The studied participants were not a random population sample and the results cannot be extrapolated directly to the general population. In many countries it is not considered ethically correct to expose healthy volunteers with X-rays to study CT signs due to mild exposures. Finding suitable study groups for this purpose can therefore be difficult. Our multivariate analyses were adjusted for the asbestos exposure, previous smoking and other potential confounders to ensure that these do not explain our findings. In these multivariate analyses, ex-smoking was related to emphysema only and asbestos exposure was related to irregular/linear opacities only, which suggests that any major residual confounding by these factors is unlikely.