Hi, i've had a rash on my chest and back for months now. is it due to sweat?

Doubtful: I wouldn't ascribe a rash that's been present that long to perspiration. A fungus could be the cause but more description is needed in order to make a differential diagnosis. Nevertheless it's time that you consulted a dermatologist for diagnosis and treatment.

VisualDX: Hi, it’s hard to say without a picture but there is a great app that may be able to help you get a diagnosis without being seen in person by a dermatologist. The app is called VisualDX and is one of the best, if not the best out there. Many clinicians, including myself, use it. Just snap a photo and it gives you a list of possible diagnoses. Good luck!