Review of the durability of inhaled fibres and options for the design of safer fibres

The hazards associated with inhaled fibres are linked with fibre shape, size, composition and durability Durability includes susceptibility both to dissolution and to mechanical disintegration. Most fibre types dissolve incongruently with some elements being preferentially leached from the fibre to leave it mechanically weakened. Incongruent dissolution is also associated with hydration of fibre surfaces and the associated volume changes can lead to mechanical failure of fibres. In general the asbestos minerals are more durable than most man-made vitreous fibres (MMVF), although some ceramic fibres may be more durable than chrysotile. The most soluble of the MMVF have relatively high alkali (Na and K) and alkali earth (Ca, Mg) contents, and low alumina and boron contents The least soluble MMVF are the high alumina silicate glasses (the ceramic fibres). The relative durability of different fibre types is governed by the physical and chemical environment which contains the fibres. The value of existing published data in determining the relative durabilities of different fibre types has been limited by the lack of comparability between studies The effective assessment of fibre durability is an important aspect in fibre safety and requires the development of better and more co-ordinated experimental design. It may ultimately be possible to design vitreous fibres which have low durabilities in the lung but are adequately durable in the intended environment of use.