Further observations on the short-term retention and clearance of asbestos by rats, using UICC reference samples

Rats exposed to UICC chrysotile ‘A’, amosite and crocidolite at concentrations close to 1, 5 and 10 mg/m 3 of respirable fibre for a total of 210 h over a 6-week period were killed in five groups at approximately monthly intervals. The asbestos retained in the lungs was estimated by infra-red spectrophotometry after ashing. The results indicated different initial fractional retentions but that the rate of clearance was independent of the type of asbestos and of the concentration during exposure.

The rates of deposition and clearance could be described in terms of a model, modified from that published by MORGAN et al . (1977). In this model it is assumed that a fraction K , of the estimated aggregate dose is deposited in three ‘compartments’, from which asbestos is subsequently cleared with half-times of 0.38, 8 and 118 days. It is also assumed that asbestos enters the three ‘compartments’ in the proportions 86.5:5.3:8.2, respectively.

The fractional deposition, K , of chrysotile was much less than for the two amphiboles but increased with diminishing concentration, from 0.17 at ca . 10 mg/m 3 to 0.36 at ca . 1 mg/m 3 . The K -value for amosite, 0.65, was less than for crocidolite, 1.0, both being independent of concentration.

In view of the nocturnal nature of rats it was thought that the normal practice of dust exposure during daylight, a period of reduced activity, might have led to misleading results. Two further experiments were therefore carried out in which rats were exposed to UICC chrysotile ‘A’ and crocidolite in a darkened chamber to simulate nocturnal conditions. The rates of clearance were closely similar to those observed after daylight exposure. Application of the model described above indicated that, although the fraction of crocidolite deposited was comparable with that observed after daylight exposure, the fraction of chrysotile deposited was approximately three times that deposited during the comparable daylight experiment.

Possible reasons for the different retention pattern of chrysotile are discussed.