Toxicology of Industrial Compounds

110 CARCINOGENIC POTENTIAL OF MAN-MADE VITREOUS FIBERS
which was not significantly elevated above reference levels. Fibers per mg
dry lung for the rat after lifetime exposure was >4000-fold higher than for
the fiber glass worker, average exposure 11 years (the average time from
last employment in MMMF manufacturing and death was 12 years). Lung
fiber dimensions in the rat study were comparable to those of fibers
recovered from the lung tissue of fiber glass manufacturing workers. From
these comparisons, it can be concluded that the exposure levels used in the
recent rat inhalation studies unequivocally achieved the goal of the studies
to exceed human exposures by several orders of magnitude.
Summary and conclusions
MMVFs are among the most studied commercial products due to their
widespread use and the concern for potential health effects of respirable
fibers. In recent animal inhalation studies RCF produced lung fibrosis,
mesotheliomas, and significant increases in lung tumors. However, it is
believed that any potential cancer risk from RCF exposure can be
minimized, if not eliminated, because of the small number of workers
exposed and the ability to use respiratory protection and engineering
controls to limit worker exposure. Both human and animal inhalation
studies have shown no association between fiber glass exposure and
disease. Although high exposure levels of rock wool (several orders of
magnitude higher than most reported workplace exposures) produced
minimal lung fibrosis in rats, no mesotheliomas and no significant increase
in lung tumors were observed. Slag wool produced no fibrosis or increase
in tumors in the animal studies. The cohort mortality studies of rock wool
and slag wool workers have also provided no clear dose-response
relationship with fiber exposure.
Results from the combined animal inhalation studies showed that
differences in lung fiber burdens and lung clearance rates could not explain
the differences observed in the toxicologic effects of MMVFs. These
findings clearly indicate that dose, dimension and durability (i.e. the
persistence of fibers in the rat lung) are not the only determinants of fiber
toxicity; chemical composition and the surface physicochemical properties
of the fibers may also play an important role. Exposure levels from animal
inhalation studies were at least three orders of magnitude higher than for
average airborne levels reported for many occupational settings.