I'm a firefighter for our area and I'm always wearing my Oakleys. Typically I wear the original Minutes or Flak Jackets to scenes (both are SI). We got called to a brush fire and the closest pair that I had were a pair of M Frames (not SI). The weather was hot and humid and I had my turnout (firefighting) gear on. I was wearing the Hybrid grey lens. For brush fires, we have backpacks with water and small nozzles the was use to pump water on the fire. I have to admit that I was disappointed with the performance of my M Frames. The nose piece kept coming loose and almost falling off. I got so frustrated that I ended up taking the sunglasses off and throwing them in my pocket. Though the lens did not fog, it did become covered in sweat (I should have used the hydrophobic solution). Wearing a helmet and the M Frames also caused some problems. The fit was a bit uncomfortable and caused the M Frames to fall out of place. In comparison, the Flak Jackets have worked very well with the helmet. (I'm using a Cairns 880). During the Fire Academy, I used the same pair of M Frames as safety glasses for vehicle extrication but with a different helmet. The weather was much cooler so less sweat was involved. I also used the clear lens. The M Frames did stay in place and held up well during that scenario. The good news, about the M Frames, was that the unobtanium worked well when I took the helmet off. After the fire was out, we were able to take off our helmets and coats when looking for "hot spots". I threw the sunglasses back on as to protect my eyes from direct smoke. During this part, the M Frames worked well and stayed in place (I left the nose piece off as I had become frustrated and stuffed it in another pocket). Lessons learned: The M Frames are okay for light duty work. Otherwise, I will be sticking to the Flak jackets (preferred) or Minute 1.0s. For Safety glasses needs, I will continue to use the SI M Frame 2.0. Hope this post/review helps.