Not knowing exactly what your job includes, probably allot of climbing around nasty places. I'm guessing that there is a laundry list of items that you could have been in contact with over the years that you didn't list. My first thought was Asbestos when you mentioned HVAC.
Old Industrial Building and Apartment Complexs still have wrapped pipes with this material. To pin point the cause of your cancer, it could be almost anything. Hopefully, you'll find the answer to that question.
My Best to You