They are unhealthy for extended periods of time if they aren't properly fashioned for such use. I have family and friends who are nurses; some on the terminal illness ward; some in the ICU, and others in the emergency room. They tell me, except for breaks and lunch/dinner, consults, they typically have them on all day for their patient's protection. The only exception to this were from those who worked in the emergency room ( whereas until it was certain patients posed no thread of disease transmission, thus it was for their own personal protection as well ), and also infectious quarantine ( such as covid-19, Ebola, etc. whereas it was for their protection as well ). So I guess that is two exceptions vs one.
I also have a very good friend who is a surgeon's assistant, and they have their masks on up to 14 hours for surgery as well.
This has been going on for decades for our healthcare workers. Masks are worn both for the protection of the patient as well as the provider. Sometimes for extended hours and sometimes limited.
One of my friends asked why, all of a sudden, is this issue so important? Because the general public has to wear them? "Welcome to our world." she said.
When I explained to her that many employers were not providing proper masks for their employees, as well as the CDC and WHO not being crystal on what type of mask, that many were purchasing inadequate ones that neither protected them nor others against transmission. She agreed that was an issue that should be immediately addressed with each employer. If it's required as a uniform ( of sorts ) then companies should indeed supply proper ones.
Albeit, we have the means to research as well; however, when funds are strapped it becomes an obstacle to protect yourself, your family, and anyone else.