In reply to sopaz: you'll need to first contact the manufacturer and confirm actually what was in it!
Then work out what your harness is made of (primarily nylon but the stitching is other stuff more akin to polyester AFAIK), then surf on line looking for "chemical resistance" "material" "sodium acetate" etc
However even this won't be definitive unless there is only one ingredient in the hand warmer as the combination of them may have a different effect than any of them alone.
When you say crystallised chemical was it dry or wet? I guess you don't know for sure when it exploded? Also once removing as much as possible from the harness wash it at first opportunity as the exposure time effects strength a lot with some chemicals.
Sodium Acetate doesn't look like it damages nylon BTW. But I'm not sure I'd trust that harness again.
Also what heat could the contents have got to? Generally harnesses come with instructions about not heating above 40-50 degrees.
In reply to CurlyStevo: Cheers for the reply. Sadly the text has worn off the handwarmer, and I've no idea where it even came from! I presume from the way it's soaked into the harness that it was wet when popped, which would suggest it probably did get hot when in contact with the narness too.
I tried washing it but it's difficult to get the crystalised stuff out. Probably one for the bin sadly. (At least it was an old spare!)