The Supreme Court has handed down its decision in Sienkiewicz v Greif. The UK's highest appeal court was reviewing the application of the "Fairchild exception" to single rather than multiple exposure cases. We reported on the Court of Appeal decision here.

In 2002, the House of Lords' decision in Fairchild v Glenhaven Funeral Services introduced an "exception" to the normal rule of causation. A victim would normally have to prove that on a balance of probabilities a defendant's negligence caused a disease; this is the "but for" test. Mesothelioma is for all practical purposes caused only by exposure to asbestos fibres. However, in circumstances where a victim has been tortiously exposed by different parties (multiple exposures), it is impossible to identify which exposure or exposures in fact caused the mesothelioma. In Fairchild, the Court relaxed the rule of causation. That relaxation was developed in the subsequent House of Lords decision in Barker v Corus and by the introduction of section 3 of the Compensation Act 2006. The Supreme Court set out the Fairchild exception in its current form: when a victim contracts mesothelioma each person who has, in breach of duty, been responsible for exposing the victim to a significant quantity of asbestos dust and thus creating a "material increase in risk" of the victim contracting the disease will be held to be jointly and severally liable for causing the disease. In broad terms, the victim can succeed by proving that a party's negligence might have caused the disease.

In Sienkiewicz, the Court looked at two cases where there had been only a single negligent occupational exposure; and that exposure had been relatively "small". There had also been "environmental exposure", a "low-level exposure to asbestos in the general atmosphere" for which no one is responsible. The Supreme Court held that the Fairchild exception applied in such single exposure cases. It was not neccesary for the victim to prove that the tortious exposure more that doubled the existing risk. The "doubles the risk" test is usually applied to epidemiological evidence.

The victims merely had to show that the tortious exposure created a material increase in the risk. The Court made it clear that a low threshold would apply. In his leading judgment, Lord Phillips said: "The reality is that, in the current state of knowledge about the disease, 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 victim's risk of contracting the disease will be where that exposure was insignificant compared to the exposure from other sources". The Court referred to the expert evidence that there is no known lower threshold of exposure to asbestos that is capable of causing mesothelioma.

The Court found the Court of Appeal was wrong in its view that section 3 of the Compensation Act created a new statutory tort of materially increasing the risk of developing mesothelioma. That legislation had been misread by the Court of Appeal. Whether and in what circumstances liability in tort attaches to a party who has materially increased that risk remains a question of common law. The common law in this area is capable of further development.