What is this?

My fiancé gets this rash all the time, sometimes worse than others and it comes and go but it's always on his chest and back. It's doesn't hurt but if I touch it he said I make it itch. I have never gotten it so it's not contagious. It happens a lot when he sweats and we joke and say he's allergic to his sweat...but could that really be the case?
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