007/s00103-008-0707-2
5. Walach H, Weikl R, Traindl H, et al. Is carbon dioxide content under
nose-mouth covering in children without potential risks? a measurement study
in healthy children. Published April 14, 2021. Accessed June 15, 2021. https://osf.
io/yh97a/?view_only=df003592db5c4bd1ab183dad8a71834f

Author Affiliations: Poznan University of the Medical Sciences, Pediatric Clinic,
Poznań, Poland (Walach); Obstetric, Gynecological, and General Practice,
Passau, Germany (Weikl); Psychotherapeutic Practice, Müllheim, Germany

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Author Contributions: Dr Walach (principal investigator) had full access to all of
the data in the study and takes responsibility for the integrity of the data and
the accuracy of the data analysis.
Concept and design: All authors.
Acquisition, analysis, or interpretation of data: Walach, Weikl, Diemer, Traindl,
Kappes, Hockertz.
Drafting of the manuscript: Walach, Traindl.
Critical revision of the manuscript for important intellectual content: Walach,
Weikl, Prentice, Diemer, Kappes, Hockertz.
Statistical analysis: Walach.
Administrative, technical, or material support: Weikl, Prentice, Diemer, Traindl,
Kappes, Hockertz.
Supervision: Weikl, Diemer, Traindl, Kappes, Hockertz.
Other–liaising with all other authors: Walach.

6. Kisielinski K, Giboni P, Prescher A, et al. Is a mask that covers the mouth and
nose free from undesirable side effects in everyday use and free of potential
hazards? Int J Environ Res Public Health. 2021;18(8):4344. doi:10.3390/
ijerph18084344

JAMA Pediatrics Published online June 30, 2021 (Reprinted)

© 2021 American Medical Association. All rights reserved.

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