Recommendation from the Scientific Committee on Occupational Exposure Limits for man made-mineral fibres (MMMF) with no indication for carcinogenicity and not specified elsewher. SCOEL/SUM/88 March 2012
SCOEL considers properly conducted inhalation studies, preferentially in rats, using fibres of rat respirable size which upon long term exposure did not induce carcinogenic effects as the best basis for setting an OEL. Fibres longer than 5 μm, shorter than 100 to 200 μm of a diameter less than 3 μm with a length/diameter ratio of at least 3:1 are considered respirable. Such studies have been performed with fibres of glass wool, rock wool, slag wool and calcium-magnesium-silicate (Table 3). In all these studies, inflammation and subsequent fibrosis of the lung have been the critical effects. In the two-year exposure studies in rats, NOAELs within the narrow range of 25 to 30 fibres/ml of inhaled air have been determined.
For fibres with insufficient data to derive a specific OEL, SCOEL proposes a general OEL of 1 fibre/ml. This value is derived as decribed before: Considering the uncertainties to extrapolate from LOAEL to NOAEL, the uncertainties of interspeciesextrapolation and possible intrinsic differences in fibre toxicity, the conservative assessment factors of 20 and 10, respectively, have been applied. The resulting values range between 1.3 and 3 (see Table 3). Based upon this information the lowest value of 1.3 fibres/ml for glass wool fibres is adjusted to a general OEL of 1 fibre/ml, which corresponds to about 0.1 mg/m3 (Schneider 1987). This OEL is applicable to MMMF without indication of carcinogenicity (see Annex I) and the characteristics: length >5 μm, diameter D <3 μm and a ratio L:D >3:1 (WHO fibres). For the fibres listed in Tables 2 and 3, for which NOAELs can be derived, SCOEL will propose specific OELs (see e.g. MMVF10, SCOEL 2000).
No fibre counting difficulties are foreseen at the recommended OELs. Fibre counting shall be carried out in accordance with the 1997 World Health Organisation (WHO) recommended method "Determination of airborne fibre number concentrations by phase-contrast optical microscopy (membrane filter method)". Theoretically, the process of counting randomly distributed (Poisson) fibres yields a coefficient of variation (CV) of 10% for 100 fibres and 32% for 10 fibres, taking into account only statistical variation. In practice, however, the actual CV will be greater because of the additional component of variation associated with subjective differences within and between microscopists.
Recomendation SCOEL
