Based on the total experience with humans and the findings of scientific research (animals, cells), it can be concluded that elongated dust particles of every type have in principle the potential to cause the development of tumours providing they are sufficiently long, thin and biopersistent. According to scientific findings inorganic fibre dust particles with a length-to-diameter ratio exceeding 3:1, a length longer than 5 μm (0.005 mm) and a diameter smaller than 3 μm (WHO-Fibres) are considered health-critical.
HTIW processed to products contain fibres with different diameters and lengths. During handling of HTIW products, fibrous dusts can be emitted. These can include fibres complying with the WHO definition. The amount depends on how the material is handled. High concentrations are usually found during removal of after-use HTIW and also during mechanical finishing activities and in the assembly of modules. Where fibre products are mechanically abraded by sawing, sanding, routing or other machining the airborne fibre concentrations will be high if uncontrolled. Dust release is further modified by the intensity of energy applied to the product, the surface area to which the energy is applied, and the type, quantity and dimensions of materials being handled or processed. Dispersion or dilution of dust produced depends on the extent of confinement of the sources and the work area, as well as the presence and effectiveness of exhaust ventilation.
Amorphous HTIW (AES and ASW) are produced from a molten glass stream which is aerosolised by a jet of high pressure air or by letting the stream impinge onto spinning wheels. The droplets are drawn into fibres; the mass of both fibres and remaining droplets cool very rapidly so that no crystalline phases may form.
When amorphous HTIW are installed and used in high temperature applications such as industrial furnaces, at least one face may be exposed to conditions causing the fibres to partially devitrify. Depending on the chemical composition of the glassy fibre and the time and temperature to which the materials are exposed, different stable crystalline phases may form.
In after-use HTIW crystalline silica crystals are embedded in a matrix composed of other crystals and glasses. Experimental results on the biological activity of after-use HTIW have not demonstrated any hazardous activity that could be related to any form of silica they may contain.