Roger W. Norman wrote:
One of the things that fiberglass doesn't do is evaporate moisture very
well, which cotton does (else why wear it during the sweaty months of
summer? g). Now I dare say that, although not dangerous in walls, wearing
fiberglass wouldn't provide this benefit to one's body. g

Cotton absorbs moisture, making it a spectacularly poor choice for cold
weather wear. You may have noticed that hot weather athletes are also
using non-wicking synthetics.