Do Masks Harm?

By Chuck Dinerstein, MD, MBA — May 16, 2023
Last week an article in City Journal wrote about a “scoping review” of the physical harms of masks. It is time for a bit of debunking. A note to the tl:dr – the too long, didn't read among us, it takes more words to correct a mistruth than to propagate one.
Image by Marcin from Pixabay

Have you ever noticed that when you misplace something, it is always found in the last place you look? Of course, that is because you then stop looking. Psychologists have a term for similar intellectual behavior, confirmation bias. The City Journal article, The Harm Caused by Masks, quoting data from a review explicitly looking at the harms of masks, is an example. Rather than pick apart the City Journal piece, let’s look at its source material and citations.

But first, a moment of medicine

One way in which physicians understand the exchange of oxygen and carbon dioxide when we breathe is to separate the portions of the respiratory tract into the functional, i.e., alveoli and lungs, from the passageways which simply move the air from the world into our bodies, i.e., mouth, trachea, bronchi. The physiologic term for these passageways is dead space. As dead space increases, the amount of air reaching the functional portions of the respiratory tract decrease leading to a rise in CO2 (hypercapnia) and a lowering of O2.

Studies of the effect of masks on our breathing all demonstrate that masks act as an increase in dead space. The acute changes in CO2 and perhaps O2 result in your blood’s acid-base balance becoming a bit more acidic, an increase in your blood pressure and heart rate, and for some, headache, fatigue, difficulty concentrating, or dizziness. The changes in your circulation are homeostatic mechanisms your body uses to return to its balance point. The symptoms result from the change in CO2 and the attempted correction. Over longer time frames, the symptoms subside as the new balance is achieved. [1]

With an understanding of the physiologic changes from masks, let’s get to the debunking.

“While concluding that wearing masks “probably makes little or no difference” in preventing the spread of viruses, a recent Cochrane review also emphasized that “more attention should be paid to describing and quantifying the harms” that may come from wearing masks."

– City Journal

While an accurate quote, it is misleading, because the Cochrane Review has more to say.

  • In comparing the use of masks with wearing no masks, “Warmth, respiratory difficulties, humidity, and general discomfort were the most frequently reported adverse events. … Unwanted effects were rarely reported; discomfort was mentioned.”
  • In comparing N95 to surgical masks, “In general, harms were poorly reported or not reported at all in trials ... General discomfort resulting in reduced wear adherence was the most frequently reported harm. …Unwanted effects were not well‐reported; discomfort was mentioned.”
  • And as a general caveat, “Relatively low numbers of people followed the guidance about wearing masks or about hand hygiene, which may have affected the results of the studies.”

City Journal can be forgiven for its inaccuracies. Perhaps the author signed off on someone else's reporting, or perhaps, they did not question the accuracy of their underlying source material. The source material was a scoping review – an exploration of the breadth and depth of research on a topic, not a critical appraisal of individual studies in a systematic review.

Consider first the harms of elevated CO2 from studies of individuals exposed to higher concentrations of CO2 than usually found.

“What can breathing too much carbon dioxide do to you? The authors write that “at levels between 0.05% and 0.5% CO2,” one might experience an “increased heart rate, increased blood pressure and overall increased circulation with the symptoms of headache, fatigue, difficulty concentrating, dizziness, rhinitis, and dry cough.” Rates above 0.5 percent can lead to “reduced cognitive performance, impaired decision-making and reduced speed of cognitive solutions.” Beyond 1 percent, “the harmful effects include respiratory acidosis, metabolic stress, increased blood flow and decreased exercise tolerance.” Again, mask-wearers are likely breathing in CO2 levels between 1.4 percent and 3.2 percent—well above any of these thresholds.”

– City Journal

Once again, an accurate quote, but with additional context missing. Here is another part of the study’s findings.

“Acute symptoms usually resolve despite continuing exposure of carbon dioxide at concentrations of up to 3%. However, in healthy adults metabolic changes are responsible for slight long-term damages (changing cellular pH, disturbing normal homeostasis of the cells leading to an acidosis and N-carboxy derivatives of peptides, proteins, and amino acids) at concentrations of <5%.”

- Possible toxicity of chronic carbon dioxide exposure associated with face mask use

Those N-carboxy derivatives sound harmful, but are a mixed bag, some with beneficial anti-oxidant effects, such as N-acetyl-L-cysteine (NAC), used in treating acetaminophen overdoses. [2] Other derivatives, like carnosine and anserine, have anti-inflammatory properties and are found in meat and fish. On the other hand, homocysteine, another derivative, is a marker, not a specific cause, for an increased risk of cardiovascular disease and cognitive impairment.

I could not access the study the scoping authors used to describe the effects of elevated CO2. However, my counter is from a study published in the International Journal of Emergency on CO2 presentations to emergency departments.  

“Carbon dioxide at low concentration has little, if any, toxicological effects. At higher concentrations (>5%), it causes the development of hypercapnia and respiratory acidosis. …

Studies have shown a wide variability of CO2 tolerance. …, suggesting that a safe CO2 exposure level cannot be characterized by a single value ….”

My take - the small but real increase in CO2, the slight hypercapnia associated with wearing masks, has no physiologically relevance. But City Journal, in parroting the scoping review, raises an even more disturbing concern – stillbirths.

“Indeed, according to the authors, there exists “circumstantial evidence that popular mask use may be related to current observations of a significant rise of 28% to 33% in stillbirths worldwide and a reduced verbal, motor, and overall cognitive performance of two full standard deviations in scores in children born during the pandemic.” They cite recent data from Australia, which “shows that lockdown restrictions and other measures (including masks that have been mandatory in Australia), in the absence of high rates of COVID-19 disease, were associated with a significant increase in stillborn births.” Meantime, “no increased risk of stillbirths was observed in Sweden,” which famously defied the public-health cabal and went its own way in setting Covid policies.”

– City Journal

Let’s begin with the Australian data. This study was a far more persuasive argument against lockdowns and their effect on maternal and fetal care than about mask-wearing. The authors considered fetal growth restriction, or FGR, a condition where a developing fetus fails to grow at the expected rate. It occurs when the fetus does not receive adequate nutrition and oxygen from the placenta, resulting in slower growth and lower-than-normal birth weight. It is a diagnosis based on the ultra-sound-determined size of the fetus. The study did identify an increase in stillbirths, from FGR, before planned delivery but a decrease in stillbirths at the time of delivery. Here is what the authors write:

“The overall prevalence of FGR across all births was no different between the exposed and control groups, nor was there any significant difference in the rate of intrapartum stillbirths. It is plausible that fewer episodes of in-person care during lockdown reduced the detection of growth-restricted fetuses via routine obstetric examination or maternal report of decreased fetal movements. these results suggest that the increase in stillbirth may be due to a failure of detection, appropriate surveillance and timely delivery for preterm FGR infants during the lockdown period, rather than a rise in the prevalence of FGR, or deficiencies in intrapartum care.”

- Increase in preterm stillbirths and reduction in iatrogenic preterm births for fetal compromise: a multi-centre cohort study of COVID-19 lockdown effects in Melbourne, Australia

Maybe the Swedish data will be a stronger argument. Spoiler alert: It is not.

“Among the few countries that do not require the wearing of masks in public is Sweden. … A Swedish nationwide study “did not find any associations between being born during a period when many public health interventions aimed at mitigating the spread of COVID-19 were enforced and the risk for any of the preterm birth categories or stillbirth. …Although society was not completely closed, Swedish authorities enforced many policies to mitigate the spread of COVID-19, such as promotion of general hygiene measures and social distancing (including remote working), ban of non-essential travel, prohibition of gatherings of more than 50 people and the closure of upper secondary schools and universities.”

- Possible toxicity of chronic carbon dioxide exposure associated with face mask use

Here are the words of the cited study:

“We compared the risk for preterm birth and stillbirth among births from 1 April through 31 May 2020, a period when Swedish authorities had enforced a range of pandemic mitigation interventions, with births from all April through May periods in the years 2015 to 2019 combined.”

- Preterm Birth and Stillbirth During the COVID-19 Pandemic in Sweden

Those “mitigation interventions,” the ones without masks, went into effect on April 1, 2020, so the impact of those effects on births is limited to a two-month window for women mid-second trimester or later. Comparison to a cohort of women over three trimesters is an apples-and-oranges comparison.

What of this claim, repeated by City Journal?

“circumstantial evidence that popular mask use may be related to current observations of a significant rise of 28% to 33% in stillbirths worldwide.”

- Possible toxicity of chronic carbon dioxide exposure associated with face mask use

The underlying citation is from The Lancet, which concludes:

“This finding suggests that the increased rate of adverse outcomes might be driven mainly by the inefficiency of health-care systems and their inability to cope with the pandemic, rather than by the stringency of pandemic mitigation measures. … Although no significant overall difference in neonatal death was observed, the data suggested that neonatal death might be increased in LMICs [low and middle-income countries] and decreased in HICs [high-income countries….”

- Effects of the COVID-19 pandemic on maternal and perinatal outcomes

Finally, what about the “reduced verbal, motor, and overall cognitive performance” in children born during the pandemic?  The scoping cited a study from Rhode Island on childhood cognitive scores in 2020 and 2021

“The scientists found that children born during the pandemic have significantly reduced verbal, motor, and overall cognitive performance compared to children born pre-pandemic, with consistent and significant reductions (p < 0.001) showing lower cognitive skills. Could there be a connection between the increased use of N95 masks by pregnant women, higher carbon dioxide re-breathing levels and the results of this recent study?”

The answer is no. 68% of the “pandemic” infants were born before the onset of the pandemic, so none of those mothers wore masks; we have no idea whether the remaining mothers wore masks either. The role of masks was mentioned more as a limitation than a causal or associated factor.

“In the absence of stress-related changes, additional factors that may explain our findings are the use of face masks during testing. Although all study visits were performed in-person, the inability of infants to see full facial expressions may have eliminated non-verbal cues, muffled instructions, or otherwise impair the understanding of test questions and instructions. Without direct comparison of performance in the same children with and without face masks, it is difficult to rule in or out the potential influence of masks.”

- The COVID-19 Pandemic and Early Child Cognitive Development

We can continue to debate the efficacy of masks in reducing the transmission of airborne infections. As the City Journal article desires, we can continue to discuss the harms of masks. But in making an argument, authors, be they scientists, journalists, or “thought-leaders,” have a responsibility of due diligence not to pass along misinformation or facts stripped of critical context. The City Journal author failed in that responsibility; that he was aided and abetted by a “peer-reviewed” study is no excuse.  


[1] Individuals with Chronic Obstructive Pulmonary Disease (COPD) destroy their functional lung tissue and create additional dead space. Their resting CO2 is much higher than those of individuals without COPD, and few have the symptoms associated with acute exposure to CO2.

[2] Our resident chemist, Dr. Bloom, wanted me to point out that NAC deactivates NAPQI, the hepatotoxic metabolite of acetaminophen, via "nucleophilic addition," whatever that means, not by an anti-oxidative process.  


Chuck Dinerstein, MD, MBA

Director of Medicine

Dr. Charles Dinerstein, M.D., MBA, FACS is Director of Medicine at the American Council on Science and Health. He has over 25 years of experience as a vascular surgeon.

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