Job title and area may therefore, in the present study, be a better predictor of cumulative exposure than duration of employment. Finally, dust may not be the best marker of the true causal exposures, since abietic acid, monoterpenes or microbial agents may be more important [19]. All of the above issues may have resulted in random exposure misclassification, but this is likely to have produced a bias towards the null hypothesis and is unlikely to explain the present positive findings [20].