the risk that the employee would contract mesothelioma. That, coupled with the fact that mesothelioma had been contracted and that it was not possible to tell when the fatal inhalation had taken place, justified, in their Lordships' view, the imposition of liability on each employer who had contributed to the risk. While Barker ruled in favour of several liability where the Fairchild principle was being applied, the approach was subsequently reversed by the UK Compensation Act 2006 which makes liability in such circumstances 'joint and several'. In Sienkiewicz v Grief (2011), the UK Supreme Court (the successor to the House of Lords), again considered the test of causation in the context of asbestos exposure. This time the increased exposure created by the defendant's breach of duty was particularly small: the judge at first instance found that the exposure to asbestos over the deceased's working life at Greif’s factory increased the risk to which environmental exposure subjected her from 24 cases per million to 28.39 cases per million – an increase of risk of 18%. Lord Phillips described the question for the court at para. 11: Knowledge about mesothelioma is based in part on medical science and in part on statistical analysis or epidemiology. These appeals raise the question of whether, and if so to what extent, the court can satisfactorily base conclusions about causation on the latter, both in mesothelioma cases and more generally. It was submitted by the defendant that the 'double the risk' test for proving causation should be applied. The theory that causation could be proved on the balance of probabilities by reference to a doubling of the risk of injury was first applied by Mackay J in the oral contraceptive litigation XYZ v Schering Health Care Ltd (2002) 70 BMLR 88. As a preliminary issue, the parties agreed that the judge should examine the epidemiological evidence relating to the risk of deep vein thrombosis arising from two different types of oral contraceptive. The claimant group could succeed only if the epidemiology showed that the risk of harm arising from the type of contraceptive they had been taking (which it was assumed they had not been warned about and would not have taken if warned) was at least twice that arising from the type which they had formerly been taking (which it was assumed they had been warned about and which risk they had accepted). The logic behind this was that, if the risk from potential cause A is x% and the risk from the other potential cause B is 2.1x%, it is more likely than not that the condition which has eventuated has been caused by B. Lord Phillips explained the circumstance in which he considered the 'doubles the risk' test might be appropriate: Where there are competing alternative, rather than cumulative, potential causes of a disease or injury, such as in Hotson, I can see no reason in principle why epidemiological evidence should not be used to show that 44