This is a timely question for me, Jill. I have a central AC on my roof that's working pretty much 24/7 much of the summer here in the desert. A technician stopped by in August to do our semi-annual inspection of the system and noticed I was doing a breathing treatment with my Aerobica and saline. He commented that his wife is a respiratory therapist and we got to discussing AC systems and respiratory problems. Beyond changing the filters often, he also said that lots of organisms grow on the moist surfaces of the interior of the machine. When he went up to inspect he took some photos of the fan and the other parts of the system. Yuck---covered in biofilm and gunk. He thoroughly cleaned the inside of the unit and subsequently installed a UV light to kill the organisms. My ID doc concurred that UV light kills MAC. I now suspect that I might have gotten MAC from the AC unit. Of course, NTM is everywhere so it's impossible to avoid completely. We can only do what we can to minimize more exposure.