Court of Appeal, Causation and the material contribution test – Divisible or Indivisible Disease?

  • Market Insight 29 November 2023 29 November 2023
  • UK & Europe

  • Insurance

The Court of Appeal held, in a claim for Parkinson’s disease where the Claimant was exposed to Trichloroethylene, that the material contribution test for causation applies to indivisible as well as divisible conditions. However, in this case the Court concluded there was insufficient evidence of a material contribution.

Holmes v Poeton Holdings Limited [2023] EWCA Civ 1377

In practice the application of material contribution to indivisible conditions, such as occupational cancers, will be difficult to prove because the causal mechanisms are often unknown or speculative. 


The claimant case was that he was exposed to unsafe levels of Trichloroethylene "TCE" (an organic solvent) whilst working for the defendant between 1982 and 2020 and was diagnosed with Parkinson’s disease in 2014. The judge’s finding at trial of breach of duty was not appealed, and the parties had agreed for the purposes of the case that Parkinson’s Disease was an indivisible condition.

At trial the judge found that TCE is neurotoxic and can act upon the dopaminergic neurons. It was also noted that it is established that damage to those neurons is the mechanism by which Parkinson's disease arises, although the precise aetiology and pathogenesis of the condition is only partially understood and remains the subject of ongoing research. 

Causation at First Instance

The claimant sought to prove causation on the basis of a material contribution in fact and did not attempt to satisfy the "but for" test. The defendant argued that the evidence could only establish an increased risk of developing the disease, which would be insufficient.

The judge found in favour of the claimant on both breach of duty and causation. He found that "the evidence taken together clearly shows that exposure to TCE created a material contribution to the risk of the claimant sustaining injury." He then concluded that on the balance of probabilities the claimant's Parkinson's disease was materially contributed to in fact by his exposure to TCE at the defendant's works.


The defendant appealed against the trial judge’s legal analysis, factual findings and overall conclusions on the issue of causation. Breach of duty was not appealed. 

Material contribution" and the "but for" test of causation

On the central question of the material contribution test and whether it applies in cases of divisible or indivisible disease, the Court of Appeal considered this was “an area that has been bedevilled by apparent inconsistency and imprecision at the highest level on multiple occasions."  

Unsurprisingly, on appeal the defendant submitted that the judge had adopted the wrong test for proving causation on the basis that it was wrong to rely upon a “material contribution” test at all, since according to the balance of the authorities such a test has no application to cases of indivisible injury (and as noted above the parties had agreed for present purposes to accept that Parkinson's disease is an indivisible condition).

The tension in the law, as those familiar with wrestling with causation issues in occupational disease will know, is that the birth of the material contribution test arose from cases in the mid-1950s to early-1970s which grappled with causation in the context of tortious and non-tortious exposures to the same substance (single agency cases), where each exposure made a contribution to the extent of the overall condition.  The familiar “ground zero” for this approach was of course Wardlaw v Bonnington Castings in 1956.  

In Bonnington whilst the Court relied upon material contribution it did not consider apportionment and therefore it was unnecessary to consider divisibility. Apportionment in divisible conditions, and thus apportioned damages, is now well established as we know from Thompson v Smiths Shiprepairers (North Shields) Ltd and Holtby v Brigham & Cowan. The material contribution test, on the other hand, leads inexorably to a 100% liability where the Defendant’s contribution cannot be quantified.  

Lord Justice Stuart-Smith, giving the lead judgment, considered the authorities and held that “it seems clear that the Bonnington principle of "material contribution" should apply to indivisible diseases.” He reached that position, after a careful review of several decades of authority, by holding that the judges in the early cases (such as Bonnington) had effectively treated the conditions concerned as if they were indivisible, even though they would today be clearly understood as divisible. This was the only way the Court concluded that logical order could be brought to the material contribution test, its historical context, and its proper modern application.  

Further, following Bailey v MoD, where the Bonnington principle (material contribution) applies “the claimant does not have to show that the injury would not have happened but for the tortious exposure for which the defendant is responsible.” In other words, it is confirmed that the material contribution test involves a flex, or relaxation, of the strict but for test. However, such a material contribution must be more than de minimis and more than just an increased risk of injury. 

Generic causation

The Court of Appeal addressed causation in two stages. The first stage, Generic Causation, was whether TCE could cause or materially contribute to Parkinson’s disease. The second stage, Individual Causation, was whether in this case the Claimant could demonstrate that his negligent exposure to TCE had caused his condition. 

The Court of Appeal described causation of Parkinson’s disease as “poorly understood.” The generic causation question was “therefore whether exposure to TCE can cause (or materially contribute to the causing of) Parkinson's disease, the mechanism of interest being the destruction of the patient's dopaminergic neurons."

The trial judge’s summary of evidence in support of the hypothesis that exposure to TCE can cause (or materially contribute to the causing of) Parkinson's disease was “weak”. The fact that TCE is neurotoxic and has been shown to damage dopaminergic cells in animals was a central element of the trial judge’s reasoning, albeit the judgment did acknowledge that animal studies must be approached with care.

However the cause-effect relationship was not substantiated by high quality case control studies, there being a deficit in clear research on the issue.  The epidemiology in this case did no more than establish that TCE exposure was a risk factor for Parkinson’s Disease and a plausible mechanism (based on animal studies) – the gap to extrapolate from the animal studies to an established causal link in the context of human occupational exposure could not be bridged on the current literature.

This ground of appeal was therefore upheld - the evidence before the judge did not justify a finding of generic causation. This is both a reminder that in novel cases a qualitative evaluation of the epidemiological and medical literature is crucial and a warning against drawing conclusions from superficially plausible but evidentially weak associations. 

Claimants may need to fall back on showing a more than doubling of risk to navigate this hurdle.  

Individual causation

Whilst generic causation had not been made out, the Court of Appeal did nonetheless address the issue of individual causation. 

It was accepted at trial that the claimant would have been exposed to many environmental factors, though nothing was known about whether, or to what extent, such other environmental factors increased the risk of him developing Parkinson's disease. A majority of cases of Parkinson's disease implicate both genetic and environmental factors (although the role of environmental factors in development of neurodegenerative diseases generally is the subject of dispute). Further numerous genetic risk factors had been identified but the claimant had not been tested. 

The Court of Appeal found there was insufficient evidence that the defendant’s tortious exposure of the claimant to TCE has caused or materially contributed to his disease:

“… no evidence was identified by the judge (and no evidence has been drawn to our attention) to substantiate a causative link between the tortious exposure and damage to Mr Holmes’ dopaminergic neurons. The difficulty this creates comes into sharp focus when one returns to the question: have Poeton’s breaches of duty caused or materially contributed to the development of Mr Holmes’ disease?...”

It is pleasing to see that the Court of Appeal was able to recognise the applicability of Wilsher v Essex Area Health Authority in this context (in other words, in multiple agency cases, to step back and assess whether it can be shown that tortious exposure contributed in fact, where it is merely one of a number of possible causes). Further, the Court of Appeal recognised the inherent difficulty, in cases where the causal mechanism is not fully understood, in assessing whether a particular occupational exposure might cross the threshold of materiality in terms of any contribution in fact. 

This analysis will be of keen interest in the context of the current sport-related neurodegenerative disease claims.


Stuart-Smith LJ therefore allowed the defendant’s appeal.

Lord Justice Underhill agreed stating that it was not open to the trial judge, on the evidence, to conclude “that such exposure as the claimant had experienced to TCE made a material contribution to his developing Parkinson's disease.” 

In addition, the “evidence about a possible causative link between TCE exposure and Parkinson's disease amounts to no more than the identification of a plausible mechanism and that that by itself is not enough.

What we can learn

  • The Court of Appeal was clear that whatever the origins of the material contribution test it was intended to address what we now accept to be indivisible conditions. The Court appeared to suggest that the logical basis of the position in relation to divisible conditions is less clear. We are considering the potential implications of this. 
  • However in practice the application of material contribution to indivisible disease, such as occupational cancers, will be difficult to prove because the causal mechanisms are often unknown or speculative. In such cases claimants will not be able to satisfy generic or individual causation - for example in cases where science is unable to establish the causal mechanism beyond an increase in risk, such as with most occupational cancers. 
  • Regarding epidemiological evidence for Parkinson’s and the link with pesticides and solvents, it was noted that “high-quality case-control studies have not substantiated the cause-effect relationship” and that “results from the studies that have been carried out have been inconsistent.” This is a reminder that the existence of some scientific support in the literature for a risk or association and a plausible theory for a causal link does not, in the absence of high quality case control studies, necessarily enable a claimant to cross the threshold required for causation. Whilst TCE is known to be a compound of interest “the evidence to prove generic causation is lacking whether one is applying the legal or a scientific standard of proof.”
  • The issue of apportionment in indivisible conditions was side-stepped, as it was not strictly necessary to decide given the outcome, but the point was left open for argument in later cases.  Underhill LJ stated that in an indivisible injury, a tortfeasor who makes a material contribution to the injury is liable for the whole. However, where such a material contribution can be quantified, our view is that the authorities show that apportionment should still occur. 
  • Ultimately, we consider that whilst Holmes clarifies that material contribution applies to indivisible conditions, the requirement to evidence a causal contribution places significant limits on its effect in practice. We consider it is unlikely that this development will result in a significant increase in successful claims, nor disturb in practice the analysis already required in considering the but for/doubling the risk test in indivisible disease claims. 



Stay up to date with Clyde & Co

Sign up to receive email updates straight to your inbox!