[Factors which may mask a possible safe threshold for asbestos fibre]

Five factors are identified which could mask a possible threshold for asbestos fibre as a cause of asbestosis: (i) the definition of asbestosis; (ii) the influence of dust other than asbestos fibre; (iii) the underestimation of the true exposure to fibres; (iv) the non-allowance for random error in the determination of disease; and (v) the overestimation of early (high) exposures coupled with the underestimation of later (low) exposures. The manner in which each of these factors may tend to mask the existence of a threshold is dealt with. It is concluded that one or more of the factors could have been responsible for some of the unsuccessful past attempts to demonstrate a safe concentration for asbestos fibre. It is recommended that in future, exposure-response studies should take these factors into account.