F.A. Everest's "Sound Studio Construction on a Budget" has an appendix on fiberglas safety, saying essentially that it's safe, as Twl suggests. But not long ago I obtained some Coincident Troubador drivers and built little boxes for them, with generous advice from Coincident's Israel Blume, and they sounded boxy with fiberglas--which I'd put in without asking him. When I asked him how much to put in, he delivered a diatribe against fiberglas--bad for sound and bad for health--and told me to use polyester fiberfil, as in pillows, obtainable from fabric shops, instead. He may be wrong about health but he was very right about sound: with the polyester fiberfil replacing fiberglas, the boxiness was all gone. I've since wondered whether it would also do better in some other applications, like inside DIY Argent Room Lenses and perforated panel absorbers--but haven't made those changes yet. Fiberfil is expensive, but certainly worth it inside speaker cabinets, and maybe elsewhere too.
DIY and fiberglass and safety
I have a friend of mine that built some bass traps and acoustic panels out of fiberglass and considering what he paid for the raw materials, they do a good job; however, I wonder how safe it is to have all that fiberglass in a listening room. Does anyone know if it is safe?