The three variables, dose, dimensions and durability, have been shown from many studies to be the factors that combine to give the potendal for a fibre to cause respiratory diseases. Considering durability, a fibre that could show increased dissoludon, possibly accompanied by chemical change, in physiological Solutions would be expected to show a reduced persistence in the lungs and have a lower potential for producing respiratory diseases. The development of such a fibre, Superwool X607, and the understanding that has been acquired in terms of solubility rate and other fibre properties is presented in this paper. The importance of in-vitro solubility rate studies to screen potendally useful fibre compositions, combined with an understanding of how these composidons can be selected using free energy of hydradon or non-bridging oxygen theories, is discussed in terms of developing new less "in-vivo" durable fibres.