Document gDg6kmrO2dvexamqbLa4kBXn9

34 S. Luxun that they indicate that the levels likely to be found in many sections of the industry engaged in this work arc below the level of 2 fibres/cm*, which is considered, in the light of past knowledge, a safe one for chrysotile asbestos. Their highest figures for the time-weigh ted average concentration over 4 hr, however, clearly show that over a period of 10 min we may have concentrations exceeding substantially the level of 12 fibres which are quoted above. 1 was interested to hear of the increasing tendency to carry out brake overhauls on a production line basis. This, dearly, must increase the exposure of particular in dividual workers, making the development of adequate precautionary measures even more important and, in this connection, I hope the devices which will be described later to-day can be improved and developed to the point where they become univer sally acceptable to the operator. Certainly we should not be blowing out brake dust, even if it contains only a few per cent of asbestos. In regard to this latter statement, perhaps I should say that work at our Industrial Hygiene laboratory confirms the figures indicated in the previous paper. We have found that samples of dust from brake drums, when subjected to X-ray diffraction analysis, contained between 1 per cent and 3 per cent of chrysotile asbestos although the original lining material contained more than 40 per cent asbestos. There seems little doubt that this is due to the considerable thermal degradation which occurs consequent on the high temperatures generated by friction at the interface between the fining and the drum. REFERENCES . l.ihestm RrguhHitms (1969) Statutory Instrument 1969 No. 690. CowMime on Hygiene Standards, British Occupational Hygiene Society (1968) Hygiene Stamianh for Chrysolite Asbestos Dust, Pergamon Press, Oxford, t Iic'Kisii, D. E. and Knight, K. L. (1969) Ann. Qccttp. Hyg., 13, 17.