I wear a mask because it is mandated (in this case in public transport), but not because I am convinced it is necessary. I wear it because I don't feel like arguing with law enforcement or self-appointed blockwarts about it. Why do I think it is not necessary? Lets see: this mask does not protect me from infection, and I hope everyone knows this. It is supposed to protect others from catching the virus from me. In my case, illogical: I am by age and precondition in the high risk group. Should I be infected (i.e. contagious to others) I would not be here, sitting in the S-Bahn, apparently healthy, but I would most likely be lying in a hospital bed or maybe even in the ICU.
Now the Austrian Government and their esteemed scientific advisers in their deep and unassailable wisdom had declared that face shields do not qualify as MNS (mouth-nose protection) because there are gaps, and that said MNS have to be reasonably tight fitting. But as I said above, they do nothing to protect me. What would protect me would be the equivalent of a N95 mask (or better, see link below), and because of my breathing problems due to my precondition, I need one with an exhale valve. I am thinking of getting one. The "regulations" did not even mention those, just face shields. And I see more of them now also on public transport, supermarkets and wherever masks are legally required.
Now understand this - the regulations talk about masks, but there is a difference between a mask and a respirator. Perhaps the Government, and its Experts (using the term rather loosely), would need a refresher course.
Furthermore, most workplace health and safety regulations require that masks should not be worn for more than 2 hours straight. A free breathing pause should be made for ideally half hour. So what about a train ride from Vienna to Bregenz, for example, that takes 9 hours, or a flight between Vienna and Calgary that takes 12 hours? You must wear a mask for the entire time!
Before anyone interjects with "what about operating room personnel that operate for hours on end" there is an answer also: they wear surgical masks that are not tight fitting, and the operating room has enhanced air circulation with oxygen enrichment.
But let us assume you are symptomatic: The normal way of getting rid of intruders from your airways is through sneezing and coughing. In this case, a virus load. You also expel viruses by simply breathing out (hence the mask requirement). But instead of getting rid of them, they hang in the mask and you then breathe them back in. So you are sort of re-infecting yourself, increasing your virus load more. This could be a problem for people who are already infected, making the infection worse. Now think about old people in nursing homes who are made to wear mask.
I used to teach respirator protocol and safety - this here is a good guide:
Masks vs Respirators
Before we go any further, let’s just clarify on a technical difference between a “mask” and a “respirator”. In day to day language we often say mask, when referring to what are technically called respirators.
In my estimation, the best resource about the subject, there is a lot of very good information and relevant links on that site, so click here or else the image above to read more.