PERSISTENCE OF NATURAL MINERAL FIBERS IN HUMAN LUNGS - AN OVERVIEW

Virtually all available data on persistence of naturally occurring min eral fibers in human lungs have been derived from studies of asbestos fiber loads. These studies indicate that, although both amphibole and chrysotile asbestos fibers are found in the lungs of the general popul ation and exposed workers, amphibole fibers are universally present in disproportionately large and chrysotile fibers in disproportionately small amounts compared to their known abundance in the original inhale d dusts. Why this should be remains unclear. Most reports have shown t hat fiber accumulation is proportional to measured exposure for amphib oles, but this is not generally true for chrysotile. Very little infor mation is available on actual fiber clearance rates from human lungs. For amosite and crocidolite, estimated clearance half-times are measur ed in years to decades, whereas for chrysotile the available, rather i ndirect, data suggest that the vast majority of fibers are cleared wit hin months, although some fibers may be sequestered and very slowly cl eared. Overall these studies suggest that the differences between amph ibole and chrysotile fiber burdens in mao reflect much faster clearanc e of chrysotile fibers, rather than a failure of chrysotile deposition . A variety of other naturally occurring fibers are commonly found in human lungs, but there are no data on their rates of accumulation or d isappearance.