NEW-TYPE OF STONEWOOL (HT FIBERS) WITH A HIGH DISSOLUTION RATE AT PH=4.5

A representative, the HT-3 fibre, of a new type of stonewool (HT fibre s), characterized by a relatively high content of alumina and a relati vely low content of silica, has recently been tested in-vivo by Bellma nn et al. to have a low biopersistence after intratracheal instillatio n in rat lungs. Several advantages of this fibre were found: - a half- time T-50 which is 2 respectively 4 times lower than that for the refe rence fibres glasswool and stonewool (after correcting to 1 mu m media n diameter), - a better ultimate clearance after 18 months, resulting in 3 % remaining for the HT-3 fibre compared to 32 % for normal stonew ool, - an effective clearance of fibres with a diameter <0.5 mu m. Fib res with a diameter belonging to this class are generally thought to b e those with the highest potency for tumour formation. The HT fibres h ave a relatively low in-vitro dissolution rate at pH = 7.5, similar to that of normal stonewool fibres, but a high in-vitro dissolution rate at pH = 4.5. The explanation of the favourable in-vivo properties of the HT-3 fibre is based on the recent investigations of the ability of alveolar macrophages to dissolve certain fibres, coupled with the obs ervation of the high dissolution rate of the HT fibres at the pH value of 4.5 to 5 prevailing inside the macrophages. All HT fibres have bee n shown to have comparable, high dissolution rates at pH = 4.5 to 5.