Is there an exposure to asbestos which does not “legally cause” mesothelioma? Evidence from: C Gilham et al. Occup Environ Med (2015);0:1–10. doi:10.1136/oemed-2015-103074 Gilham et al, compared asbestos fibre burdens from the lungs of mesothelioma and lung cancer cases and from these, developed risk equations. 79% of the fibres they observed were amosite; the subjects were all resident in the UK. Fibre burden and diagnosis were objective and precise; which means the main uncertainties were in unintentional bias and biological variability, about which not much more could be done except by sub categorisation by genetic and epigenetic profile (which would require a much bigger study). Using mathematical methods adapted from the design of optical telescopes and electronic circuits, we have extended the reported analysis to make an original estimate of de minimis. Compared with background, and for asbestos fibres which are longer than 5 micrometres, the smallest detectable increase in the