A seven member panel of the Supreme Court has held in Karen Sienkiewicz (Administratix of the Estate of Enid Costello Deceased) v Greif (UK) Limited; Knowsley MBC v Willmore  UKSC 10 that any culpable exposure to asbestos which is not insignificant will be sufficient causation to allow a claim. It is not necessary for the culpable exposure to have given rise to a risk which is greater than the risk caused by general environmental exposure.
The decision was, however, reached with reluctance from some members of the panel and the position as regards other diseases caused by cumulative exposure, particularly indivisible diseases such as lung cancer, has been left less clear.
In Greif, Mrs Costello had been an office worker at a factory when employed by Greif's predecessor. During that employment, she had been exposed to asbestos due to her employer's breach of duty. Mrs Costello was not exposed to asbestos during the course of any other employment. Her occupational exposure was modest compared to her total environmental exposure, increasing by only 18%, the risk of mesothelioma that would have arisen in any event given the environmental exposure.
In Knowsley, the defendant local authority was responsible for a school attended by Mrs Willmore as a pupil where, it was alleged, she was exposed to asbestos contained in the roof tiles covering part of the school's ceiling.
Decisions of the lower courts
In Greif, the judge at first instance held that in order to prove causation, Mrs Costello's estate must show that the occupational exposure at least doubled the risk of contracting mesothelioma which Mrs Costello would have faced in any event as a result of environmental exposure. The Court consequently dismissed the claim.
The Court of Appeal, applying section 3 of the Compensation Act 2006 and the House of Lords' decisions in Fairchild v Glenhaven Funeral Services Ltd  1 AC 32 and Barker v Corus UK Limited  2 AC 572, upheld the appeal on the basis that it is only necessary for the claimant to show that the exposure for which the defendant was responsible led to a more than minimal increase in the risk of contracting mesothelioma in order to found a claim.
The Court of Appeal decision in Knowsley proceeded, consistently with Sienkiewicz (although without reference to it), on the basis that it was necessary to show that exposure to asbestos caused a material increase in the risk of contracting mesothelioma. Despite the evidence that exposure had actually occurred being sparse, the first instance finding that there had been exposure which had caused a material increase in risk was upheld.
The decision of the Supreme Court
It was unanimously held by each of the seven judges that in relation to mesothelioma claims, the correct test for causation was whether the culpable exposure for which the defendant was responsible had led to a material increase in the risk of the claimant contracting the disease. If it had, the defendant was liable for the full loss. A material increase for these purposes means more than minimal. Lord Phillips concluded "[t]he reality is that… the only circumstances in which a court will be able to conclude that wrongful exposure of a mesothelioma victim to asbestos dust did not materially increase the risk of contracting the disease will be where that exposure was insignificant compared to the exposure from other sources".
The effect of this ruling caused some of the Supreme Court judges considerable disquiet – none more so than Lord Brown who regarded the position as "unsatisfactory" and who indicated that mesothelioma cases must now be considered from the defendant's standpoint "a lost cause". Lady Hale also pointed out that the resulting position was that "a defendant who may very well not have caused the claimant's disease – indeed probably did not do so – is fully responsible for its consequences".
Perhaps partly as a result of these misgivings, the Court was keen to emphasise that the Courts should not take a lax approach to the requirement of evidence to show that culpable exposure had occurred. It is only in exceptional circumstances that the Supreme Court will disturb findings of fact and, with some hesitation, it was unwilling to do so in respect of the appeal in Knowsley. It was clear, however, that the Court did not approve of what Lord Mance described as the "slender and speculative" basis for the finding that exposure had actually occurred.
Diseases other than mesothelioma
While the claims in issue were both in respect of mesothelioma, the judgment could have some wider relevance to diseases caused by the cumulative effect of exposure. In the Court of Appeal, Smith LJ had indicated that as a matter of principle, in a case of multiple potential causes, a claimant can demonstrate causation by showing that the tortious exposure has at least doubled the risk from other causes. The Supreme Court disapproved of that indication. One reason for this was the doubt that the Court expressed as to over-reliance on evidence specifying the level of risk that an individual would contract a disease as a result of exposure to a particular potential cause for a particular period of time. It was also held, however, that where two causes operated cumulatively in causing (or increasing the risk of) disease, the correct test was whether the culpable exposure caused a material contribution to the risk of the disease. This was less stringent than a requirement that the culpable exposure doubled the risk from other causes.
Lord Phillips indicated that where there has been a material increase in risk in relation to a divisible claim, such as asbestosis, the defendant would be liable in respect of the share of the disease for which it was responsible. In respect of indivisible diseases, such as lung cancer, Lord Phillips indicated that a defendant which has tortiously contributed to the cause of the disease will be responsible in full.
The decision of the Supreme Court as it relates to mesothelioma has provided clarity, even if it has been reached with some misgivings as to the fairness of the outcome. It seems that any culpable exposure to asbestos, if it can be proved, is highly likely to give rise to a potential mesothelioma claim. This is not limited to occupational exposure. Going forward, it may well be that mesothelioma claims will not just be brought against employers involved in the usual defendant activities of running shipyards or metal works. Claims could be brought against a variety of employers or others who have been responsible for exposing individuals to asbestos. This will be of potential interest to Employers' Liability insurers and Public Liability insurers alike.
The position in respect of diseases other than mesothelioma which are caused by the cumulative effect of exposure is left less clear. This is particularly the case in respect of indivisible diseases such as lung cancer. The 'double the risks' test to which Smith LJ referred in the Court of Appeal cannot be relied on. However, the position that remains is potentially confusing. Lord Phillips clearly indicated that defendants responsible for exposure which has materially contributed to the risk of indivisible disease will be liable in full. This is effectively the same position as in relation to mesothelioma claims. This does not, however, sit comfortably with Lord Brown's insistence that "mesothelioma cases are in a category all their own". It also seems to ignore that Section 3 of the Compensation Act 2006, along with Fairchild and Barker, expressly apply only to mesothelioma claims. It remains to be seen to what extent this leads to uncertainty in future cases. In the meantime, insurers with exposure to lung cancer losses may be advised to watch this space.