Causation of indivisible diseases – material contribution test applies, says Court of Appeal

In Holmes v Poeton Holdings Ltd [2023] EWCA Civ 1377, the Court of Appeal has attempted to bring clarity to the notoriously slippery legal concept of “material contribution”.

The judgment reviews the leading cases in some detail and provides clear guidance on an area which, the court observed, has been “bedevilled by apparent inconsistency and imprecision at the highest level on multiple occasions.” (para. 30).

The case concerned an individual who alleged that his Parkinson’s Disease was caused, at least in part, by exposure to the organic solvent Trichloroethylene (TCE) at work over a number of years. It was agreed by the parties that Parkinson’s was an indivisible disease. The claimant succeeded at trial on both breach of duty and causation. The judge accepted the claimant’s submission that the exposure had made a “material contribution” to the disease, not merely to the risk of the disease eventuating. On that basis, the judge concluded, causation was established.

The finding on breach was uncontested, but the defendant appealed in relation to causation.

The defendant argued that the material contribution test did not apply in cases of indivisible injury and that the available scientific evidence was insufficient to prove either that such exposure was capable of causing Parkinson’s Disease in general (“generic causation”) or that it had in fact been a cause of the claimant’s Parkinson’s Disease specifically (“individual causation”).

The Court of Appeal found that the test of material contribution could be applied to both indivisible and divisible diseases. Indeed, that this had been the position since Bonnington Castings v Wardlaw [1956] AC 613, but that there had been a misunderstanding of the House of Lords’ reasoning in Bonnington in certain subsequent cases, creating confusion.  

However, in Holmes, the available scientific evidence did not prove that exposure to TCE was anything more than a risk factor for Parkinson’s, and so the claim failed on causation.

Divisible and Indivisible diseases

A divisible disease is one which, once contracted, is made worse by further exposure to the harmful agent. Examples include asbestosis and Noise Induced Hearing Loss. They are said to be “dose related.”

An indivisible disease, once contracted, is not made worse by further exposure, although the likelihood of onset may be related to the level of exposure. The extent of injury is not dose related. Examples include mesothelioma and lung cancer. An indivisible disease may have a single cause or be the result of cumulative causes.

Both assessments are, of course, made on the basis of the prevailing scientific understanding of the cause and development of the diseases in question.

Causation of injury – “but for” test and material contribution

The starting point for determining whether the defendant’s breach of duty caused the injury for the purposes of establishing liability is the “but for” test. The claimant must prove that, “but for” the defendant’s breach, the injury would not have been sustained.

That standard test is modified to cope with injuries where there is more than one cause, or where the current state of scientific understanding is such that it is impossible to say which of a range of potential causes was responsible. That challenge is typically encountered in disease and clinical negligence litigation.

It is generally not necessary to prove that the defendant’s breach was the sole cause of the injury. If medical science cannot establish the extent of the contribution made by the breach, it is sufficient to prove that it was a “material” cause.

Where the disease is indivisible, the defendant whose tortious conduct is found to have made a material contribution to the onset of the disease will be liable for the full extent of that disease. Where it is divisible, the defendant will only be liable for that share of the disease its failure was responsible for (caused).

What the Court of Appeal found, then, is that although an indivisible disease is not made worse by further exposure, its onset may be the result of an accumulation of causes. Each will be “a cause” of the indivisible disease and, if the defendant’s breach was a material cause, liability will be established.  The challenge for the claimant will be to prove, on the balance of probabilities, that the defendant’s breach was a material cause of the disease.

Implications of the Court of Appeal’s findings

In concluding that the test of material contribution does indeed apply in cases of indivisible injury, the Court noted it was not necessary, in such cases, for the claimant to prove that the injury would not have happened “but for” the tortious exposure. The standard “but for” test is therefore modified in cases of indivisible injury very much in favour of claimants.

In reaching that conclusion, the Court rejected an earlier Court of Appeal judgment, AB v Ministry of Defence [2010] EWCA Civ 1317, regularly relied on by defendants, which was clear in its conclusion that “the principle applies only where the disease or condition is “divisible” so that an increased dose of the harmful agent worsens the disease” (para 130).

It seems extremely likely that this difficult issue will eventually find its way to the Supreme Court, whether in this case or another. The reasoning in Poeton does not seem immune from reasonable challenge and a definitive decision in the Supreme Court would be welcome.

As was clear in Poeton, winning the argument on material contribution doesn’t get the claimant home; they will still need to prove that the breach was, as a matter of fact, a cause of the injury on the balance of probabilities. This will often pose a considerable evidential challenge. 

It is essential that medical experts are given clear instructions. Material contribution is a legal concept often not well understood by even very experienced experts, who might misidentify exposures which (merely) increase the risk of injury as material contributions to the injury. Opinions on the issue of divisibility/indivisibility and material contribution to injury will need to be teased out with care.