Yes, wearing masks is harmful
The question of masks and their efficiency or uselessness comes up again and again, now with the renewed demand that children should wear masks at school and - as the director of the NIH just suggested - parents should wear masks at home.
A German study that showed, among other harmful effects, the massive build-up of CO2 under face masks after just a few minutes and demanded the immediate end of masking for children, was withdrawn by the JAMA journal of pediatrics. The author rejects their claim that there was something wrong with the study and instead claims that the withdrawal was entirely political:
A surgical mask does not "trap" CO2, as some people promoting masks mockingly claim, it slows down the airflow, which is now mostly going through the open edges of the mask, while the air flow through the mask is restricted.
This means that respiration cannot evacuate all the exhaled air completely before the user breathes in again. That's why the concentration of CO2 increases very rapidly. So you breathe in less oxygen and more CO2:
Masks lead to deoxygenation of the wearer's blood, measurable after already 1 hour:
https://pubmed.ncbi.nlm.nih.gov/18500410/
Wearing masks over prolonged periods causes headaches:
Facemask headache: a new nosographic entity among healthcare providers in COVID-19 era
https://link.springer.com/article/10.1007%2Fs10072-021-05075-8
Changing the mask must be done expertly, otherwise all the pathogens that accumulated on the outside will transfer to your hand and then to your face, so the droplet protection will have been for nothing:
Contamination by respiratory viruses on outer surface of medical masks used by hospital healthcare workers
https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-019-4109-x
Masks offers a moist, warm breeding ground for pathogens - mostly bacteria and fungi, which can then lead to a self-infection of the wearer, unless he changes the mask very frequently:
A mask can lead to more particles being deposited in the airways:
Effects of mask-wearing on the inhalability and deposition of airborne SARS-CoV-2 aerosols in human upper airway
https://aip.scitation.org/doi/10.1063/5.0034580
"The results of this study show that wearing a zero-filtration mask can lead to a higher deposition rate of particles smaller than 10 µm (i.e., PM10) in the upper airway for all flow rates (15 l/min–60 l/min) and mask resistance matrices considered. This seemingly counterintuitive observation may be attributed to the altered pressure and airflow fields caused by the mask, which further changes the inhalability of the particles and subsequent deposition in the upper airways. The overall lower speeds of the respirable particles after wearing a mask, as well as an increased area of respiration, can increase the chance of respirable particles to land on the face or being inhaled into the mouth and nose. This unexpected finding raises an alarm that wearing masks with very low filtration efficiencies may lead to a higher chance of deposition of ambient aerosols and thus can do more harm than protection".
From the New England Journal of Medicine:
https://www.nejm.org/doi/full/10.1056/NEJMp2006372
“We know that wearing a mask outside health care facilities offers little, if any, protection from infection. Public health authorities define a significant exposure to Covid-19 as face-to-face contact within 6 feet with a patient with symptomatic Covid-19 that is sustained for at least a few minutes (and some say more than 10 minutes or even 30 minutes). The chance of catching Covid-19 from a passing interaction in a public space is therefore minimal. In many cases, the desire for widespread masking is a reflexive reaction to anxiety over the pandemic.”
Here are some blog posts that link to many more studies regarding masks and their efficiency:
Wearing a Mask Can Harm Your Health
https://www.jennifermargulis.net/wearing-mask-can-harm-your-health/
Masks Don’t Work: A Review of Science Relevant to COVID-19 Social Policy