Refractory materials, including bricks, castables and man made mineral fiber products, are usually composed of silicates - silicon in various combinations with oxygen and other elements. Most HTIWs consist of amorphous glass fibers. Although forms of silica may be used in their manufacture, none of these contain any free crystalline silica in the product as sold or installed.
When amorphous HTIW products are used in high temperature applications, depending on time and temperature, different stable crystalline phases may form. This process is called devitrification and the product then is referred to as "after use."
In 1997, IARC classified crystalline silica when inhaled in the form of quartz or cristobalite dust from occupational sources as a Group 1 carcinogen. Silica related fibrosis and cancer in humans have been observed following exposures to freshly cleaved respirable free silica dust.
In making their overall evaluation, the IARC Working Group noted that carcinogenicity in humans was not detected in all industrial circumstances. Carcinogenicity may be dependent on inherent characteristics of the crystalline silica or on external factors affecting its biological activity or distribution of its polymorphs.
Scientific studies (in-vivo and in-vitro) on after use fibers have shown no effect related to the presence of crystalline silica.
In the 1980’s, when RCF’s were tested in a series of animal experiments (the so-called RCC studies), the samples tested included a specimen of heated (devitrified or crystallised) RCF estimated as containing 27 % cristobalite to simulate after use fibers. This sample caused less lung effects than any other sample tested and no excess of tumors. These early results with RCF gave an early indication that devitrified end of life fibers do not constitute a health hazard. Further studies at the IOM in Edinburgh also found this sample to be inert when injected into the peritoneum of rats.
Recently at the Fraunhofer Institute for Toxicology and Experimental Medicine, four samples of AES with classification temperatures between 1100° C and 1300° C were heated to their classification temperature and normal maximum continuous use temperatures (approximately 150° C below their classification temperatures). These heated samples contained between 3 and 32 % crystalline silica. Unheated and heated fibers were then tested in cell cultures, showing that no hazardous activity can be linked to the formation of crystalline silica in the fibers.
This, coupled with the inability to detect airborne crystalline silica during most after use activities (for example, during furnace maintenance or wrecking) leads to the conclusion that there is unlikely to be any risk of crystalline silica related disease associated with exposure to after use fibers.