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 risk of mesothelioma occurs at a lung fibre burden of between 19,000 and 20,000 fibres per gram.
- For high exposures, e.g. > 700,000 fibres per gram, the probability that further exposure increased the risk is always below 50%. In other words once the lungs are exposed to a certain level, further exposure is causally innocent (in statistical terms).
Using the data for lung cancer, the above figures can be converted into the traditional measure of exposure i.e. cumulative exposure. The risk detection threshold is ~ 1.1 f.years/ml.
Surveys suggest that 42% of men born in the UK around 1945 will have fibre burdens of more than this and that 0.86% of these will develop mesothelioma. If using this threshold test, these would have no problem establishing legal causation. Those with lung fibre-burdens below this would have to persuade the court by some other means.
Mesothelioma is a dreadful disease which is compensated at common law when both breach of duty and material increase in risk can be shown to the satisfaction of the court. By UK statute, it is settled on a joint and several basis.
UK courts have never defined ‘material increase in risk’ in a way which could be addressed in a rational way by science experts. It would take some political will to do so as it could lead to some failed claims, and, would partly restrict the latitude that the courts enjoy. On the other hand, duty-holders who have negligently exposed people to trivial amounts of asbestos are being asked to pay 100% of the damages. The creation of an objective test of de minimis provides an opportunity for the courts to consider the situation in the light of greater factual certainty.
The calculated threshold could be refined. One option would be to use a test other than the long-established, balance of probabilities. For public policy reasons, UK mesothelioma case law already breaks legal conventions, so, to use a non-standard test of fact would not be completely unimaginable.
The mathematical method used to make this estimate of de minimis can be applied quite generally if a quantitative risk relationship is known.