Chronic inhalation studies of two types of stone wool fibers in rats

A summary is given of the pathology results after long-term inhalation in r ats of insulation wool representing the new biosoluble types. The pathology results are compared with previously conducted long-term inhalation study with MMVF21 ( traditional stone wool). The biosoluble fiber MMVF34/HT ( HT) is characterized by a relatively high content of aluminum and a relatively low content of silica compared to the older MMVF21. HT has a high in vitro dissolution rate at pH 4.5, and a relatively low dissolution rate at pH 7. 5. Male Fischer 344 rats were exposed at one exposure level of 30 mg/m(3) b y nose-only inhalation of a well-characterized fiber test atmosphere. The f ibers had been size selected to be largely rat respirable. The negative con trol group was exposed to filtered air. The exposure duration was 6 h/day, 5 days/wk for 104 wk, with a subsequent nonexposure period lasting until ap proximately 20% survival in the air control group. Interim sacrifices were performed at wk 13, 26, 52, 78, and 104 to monitor the progression of pulmo nary change and fiber numbers. Effectively the main protocol for the previo usly conducted chronic study with MMVF21 was the same, except that there we re three concentration levels ( 3, 16, and 30 mg/m(3)). In addition to the endpoints measured in the previous study, slides from both studies were eva luated for collagen deposition using a quantitative morphometric method. Th e results of the comparative study clearly showed a marked difference in th e pulmonary pathogenicity of the MMVF21 and HT in terms of their fibrogenic potential. MMVF21 caused pulmonary fibrosis, but the HT fiber did not. The incidence of tumors for both the HT and the MMVF21 fiber was comparable to the control groups.