• Some forms of silica are used in the manufacture of HTIW products, however Crystalline Silica (CS) is not present in any products as sold and 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".
• IARC classified CS 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.
• Silica related fibrosis and cancer in humans have most clearly been observed following exposures to freshly cleaved respirable free silica dust.
• In after use HTIW products, CS can be present in the form of crystals that are embedded in a matrix composed of other crystals and glasses.
• Scientific studies (in-vivo and in-vitro) on after use fibres have shown no effect related to the presence of CS.

Refractory materials, including bricks, castables and Man Made Mineral Fibre products, are usually composed of silicates - silicon in various combinations with oxygen and other elements. Most HTIWs consist of amorphous glass fibres. Although forms of silica may be used in their manufacture, none of these contain any free CS in the product as sold and installed.

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 CS or on external factors affecting its biological activity or distribution of its polymorphs.

When, in the 1980’s, ASW/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) ASW/RCF estimated as containing 27 % cristobalite to simulate after use fibres. This sample caused less lung effects than any other sample tested and no excess of tumours. These early results with ASW/RCF already gave an indication that (devitrified) end of life fibres 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 1.100° C and 1.300° C were heated to their classification temperature and normal maximum continuous use temperatures (approx 150° C below their classification temperatures). These heated samples contained between 3 and 32 % of CS. Unheated and heated fibres were then tested in cell cultures, showing that no hazardous activity can be linked to the formation of CS in the fibres.

This, coupled with the inability to detect airborne CS during most after use activities, means that there is unlikely to be any risk of CS related disease associated with exposure to after use fibres (i.e. during furnace maintenance or wrecking).