The material contribution test and indivisible injury
In Holmes -v- Poeton Holdings Ltd  EWCA Civ 1377 Court of Appeal considered whether the rule in Bonnington Castings v Wardlaw  1 AC 613 applies to cases of “indivisible” injury. They held that it does, resolving a conflict of decisions at first instance and of dicta at higher level. Therefore a wrongdoer whose tort has made a “material contribution” to a claimant’s injury can be liable for all the damage done if that injury is properly characterised as “indivisible” (a question on which they also give some guidance) even if “but for” the breach of duty the injury or illness might still have occurred. However this is only a partial exception to the “but for” test. First the claimant must prove “generic causation”, i.e. that the insult caused by the breach—in this case exposure to a solvent used for metal degreasing—was capable of causing the injury or illness complained of; if it is, it is then necessary for the claimant to prove “individual causation”, that the insult created by the breach of duty has in fact made a contribution to their injury or illness that is more than minimal. A Claimant arguing material contribution to injury or illness is not released from the burden of proving by evidence that a tortious exposure did play a part in the onset.
In many types of indivisible injury such as cancers – or in this case Parkinson’s disease – proof of both generic and individual causation may be problematic: it is not enough to show that the exposure in breach of duty made more than minimal addition to the overall exposure. In cases of divisible injury (for example asbestosis) it is normally straightforward to prove that each exposure to the toxic chemical has in fact made a material contribution by causing additional damage even though the court may be required to approach the necessary apportionment exercise in a broad brush way. In true indivisible injury cases even if generic causation is admitted or proved, proof of individual causation is likely to be problematic, requiring proof of the mechanism of the injury or disease, potentially at a cellular level. Indeed in conditions such as cancers or, as in this case, Parkinson’s disease where there may be many other potential causes (some unknown), a claimant is likely to find the task impossible unless he or she can show that the breach of duty more than doubled the risk of the injury occurring or the case falls within the Fairchild rule (not available in a case where there may be a number of competing alternative potential causes when the case is caught by the rule in Wilsher v Essex AHA  1074). This decision is therefore likely to have implications going well beyond the toxic chemical exposure cases.
The CA explicitly left open for another case the question of the possibility of apportionment in indivisible injury cases, noting the line of authority leading up to BAE Systems (Operations) Ltd v Konczak  ICR 1 where in a wide-ranging judgment Underhill LJ (who also gave one of the judgments in Holmes v Poeton) seemed to permit of a category of indivisible injury where, if the facts and the evidence permit, apportionment should be attempted between tortfeasors, causes and none.