2. Relevance to Public Health 2.1 Background and Environmental Exposures to Synthetic Vitreous Fibers in the United States

Synthetic vitreous fibers are inorganic fibrous materials, manufactured principally from glass, rock, minerals, slag, and processed inorganic oxides. Synthetic vitreous fibers are manufactured by several processes, all of which involve cooling of a stream of high-temperature, molten inorganic oxides. Commercially important synthetic vitreous fibers are primarily silica-based, but contain various amounts of other oxides (e.g., aluminum, boron, calcium, or iron oxides). Synthetic vitreous fibers have amorphous molecular structures, while naturally occurring mineral fibers, such as asbestos, possess crystal structures. In the past, synthetic vitreous fibers were classified into three categories: fibrous glass; rock wool and slag wool (sometimes collectively referred to as mineral wool); and refractory ceramic fibers. The fibrous glass category included continuous filament glass fibers (sometimes called textile fibers) and glass wools. Recently, the World Health Organization (WHO) IARC classified synthetic vitreous fibers into two broad categories: filaments and wools. A schematic of this classification scheme is shown in Figure 2-1. The filaments category refers to glass fibers that are produced by extrusion (continuous glass filaments). IARC noted that more than 98% of currently produced continuous glass filaments are for electrical applications. The wools category includes five subgroups: glass wool, rock wool, slag wool, refractory ceramic fibers, and other fibers. Included in the glass wool category are the subgroups, insulation wools and special purpose fibers. The special purpose fiber group includes glass fibers produced by flame attenuation for special applications such as high-efficiency air filtration and include special fine-diameter glass fibers. The other fibers group includes fibers such as alkaline earth silicate wools and high-alumina, low-silica wools that have been recently developed to be more biosoluble than older high-temperature synthetic vitreous fibers such as refractory ceramic fibers or rock wools.