The biopersistence and pathogenicity of Man-made Vitreous Fibres after short- and long-term inhalation

A summary is given of the biopersistence and pathology after inhalation by rats of two different Man-made Vitreous Fibres, MMVF21 (traditional stone wool) and MMVF34 (HT stone wool), and the results are discussed in relation to biopersistence measured after intra-tracheal instillation. The results are given from a short-term inhalation biopersistence study, a completed chronic inhalation study, and interim results from an on-going chronic inhalation study. In both the short-term and chronic studies, laboratory rats were exposed by nose-only inhalation to well-characterised fibre test atmospheres that had been selected to be largely rat respirable. The short-term inhalation study included groups exposed to aerosols targeted at 150 fibres longer than 20 microns per cm3. The exposure duration was 6 hours/day for 5 days, with subsequent post-exposure periods lasting up to 12 months. For lung burden analyses, interim sacrifices were performed at regular intervals. The ongoing chronic study comprises a group of rats exposed to the MMVF34 fibre at one exposure level of 30 mg/m3. The negative control group is filtered air. The exposure duration is 6 hours/day, 5 days/week for 2 years, with a subsequent post-exposure period lasting until approximately 20% survival in the test fibre group. Interim sacrifices are performed at months 3, 6, 12, 18 and 24 and biopersistence monitored for rats exposed for 3 and 12 months, with subsequent post-exposure periods lasting 6 months. Effectively the main protocol for the previously conducted chronic study was the same, except that there were 3 fibre exposure groups (3, 16 and 30 mg/m3) and no specific biopersistence satellite groups were included. For MMVF34, the inhalation tests of different duration show a similar biopersistence pattern, while the intra-tracheal test gives longer elimination half-times especially for long fibres. The MMVF34 fibre is considerably less biopersistent than the traditional MMVF21 fibre when comparing the calculated elimination half-times after short-term inhalation. When comparing the pathology after 3, 6, 12 and 18 months exposure, MMVF34 showed minor histopathological changes compared to MMVF21. The carcinogenicity and toxicity results of the chronic study with MMVF21 suggest that this fibre does not pose a significant health risk to humans and the current results with MMVF34 indicate that this fibre consequently should pose an even smaller risk, if any.