When any synthetic vitreous fibre is heated at prolonged high temperatures it can ‘devitrify’ to form cristobalite, a type of crystalline silica. Since crystalline silica is classified by IARC as carcinogenic, this has led to the speculation that ‘after-service’ (heated) HTIW may be dangerous, even if the material is safe in its original form. However, studies have demonstrated very clearly that such devitrified fibres are not more toxic, and that the cristobalite they contain appears inactive or not bioavailable. Exposure measurements undertaken during demolition works have shown no or very low levels of cristobalite in the workplace air. Note: These considerations are not relevant to PCW because it is not a vitreous fibre. Also, no HTIWs contain crystalline silica in their newly manufactured form.