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 1997 IARC reviewed the available literature on CS exposure and concluded that there was sufficient evidence in humans for the carcinogenicity of inhaled crystalline silica in the form of quartz or cristobalite from occupational sources; IARC therefore classified CS in these situations as a Group 1 carcinogen.
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 1100° C and 1300° 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 and the results showed that no hazardous activity can be linked to the formation of CS in the fibres.
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, the CS is embedded in a matrix of other crystals and glasses and does not seem to be biologically available or capable of damaging the lung.
This, coupled with the inability to detect airborne CS during most after use activities (e.g. during furnace maintenance or wrecking), means that there is unlikely to be any risk of CS related disease associated with exposure to after use fibres.
Download and read more detailed information on Crystalline Silica in HTIW products after use in high temperature applications
AES and ASW may transform into a mixture of crystalline phases, but PCW do not contain CS even after use.