In countries with a mask mandate due to high COVID-19 incidences there is often some side rule in effect where you can get a medical certificate if you have some medical condition that would contraindicate wearing a mask.

Sure, on a general level, having an exception isn't that surprising since most rules come with exceptions, nothing is black and white, etc.

But in that case, I have trouble thinking of medical conditions where wearing a surgical mask would really harm you.

For example in Germany, there are stories in the press about some people having such a medical certificate because of asthma. But even if you have trouble breathing due to asthma, a standard surgical mask doesn't seem to make breathing any harder, in any way. FFP2/FFP3 masks are a different story, they make it somewhat harder to breath (i.e. it requires somewhat more energy), and thus, some work protection policies require pausing after half an hour or so wearing an FFP2 mask.

But the mask mandate (in Germany) just requires a surgical mask or better. Thus, such a medical certificate is required if and only if you cannot even wear a simple surgical mask.

So, are there any medical condition that make wearing a surgical mask impossible or where using a surgical mask would add harm?


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    Is this a 'notable question'? You broadly inquire about a lot of possible medical reasons (which are opinions by doctors we call "diagnosis"). But this isn't really about hard & fast 'reasons' but about opinions lawmakers write into measures. Being 'disabled' auto-exempts you theoretically, breathing-, cardiac probs etc should as well. Psychological reasons (trauma) should. But ultimately the question is about 'legitimacy' of individuals claiming exemption. I am not sure this can be answered well here. MedSci, Pol & LawSE all seem like a conceptually better fit? – LangLаngС May 16 at 13:27
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    In UK you do not need a medical certificate and there are no official exemption cards. The UK government says: ... you do not need to seek advice or request a letter from a medical professional about your reason for not wearing a face covering ... Carrying an exemption card or badge is a personal choice and is not required by law ... Please note that the government is not able to provide physical exemption cards or badges. – Weather Vane May 16 at 16:00
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    "a standard surgical mask doesn't seem to make breathing any harder, in any way." Yeah, sure. Aside from that odd statement, there's a lot of information in this question to unpack. Probably could do with a significant content cutting. – fredsbend May 16 at 22:12
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    Actually, the question answers itself. Apparently, according to these same authorities, there are reasons, such as asthma. So you're apparently looking for a medical opinion that says there is no legitimate medical reason. It's pretty close to violating our rule against medical advice. – fredsbend May 16 at 22:20
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    A simple medical reason: You can't remove the mask on your own. It is not a direct issue but throwing up with a mask on would be quite dangerous if you couldn't remove it. Likewise, a sufficient spill onto the mask could likewise prove dangerous. – Loren Pechtel May 17 at 1:18

As the other answers don't seem to make this link too clear from experiments (or at least the comments are in that direction); there aren't a lot of those it seems, but there's one cited in a review of Matuschek et al. (Aug 2020)

the role of N95/FFP-2 masks was tested in 97 patients with advanced COPD while undergoing a 6-min walk test. Seven patients did not tolerate the test and stopped prematurely. The respiratory rate, oxygen saturation and CO2 levels changed significantly while wearing N95/FFP2 masks. These results demonstrated the potential risks of wearing this type of mask in the presence of advanced COPD.


Kyung SY, et al. Risks of N95 Face Mask Use in Subjects With COPD. Respir Care. 2020;65(5):658–64.

So, the irony is that those who already have impaired lung function and are thus more at risk from Covid-19 have also more difficulty wearing an (unpowered) respirator.

There are a few case studies that point to the benefit of a PAPRs (powered respirator) in people with COPD, e.g.

PAPR are air-fed respirators that supply filtered air and are equipped with a visor. PAPR are appropriate for workers with COPD as they provide a continuous supply of oxygen, even in enclosed spaces with reduced oxygen levels.

Note that there are some PAPRs in semi-mask form (no visor).

On the other hand, medical/surgical masks definitely impair respiration less, but also provide less protection.

Until (very) recently, there were not studies specifically on COPD and surgical masks (it seems), but the CDC nonetheless (in one of the many iterations of their guidelines) was paraphrased as giving this advice:

There are three reasons that the CDC approves not wearing a mask. They state that:

Cloth face coverings should not be worn by:

  • Children younger than 2 years old
  • Anyone who has trouble breathing, including those with chronic obstructive pulmonary disease (COPD)
  • Anyone who is unconscious, incapacitated, or otherwise unable to remove the cloth face-covering without assistance

On the other hand, Web MD cites some experts disagreeing with the CDC's advice:

“There are a lot of myths floating around about who should wear a mask, and how and when they should wear it,” says Frank Coletta, MD, director of pulmonary medicine and chief of critical care at Mount Sinai South Nassau in Oceanside, NY. Read on to get the facts on masks and lung disease.

Myth: Masks Aren’t Safe for People With Lung Disease

Fact: Almost all people with lung disease can and should wear a mask. “They don’t restrict your breathing,” Coletta says. “They might feel slightly uncomfortable, but that’s not a sign that you’re getting less oxygen.”

Or at least relative to the risks:

“Studies show that masks don’t impact air exchange, even for people with severe emphysema,” says Bradley Hayward, MD, a pulmonologist at Weill Cornell Medicine and NewYork-Presbyterian in New York City. “Your breath might feel warmer. But if you think that’s uncomfortable, imagine being on a ventilator because you got COVID-19.”

And actually there is one study published in Mar 2021 on medical masks and COPD, which actually seems to back up the latter view (of such masks not having a significant effect):

To demonstrate the changes in end-tidal CO2 and oxygen saturation as measured by pulse oximetry before and after wearing a surgical mask, we used a convenience sample of 15 house staff physicians without lung conditions (aged 31.1 ± 1.9 yr, 60% male) and 15 veterans with severe chronic obstructive pulmonary disease (COPD) (aged 71.6 ± 8.7 yr, forced expiratory volume in 1 second [FEV1] 44.0 ± 22.2%, 100% male). The patients needed to have a postbronchodilator FEV1 <50% and FEV1/forced vital capacity <0.7 and were enrolled from the pulmonary function laboratory during a scheduled 6-minute walk test ordered to assess the need for supplemental oxygen. In our institution, the 6-minute walk tests are done with arterial blood analysis before and immediately after the walk to assess the need for long-term oxygen. Because of the COVID-19 pandemic, the 6-minute walk tests are done with subjects using a surgical mask. [...]

At 5 and 30 minutes, no major changes in end-tidal CO2 or oxygen saturation as measured by pulse oximetry of clinical significance were noted at any time point in either group at rest (Table 1). With the 6-minute walk, subjects with severe COPD decreased oxygenation as expected (with two qualifying for supplemental oxygen). However, as a group, subjects with COPD did not exhibit major physiologic changes in gas exchange measurements after the 6-minute walk test using a surgical mask, particularly in CO2 retention.

Granted this was a small study on a convenience sample, but then the one by Kyung et al. on COPD with N95 was of similar nature, albeit with a somewhat larger sample. Also of some note, the level of impairment for which N95 could not be tolerated was fairly high in that study:

FEV1 < 30% predicted (odds ratio 163, 95% CI 7.4 to >999.9; P = .001) was associated with a risk of failure to wear the N95.

Alas from the 2021 study by Samannan et al. on surgical masks in COPD, I can't tell if they had any patients with FEV1 < 30% in their sample, as they only seem to have published the cutoff for enrollment (FEV1 <50%) but no other sample stats in that regard...

Also, the criteria for COPD differ between (e.g.) European Respiratory Society and UK' National Institute for Clinical Excellence; the former sets a FEV1 around <88-89% as theshold for COPD disgnosis while in the US it's 75%. But still, a cutoff FEV1 <50% (as in the 2021 study) is a farily substantially below either of these. According to Healthline, it seems that in the US the GOLD COPD stadard is used; on this FEV1 30-49% qualifies as severe COPD, so the latter (Florida study) probably used this threshold (<50%) for that reason. FEV1 < 30% would qualify as "very severe" COPD and 50-79% as moderate according to the same (Healthline/GOLD COPD) source. The GOLD guideline itself also says right after giving those cutoffs: "It should be noted that there is only a weak correlation between FEV1, symptoms and impairment of a patient’s health status. For this reason, formal symptomatic assessment is also required."

  • Did any of those tests include other style masks that are not as restrictive for breathing as a n95 mask is? – Joe W May 17 at 20:52
  • IS there a good reason for two separate answers? Can't they be consolidated into one? – Oddthinking May 18 at 1:04
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    The expert is a cynical polemicist (breath isn't just warmer, surgical mask vs ventilator., srsly?) surgical mask study is useless (6 min! and no other parameters measured in this tiny sample (even in healthy surgeons O2 gets low enough to neg impact operations). To retain the ones looked at, other compensatory things kick in, ie heart rate etc That test is a poor proxy for RL). In short: those apologetic pieces here are having a totalitarian agenda: masks4all. – LangLаngС May 18 at 8:39
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    @LangLаngС: if you're talking about the Turkish study by Beder et al. that you've quoted in your answer, it wasn't a controlled study "This early change in SpO2 may be either due to the facial mask or the operational stress. " – Fizz May 20 at 4:30
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    @LangLаngС: And here is much better controlled study (cross-over RCT) under much more strenuous conditions (cycling to exhaustion) that found no effect of [surgical] mask wearing on either oxygenation or performance. The narative review by Kisielinski et al. obviously left out such studies that disproved the point they were trying to show... – Fizz May 20 at 5:32

Yes, there are several medical conditions that are directly triggered by wearing a mask. Physical problems are most commonly caused by the increased humidity (and temperature to a lesser degree) of the air that is trapped in the mask and gets breathed in again. Psychological problems can also be caused by the way the mask feels on your face (like claustrophobia or sensory overload).

All emphases in all citations are added by me.

All types of masks increase the humidity of the air that is breathed in, as shown by this press release:

The study, led by researchers in the NIH’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), found that face masks substantially increase the humidity in the air that the mask-wearer breathes in. [...] The results showed that all four masks increased the level of humidity of inhaled air, but to varying degrees.

First and most often cited is asthma. There are nerves in our lungs that constrict the air ways if the air we breathe is too hot and humid. Wearing masks, even a surgical one, always traps some hot and humid air which is then breathed in again.

The study Bronchoconstriction Triggered by Breathing Hot Humid Air in Patients with Asthma - Role of Cholinergic Reflex not only cited several earlier studies, but added its own results:

An earlier study by Aitken and Marini (5) has shown that, after hyperventilation of the air with different combinations of temperature and humidity in patients with asthma, the most intense bronchoconstriction occurring immediately was generated by breathing hot humid air, which caused an almost 2-fold increase in airway constriction generated by cold dry air at the same time point.

The results of this study showed that hyperventilation of HA (humidified air at 49°C) triggered an immediate and reversible increase in airway resistance in patients with mild asthma but caused either only a very small or no response in healthy subjects. [...] Breathing HA also triggered coughs consistently in these patients, suggesting an involvement of the airway sensory nerves that are responsible for eliciting the cough reflex.


COPD (chronic obstructive pulmonary disease) is another disease that is directly affected by the temperature and humidity of the air. COPD flare-ups are primarily caused by cold air, but humidity is a secondary factor that increases the risk.

Synergistic effects of temperature and humidity on the symptoms of COPD patients found that:

There was a significant interactive effect between temperature and humidity (p < 0.0001) on COPD patients. [...] Low temperature was a risk factor for COPD patients, and high humidity enhanced its risk on COPD.


There are also psychological conditions like anxiety disorder that are affected by wearing a mask.

There's a study Influence of Anxiety Level on Work Performance With and Without a Respirator Mask that found:

Twenty subjects were tested for trait anxiety levels and performed on a treadmill at 80–85% of their maximum heart rates until they reached voluntary end-point. [...] Anxious subjects experience more discomfort, perform for shorter times, and accomplish less total work than their lower anxiety counterparts, even when rate of work can be adjusted.

And this article Mask anxiety, face coverings and mental health summarizes some symptoms people with mental health problems might experience when wearing a mask, regardless of how well the air can flow through it:

  • You might feel anxious or panicky, as covering your mouth and nose might affect the air you breathe. This can cause symptoms like feeling dizzy or sick, which you may then associate with your mask.
  • You might feel trapped or claustrophobic.
  • Covering your face changes the way you look, which may cause negative feelings around your identity or body image.
  • Certain materials touching your skin might feel very hard to cope with (which may create sensory overload).
  • If you wear glasses, they might steam up so you can't see clearly. This might add to feeling overwhelmed.

The list goes on but I concentrated mostly on the effects of wearing a mask, not on being reminded of the pandemic or other traumatic events.


These are just a few conditions for which I could find objective reasons that might make wearing a mask impossible. There are certainly more conditions that cause problems on a personal scale. That's why (originally) a medical doctor should assess whether an individual person is to be exemted from wearing a surgical mask.

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    "I concentrated mostly on the effects of wearing a mask" - but the only parts of your answer dealing with masks are about anxiety. The first half of your answer dealing with actual physical effects doesn't mention masks at all. – pipe May 17 at 12:42
  • @pipe I added another source that proves that wearing a mask increases the humidity of the air you breathe in. The physiological problems are not directly caused by the mask itself, but by the increased humidity. – Elmy May 17 at 13:29
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    But it would be impossible for a mask to heat the temperature even close to the 49°C described in one report, so I don't see how relevant that paper is. They don't seem to test the humidity aspect in isolation. And nothing here really tells me how much "high" is. If the increase is from 10% to 20% it's a high increase, but that doesn't matter if the problematic level is 90%. – pipe May 17 at 15:01

Q: Are there medical reasons for not being able to wear a surgical mask?

In countries with a mask mandate […] there is often some side rule in effect where you can get a medical certificate if you have some medical condition that would contraindicate wearing a mask.

This is primarily a legal question, less a medical one.

The situation described in question pertains to Germany, and how the reasoning there is given and applied. It will differ in different jurisdictions, whether in other countries, or within Germany itself.

"Are there reasons" of a medical nature? Yes, and some of them are directly cited in the regulations that mandate the masks.

Clarification from OP in comments:

It would be sufficient to come up with one concrete medical condition (or several if you like) which make wearing a surgical mask impossible or under which wearing a surgical mask would harm you. Plus some scientific justification.

Chronic obstructive pulmonary disease (COPD) is

is a chronic inflammatory lung disease that causes obstructed airflow from the lungs. Symptoms include breathing difficulty, …

Since there is no mandated mask in existence that makes breathing easier for anyone…

As such, different federal states have mandated wildly different mask mandates, and wildly differing exemptions. Most often not indeed not listing any specific condition, but a few do, shown below. Seldom giving any 'scientific' reasoning to accompany this.

Most regulations just vaguely state "may be exempt for medical reasons" — making this in writing a strictly private and confidential thing to be stablished between doctor and patient. That this is often ignored is a matter frequently brought to court. No federal state seems to provide an exhaustive list of concrete conditions that would fall under this regime.

Whether a legitimate medical reason is present and cited as grounds for being exempt from wearing a mask is a medical opinion called "diagnosis" and thus made on an individual basis, as made quite explicit in this clarification:

Whether a health impairment or illness prevents the use of a mask is a medical question that is left to the doctor's assessment. Contrary to the opinion of some court decisions in other federal states, requesting more detailed reasons or even diagnoses in the context of submitting a medical certificate is generally neither appropriate nor necessary and thus violates European law requirements (principle of data minimisation, Art. 5(1)(c) GDPR).
Concerns about possible certificates for convenience or from the internet are understandable. However, the collection of individual health data beyond the medical determination is not necessary. As a rule, the burden of proof for the reason for exclusion should be sufficiently met by a simple medical certificate. If there is reasonable doubt, an official medical certificate can be demanded as a mitigating measure in a particular individual case. In the case of cursory access controls, it also does not seem practicable to check medical details or even diagnoses. Moreover, the explanatory memorandum of the 12th SARS-CoV-2-EindV mentions that special medical certificates are "expressly not required". The requirements should therefore be set at a low threshold so that the exceptions are not invalidated by excessive requirements at the admission control. A significant encroachment on fundamental rights through the interrogation of sensitive medical information by non-medical staff is therefore not justified as a rule.
— Hinweise zur Befreiung von der Pflicht zum Tragen von Mund-Nase-Bedeckungen, State of Sachen-Anhalt, Mya 2021 (PDF)

Different regulations for different federal states:

  • Baden-Wuerttemberg
    "There is no obligation to wear an oral-nasal covering for persons for whom the wearing of an oral-nasal covering is not possible or not reasonable for health or other reasons, whereby the prima facie evidence of health reasons must usually be provided by a medical certificate," (Corona Ordinance, § 3, version of 25.01.21)
    —— Source: Baden-Wuerttemberg State Government

  • Bavaria
    "Persons who can credibly demonstrate that wearing a mouth-nose covering is not possible or unreasonable for them due to a disability or for health reasons are exempt from the obligation to wear it; in the case of health reasons, the credible demonstration is made in particular by means of a medical certificate, ...

    Removal of the mouth-nose covering is permissible as long as it is necessary for identification purposes or for communication with people with hearing disabilities or for other compelling reasons."
    ——Source: Bavarian State Chancellery

  • Berlin
    "The obligation to wear a mouth-nose covering does not apply to persons who cannot wear a medical face mask or mouth-nose covering as certified by a doctor due to a health impairment, a chronic illness certified by a doctor or a disability, ... or for deaf and hard of hearing persons and persons who communicate with them, as well as their accompanying persons."
    —— Source: Berlin SARS-CoV-2 infection control measures ordinance.

  • Bremen
    "There is no obligation to wear an oral-nasal covering for deaf or hard-of-hearing persons, their accompanying persons and, if necessary, for persons who communicate with them, and for persons for whom the use of a mouth-nose covering is not possible or not reasonable because of a disability, pregnancy or health reasons."
    —— Source: 23rd Corona Ordinance of the Hanseatic City of Bremen.

  • Brandenburg
    "The following persons are exempt from the obligation to wear an oral-nasal covering or a medical mask: 2. deaf and hard of hearing persons, their accompanying person and, if necessary, persons communicating with them, 3. persons for whom the use of a mouth-nose covering or a medical mask is impossible or unreasonable because of a disability or for health reasons; this must be proven on site by an original written medical certificate."
    —— Source: Fifth SARS-CoV-2 Containment Ordinance of the State of Brandenburg.

  • Hamburg
    "For the mask requirement, the following applies: ... 2. persons who can credibly demonstrate that wearing an oral-nasal covering is not possible or unreasonable for them due to a disability or for health reasons are exempt from the obligation to wear one, 3. removal of the mouth-nose covering is permitted as long as it is necessary for identification purposes or for communication with persons with hearing disabilities,"
    —— Source: Ordinance to contain the spread of the SARS-CoV-2 coronavirus in the Free and Hanseatic City of Hamburg.

  • Hesse
    "Not required to wear an oral-nasal covering are (...) persons who cannot wear a mask for health reasons."
    —— Source: Hesse State Government

  • Mecklenburg-Western Pomerania
    "The mask obligation does not apply to (...) persons who cannot wear a mouth-nose covering due to a medical or mental impairment or due to a disability and who can prove this by a medical certificate."
    —— Source: Corona State Ordinance Mecklenburg-Western Pomerania

  • Lower Saxony
    "Persons for whom the wearing of a mouth-nose covering is not reasonable due to a physical, mental or psychological impairment or a previous illness, for example a serious heart or lung disease, and who can substantiate this by a medical certificate or a comparable official certificate, [...] are exempt from the obligations [...]. "
    —— Source: Lower Saxony Corona Ordinance

  • North Rhine-Westphalia
    "Exempted from the obligation to wear a mask are persons who cannot wear an everyday mask for medical reasons. The existence of the medical reasons must be proven by a medical certificate, which must be presented on request."
    —— Source: Corona Protection Ordinance NRW

  • Rhineland-Palatinate
    "The distance requirement and the obligation to wear a mask do not apply to (...) 2. to persons for whom this is not possible or unreasonable due to a disability or for health reasons; this is to be proven by medical certificate, 3. to the extent and for as long as it is necessary for communication with persons with a hearing or visual impairment, for identification purposes or in connection with the performance of the functions of the administration of justice (including notarial and legal offices)."
    —— Source: 15th Corona Control Ordinance Rhineland-Palatinate.

  • Saarland
    "(...) a mouth-nose covering is to be worn, unless there are health reasons to the contrary."
    —— Source: Ordinance amending ordinances under infection law to combat the Corona pandemic.

  • Saxony
    "Exempted from the obligation to wear a mouth-nose covering are (...) people with disabilities and those with health restrictions, unless they are unable to do so. To substantiate an exemption from the obligation, it is sufficient to grant access to a medical certificate."
    —— Source: Information on wearing a mouth-nose covering, Saxon State Ministry for Social Affairs and Social Cohesion.

  • Saxony-Anhalt
    Exempt from the obligation to wear a mouth-nose covering are (...)

    • Deaf and hard of hearing people, as they depend on being able to read the lips of the other person in their communication. The same applies to their companions and, if necessary, to persons who communicate with them.

    • Persons for whom the use of a mouth-nose covering or medical mouth-nose protection is not possible or unreasonable due to a disability, pregnancy or health reasons; this is to be made credible in a suitable manner (e.g. by plausible oral statement, severely disabled person's card, medical certificate)."
      —— Source: Ministry of Health Saxony-Anhalt

  • Schleswig-Holstein
    Exempted from the obligation to wear a mouth-nose covering are "persons who cannot wear a mouth-nose covering due to a physical, mental or psychological impairment and can credibly demonstrate this. " In addition, "(...) the use of a visor covering the entire face by sign language interpreters, communication assistants or communication aides working for persons with hearing impairments is sufficient."
    —— Source: Corona Control Ordinance Schleswig-Holstein

  • Thuringia
    "People for whom the use of an oral-nasal covering is not possible or unreasonable due to disability or health reasons are exempt from the obligation. This is to be made credible in a suitable manner."
    —— Source: Thuringian Ministry of Labour, Social Affairs, Health, Women and Families

(Adapted from Ausnahmen für Menschen mit Behinderung von der Maskenpflicht)

In summary:

Those regulations legally exempting explicitly named conditions include:

In Saxony, it is said that for example chronic bronchitis, asthma and anxiety disorder fall into that category.

For a discussion into the legalese differentiating different types of exemptions, specifically whether there might be a graded legitimacy for being exempt from wearing a surgical mask as well as for being exempt from wearing a FFP2 mask (which is a mandatory category to wear under §Viertes Gesetz zum Schutz der Bevölkerung bei einer epidemischen Lage von nationaler Tragweite if the so called 'rolling-7-day-incidence' exceeds '100'), the example of some Berlin bureaucrats discussing some recent court verdicts and attempting to read the law properly — or failing to do so? — might give a hint of the chaos and legal uncertainties involved.

Should anyone believe that 'a surgical face mask is just such a small problem, literally everyone should be able to wear one', then the medical side giving some hints might indeed round this off with this tip of an iceberg:

[… ]the use of surgical face masks was associated with a significant increase in airway resistance, reduced oxygen uptake, and increased heart rate […]
Further studies in the elderly and in persons with pulmonary or cardiac diseases are necessary.
— Lässing, J., Falz, R., Pökel, C. et al. Effects of surgical face masks on cardiopulmonary parameters during steady state exercise. Sci Rep 10, 22363 (2020). https://doi.org/10.1038/s41598-020-78643-1

It should be quite clear and certainly self-evident that really nobody just barely surviving with getting enough oxygen while for example suffering from a diagnosed COPD on a good day and every day does not need any additional breathing resistance added on top of that condition forced upon the person by a totalitarian mask mandate.

The list of medical contraindications is long and still not exhaustive:

Slightly elevated CO2 levels are known to increase heart rate, blood pressure, headache, fatigue and concentration disorders. Accordingly, the following conditions were listed as exclusion criteria for mask use: any cardiopulmonary disease including but not limited to: asthma, bronchitis, cystic fibrosis, congenital heart disease, emphysema; any condition that may be aggravated by physical exertion, including but not limited to: exercise-induced asthma; lower respiratory tract infections (pneumonia, bronchitis within the last 2 weeks), anxiety disorders, diabetes, hypertension or epilepsy/attack disorder; any physical disability due to medical, orthopedic or neuromuscular disease; any acute upper respiratory illness or symptomatic rhinitis (nasal obstruction, runny nose or sneezing); any condition with deformity that affects the fit of the mask (e.g., increased facial hair, craniofacial deformities, etc.). […]

The American Asthma and Allergy Society has already advised caution in the use of masks with regard to the COVID-19 pandemic for people with moderate and severe lung disease. Since the severely overweight, sleep apnea patients and overlap-COPD sufferers are known to be prone to hypercapnia, they also represent a risk group for serious adverse health effects […] This is because the potential of masks to produce additional CO2 retention may not only have a disruptive effect on the blood gases and respiratory physiology of sufferers, but may also lead to further serious adverse health effects in the long term. Interestingly, in an animal experiment an increase in CO2 with hypercapnia leads to contraction of smooth airway muscles with constriction of bronchi [166]. This effect could explain the observed pulmonary decompensations of patients with lung disease under masks.
Patients with renal insufficiency requiring dialysis are, according to the literature available, further candidates for a possible exemption from the mask requirement.
According to the criteria of the Centers for Disease Control and Prevention, GA, USA (CDC), sick and helpless people who cannot remove a mask on their own should be exempted from the mask requirement. […]

[…] children react even more sensitively to masks, the literature suggests that masks are a contraindication for children with epilepsies (hyperventilation as a trigger for seizures). In the field of pediatrics, special attention should also be paid to the mask symptoms described under psychological, psychiatric and sociological effects with possible triggering of panic attacks by CO2 rebreathing in the case of predisposition and also reinforcement of claustrophobic fears.
— Kai Kisielinski 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, 18,4344. doi

Experts weighing in:

Wearing a mask is not safe for everyone. This applies to all patients with symptomatic and unstable angina pectoris and symptomatic chronic obstructive pulmonary disease (COPD) or impaired lung function.

And most specifically one concrete example as requested, COPD patients wearing "just" a surgical mask gives:

Results: Surgical mask using increased the levels of PaCO2 (8.98mm Hg; p = 0.004), HCO3-act (4.1mmol/L; p =0.006), BE (3.01mmol/L; p =0.019) and systolic blood pressure (11.39mm Hg; p = 0.01) in patients with AECOPD compared to last hospitalization. Surgical mask using for 30 to 120 minutes is associated with hypercapnia. […]
Conclusions: In this study, we found that systolic blood pressures and PaCO2, HCO3-act, BE were signi cantly elevated in AECOPD patients using masks compared to the other groups without masks. In addition the changes in PaCO2,HCO3-act,BE is closely related to serum chloride concentration.
— You Mo: "Risk and impact of using mask on COPD patients with acute exacerbation during the COVID-19 outbreak: a retrospective study" 2020 doi

This shouldn't be surprising, since even in perfectly healthy surgeons oxygen levels do decrease after while, as the compensatory mechanisms to main blood gas levels increase the work load faced with this increase breathing resistance and increased dead space with a build-up of CO2 behind the mask and increased re-breathing:

As the duration of the operation increases, oxygen saturation of hemoglobin decreases significantly. […]
this is the first clinical investigation reporting a decrease in blood O2 saturation and an increase in pulse rates of the surgeons after the operations due to surgical mask usage. doi

Some bought science "because of the 'pandemic'" may create an artificial scenario — for tiny numbers of test subjects only measuring the harm these masks present for everyone forced to wear them — by stopping the experiment after just 6 minutes. Mandates require often to wear the mostly useless to contain the spread masks for much longer. In a more realistic scenario masks are worn for extended periods of time. They then show their negative impact and actual harms in a wide range of symptoms. One example:

— Mask-associated ‘de novo’ headache in healthcare workers during the COVID-19 pandemic. doi
— Ong et al.: "Headaches Associated With Personal Protective Equipment - A Cross-Sectional Study Among Frontline Healthcare Workers During COVID-19", Headache: The Journal of Head and Face Pain. 2020 May;60(5):864-877. doi: 10.1111/head.13811.


(91.3%) of respondents with pre-existing headache diagnosis either "agreed" or "strongly agreed" that the increased PPE usage had affected the control of their background headaches, which affected their level of work performance. Conclusion: Most healthcare workers develop de novo PPE-associated headaches or exacerbation of their pre-existing headache disorders. — PK Purushothamanet al.: "Effects of Prolonged Use of Facemask on Healthcare Workers in Tertiary Care Hospital During COVID-19 Pandemic", Indian J Otolaryngol Head Neck Surg 73, 59–65 (2021). https://doi.org/10.1007/s12070-020-02124-0

Mandated mask usage is just not evidence-based for its alleged protective effect and the downsides and harms often just ignored:

Harms From Nonevidence-Based Measures
Given the uncertainties, one may opt for abundant caution and implement the most severe containment measures. By this perspective, no opportunity should be missed to gain any benefit, even in the absence of evidence or even with mostly negative evidence.

This reasoning ignores possible harms. Impulsive actions can indeed cause major harm. One clear example is the panic shopping which depleted supplies of face masks, escalation of prices and a shortage for medical personnel. Masks, gloves and gowns are clearly needed for medical personnel, and their lack poses healthcare workers' lives at risk. Conversely, they are meaningless for the uninfected general population.

— Coronavirus disease 2019: The harms of exaggerated information and non-evidence-based measures (Eur J Clin Invest. 2020;50 DOI: 10.1111/eci.13222)

Even seemingly small effects on their own pile up and tend to have consequences:

All participants wore either surgical masks or N95 respirators for a minimum of 4h per day. […] A total of 250 healthcare workers participated in the study […] The acquired results were excessive sweating around the mouth accounting to 67.6%, difficulty in breathing on exertion 58.2%, acne 56.0% and itchy nose 52.0%. This study suggests that prolonged use of facemasks induces difficulty in breathing on exertion and excessive sweating around the mouth to the healthcare workers which results in poorer adherence and increased risk of susceptibility to infection.” — Elisheva Rosner E (2020) “Adverse Effects of Prolonged Mask Use among Healthcare Professionals during COVID-19”. Journal of Infectious Disease and Epidemiology 6:130. doi.org/10.23937/2474-3658/1510130

That comprehensive review of really looking for the downsides of harmfiul maks mandates is still not being done as of now. After a screening of more than 5000 articles, the actual harms done by mandated masks is often sinpmply not cared for:

For some of the clearly present added by any mask, we can add a host of examples found in the literature: discomfort, warmth, headache, shortness of breath, difficulty breathing, pressure on the nose, itch, rash, skin injury, de-novo headache, increased migraines, decreased O2, increased CO2, hypercapnia, dyspnoea, a very long list of psychological problems (from:)

New research on face masks should assess and report the harms and downsides. Urgent research is also needed on methods and designs to mitigate the downsides of face mask wearing, particularly the assessment of possible alternatives.

— Mina Bakhit et al.: "Downsides of face masks and possible mitigation strategies: a systematic review and meta-analysis", Infectious diseases, BMJ Open, 2020 doi

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    There is no need to bring in a strong hatred of masks into this answer. Your final paragraph seems like an edge case where the person would not only have a valid reason to not wear a mask but also has the chance of getting oxygen from an external source. I would guess that the number of people in cases like this are small compared to the number of people who just don't want to wear a mask. Masks are worn by plenty of professionals in various areas that are doing more active work and have a higher demand for oxygen than is required in most cases people are complaining about wearing a mask. – Joe W May 16 at 23:18
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    I wouldn't know how much the percentages are between "'can't' — as defined per laws" and anti-maskers (non-acute/potential-own-medical resaons). If you know a better word for 'totalitarian' here, but retaining the 'must mask every last one of em'-angle, feel free to suggest an edit. To my tastes, it was a great fit, but another one might be better. – LangLаngС May 16 at 23:37
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    Mandating masks for public health safety is in no way a totalitarian action. Besides this question was asking if there are medical reasons for not being able to wear a surgical mask. Your answer is talking about legal reasons and not answering the actual question about medical reasons. – Joe W May 16 at 23:53
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    The question is explicitly medical, you're answering on a legal basis. – Loren Pechtel May 17 at 1:15
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    If you're really having trouble finding a neutral term for a hypothetical mask mandate with no exceptions, might I suggest "absolute"? – Nate Eldredge May 17 at 4:26

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