The first instance of medfet for me (that I remember) was before I understood sexuality and fetish. I remember two early moments.
One was when I was in the days leading into a surgery, and my mot-her had bought me a book about what the surgery experience would play out like. In it was a picture of a girl my age who was lying on an OR table being anesthetized by "black beauty" mask. I read the whole book, but remember fixating relentlessly on the sleeping girl underneath the big, shiny black mask with corrugated tubing going from it to a generator for anesthetic.
Another was around that time as well, and my mother found me a kids group for those facing surgery (I grew up going into OR a lot due to spina bifida and hydrocephalus). Part of it entailed sitting in a circle in a group and passing around things related to surgery. One of them was a "black beauty" mask. We were encouraged to put it on for a few seconds and breathe, just to see what it was like. I was trembling. My heart was racing. I put it on for less than a second then passed it to the girl next to me.
Later on, I started watching medical documentaries and fundraisers that were likely to have masking scenes of patients my age. The same sort of thing happened. My heart would begin to race if I saw a pretty girl either being anesthetized by mask or having to mask due to a low immune system.
I really should have started keeping a sexual diary. There are gaps in my memory of all this. But the next pivotal thing for me was when the SARS epidemic occurred in 2003. Lots of pretty young women in masks due to a mandate for that. At that point, IIRC, I started becoming aware of brands of masks. 3M, MSA, Moldex, and some others that aren't even around anymore. I started paying attention to the colors of masks, the materials, the various parts, and learning what I liked and what I didn't like.
I like half and quarter masks, from surgical earloop and tie-on masks all the way up to an elastomeric half mask cartridge respirator (sopmetimes called a P100 due to the most severe cartridges it can handle). I like watching a girl breathe through a pleated surgical mask, and seeing it collapse on her face each breath. On elastomeric respirators, I like the "gasket", which is the area that touches the face when it is strapped on the wearer. I also like the thick straps. I do NOT like eyeholes/visors. I only like the girl's mouth and nose covered.
So, especially since COVID started, I am fascinated by the machinery of masks, and the various types and brands of them on the market. And all that helps inform what I look for when I look at masked girls on the web. But also, masks have become so political, and that sometimes makes me mad. People have a right to be protected, or at least should.
So, before I say any more, the TLDR is that it remains highly sexual for me, but it has slowly become technical and medical and political and whatever else as well.