Man-made vitreous (silicate) fibres with random orientation with alkaline oxide and alkali earth oxide (Na2O+K2O+CaO+MgO+BaO) content greater than 18 % by weight

Endpoint summary

Description of key information

Short description of key information on bioaccumulation potential result: 
It is assessed that FMMVF fibres are not taken up into the body following inhalation, ingestion or dermal contact. Inhaled fibres are disintegrated in the lung to smaller fibres, that will be cleared and expelled via coughing, or swalloved and cleared via the gastrointestinal system.  No systemic exposure is expected. The substance have no bioaccumulation potential, When exposure is ceased the accumulated substance in the lungs will gradually clear.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

The clearance of fibres deposited in the respiratory tract results from a combination of physiological clearance processes (mechanical translocation/removal; macrophage mediated clearance) and physico-chemical processes (chemical dissolution and leaching, mechanical breaking). Long and short fibres differ in the way in which their elimination from the respiratory tract is affected by each of these mechanisms. Short fibres are taken up by macrophages and subjected to chemical dissolution/leaching within an acidic milieu while at the same time they are actively removed via the tracheal bronchial tree and the lymphatic system. In contrast, long fibres (longer than approximately 20 µm) which can only be incompletely phagocytised by macrophages, cannot be efficiently removed from the lung parenchyma by physical translocation but may be subjected to chemical dissolution/leaching at acidic pH where macrophages attach to the long fibres, and at neutral pH when in contact with the lung surfactant.

Fibres which deposit in the bronchial tree are removed from the lung via the mucociliary escalator and either expelled from the body by coughing or swallowed. If swallowed, the MMVF (FMMVF) fibres will incongruently leach alkali earth metals at acidic gastric pH, thereby creating a silicate enriched brittle crust, and the fibre will tend to break perpendicular to the length, into smaller pieces.

No data has been identified on absorption following exposure via the dermal or oral route, or via inhalation.FMMVF fibres have low potential for crossing biological membranes and it is therfore evaluated that systemic exposure is negligible.

Distribution:No data have been identified on distribution.

Metabolism:The fate of fibres deposited on surfaces within the respiratory system depends on two different but simultaneously occurring processes: dissolution and disintegration((Hesterberg et al., 1996);(Hesterberg et al., 1998b);(Muhle and Bellmann, 1997);(Zoitos et al., 1997)).

Dissolution:FMMVFfibres exhibit a small degree of leaching (incongruent dissolution), in which certain components dissolve more rapidly than others(Cullen et al., 2000). Hence, following inhalation by rats, it has been demonstrated that the composition of FMMVFfibres changes with time in the lungs, with a depletion in Na₂O, CaO and MgO and a corresponding relative enrichment in SiO₂and Al₂O₃(Cullen et al., 2000). Furthermore, FMMVF fibres (475) have been shown to develop severe surface etching and deterioration starting as early as 1 week post exposure to the rat lung environment in vivo(Hesterberg et al., 1998a).

Studies show that FMMVFfibresdissolve significantly slower than MMVF note Q fibresin vitroat pH = 4.5(Kamstrup et al., 1998;Guldberg et al., 2002). For this fibre type the dissolution rate k_disis 620 ng x cm^-2x hour^-1, which is approximately 10 times higher than dissolution rates, 55 ng x cm^-2x hour^-1, obtained from FMMVF fibres (MMVF 21) in the same study. Consequently, the elimination half-lifetime (WT_½) for MMVFnote Q fibresis approx. 10 times lower than that of non-note Q fibres.

Biopersistence of MMVF 21 and MMVF 33 (475) fibres were reported by Hesterberg, et. al., 1998a, following 5 days inhalation exposure on rats. The fibre concentration was 58 and 51 mg/m³, corresponding to 466 and 371 WHO fibres per cm³, for MMVF 21 and MMVF 33, respectively. At 365 days post-exposure, 7% and 8% of the deposited fibres still remained in the lungs. The weighted half-lifetime for fibres longer than 20 µm was calculated as 67 and 49 days, for MMVF 21 and MMVF 33, respectively. In another study by Hesterberg et al. (1996) it was demonstrated that biopersistence was inversely correlated with fibre length, fibres longer than 20 µm, and shorter than 5 µm, showed half-lifetimes of 56 and 174 days, respectively.

 

Disintegration:Since FMMVF fibres dissolve slowly at neutral pH (eg. In the extracellular lung fluid) it is likely that acid attack from alveolar macrophages are causing the longer fibres to dissolve and break into smaller fibres(Eastes et al., 2007).The shorter fibres will be removed from the lung either by the lymphatic system or via the mucociliary escalator and either expelled from the body by coughing or swallowed. If swallowed, the FMMVF fibres will dissolve slowly at acidic gastric pH(Bernstein, 2007).

Excretion:It is expected that FMMVF fibres when swallowed, will be mixed with the gut content and be expelled via the gastric system, due to the low solubility.

 

Conclusion:The most likely routes of exposure for FMMVFfibres is evaluated to be by inhalation. Fibres have been shown to disintegrate slowly in acidic environment. Inhaled fibres are subjected to breakage, leading to shorter fibre length. Due to the inert nature of the substance, and the fact that the substance does not cross biological barriers, systemic exposure leading to toxic reactions is evaluated to be very unlikely.

Discussion on bioaccumulation potential result:

The most likely routes of exposure for FMMVF fibres is evaluated to be by inhalation. Fibres have been shown to disintegrate slowly in acidic environment. Inhaled fibres are subjected to breakage, leading to shorter fibre length. Due to the inert nature of the substance, and the fact that the substance does not cross biological barriers, systemic exposure leading to toxic reactions is evaluated to be very unlikely.