Courtesy of NR Murphy, Ltd. Fiberglass dust is a possible carcinogen. According to the American Lung Association has no longer classified fiberglass dust as a "possible carcinogen". Visit this site NOTE: this article is talking about fiberglass insulation in particular However, who wants to take the risk? Even if it doesn't give you cancer; it will make your skin, mouth, throat, and lungs hurt and itch if you don't properly protect yourself. ALWAYS wear an approved mask while sanding / cutting fiberglass; or anytime when you might be exposed to fiberglass dust.
