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Holmes v Poeton Holdings: Indivisibility and causation in the Court of Appeal

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By Will Potts and Daniel Hobson


Published 07 December 2023


In the case of Holmes v Poeton Holdings Limited [2023] EWCA Civ 1377, in which DAC Beachcroft's Head of Disease, William Potts assisted by Daniel Hobson, represented the defendants and it's insurers, the Court of Appeal has handed down a landmark ruling in which provides further clarity on whether Bonnington Castings v Wardlaw applies to indivisible disease cases and if so how the court should approach causation.

The decision of the Court of Appeal is a complex and important one. In this note set out at the beginning a short "quick read" summary of the headline issues followed by a more in-depth analysis for those with an interest in the detail of the case.

Quick read summary

The claimant had contracted Parkinson's Disease (PD) and claimed damages from the defendant, his former employer, alleging that the latter had breached its statutory and common law duties by exposing him, during the course of his employment, to unsafe levels of trichloroethylene (TCE). The claimant succeeded at trial and the defendant appealed alleging, amongst other things, that:

  • The judge had adopted the wrong legal test for establishing causation of what was acknowledged on all sides to be an 'indivisible disease';
  • The judge misunderstood the evidence before him in assessing whether exposures to TCE in excess of occupational exposure limits occurred; and
  • The finding of individual causation in the case was speculative and unsupported by any evidence.

In short, the appeal was allowed. The Court of Appeal, amongst other things, held that while it had been established that Trichloroethylene (TCE) was a risk factor in the development of Parkinson's Disease (PD) the judge's finding that the exposure had made a material contribution to PD was not supported by the expert evidence.

In depth

As can be imagined, the quick read summary set out above sets out merely the headlines in a broad manner while the detail of the case and its implications are significantly more complex and nuanced. Below we set out a more detailed consideration of the case and its implications.


The claimant sought damages from the defendant in respect of his alleged exposure to TCE while employed by them, contending that his exposure was in breach of the duty of cared owed to him and that the exposure was implicated in his subsequent development of PD.

The defendant did not dispute that the claimant had been exposed to TCE during his employment but denied that the exposure was in breach of any duty owed and, further, that there was insufficient evidence to link any exposure that may be found to be in breach of duty to the claimant's development of PD.

The trial

The matter came before His Honour Judge Harrison in the Cardiff County Court in May/June 2022 for a liability only trial, judgment being handed down in September 2022. The judge found in favour of the claimant holding the defendant liable in full for the claimant's development of PD with damages to be assessed.

The defendant appealed, permission being granted by Lane J. in November 2022, the case being "leapfrogged" to the Court of Appeal as it "raises an important point of principle or practice"

The basis of the appeal

The defendant's case was that in coming to the decision that he had, the trial judge had misdirected himself as to the legal test for causation and has therefore allowed the claimant to succeed without needing to make a finding that, but for the exposures to TCE which the judge has found to be in breach of duty, the claimant would not have suffered PD (or would not have contracted it when he did) or that the negligent exposure had led to a doubling of the risk of developing PD.

While the claimant indicated at trial that he was not at this stage suggesting that the case fell under the Fairchild exception, the judge was still prepared, despite there being very limited scientific evidence to support such a causative link, to make findings of fact that the exposure to TCE was sufficient to establish causation on the basis that there was likely to have been damage to his dopamine producing brain cells because of the possible neurotoxic effect of TCE.

The approach to the issue of causation that had been adopted by the trial judge was, the defendant submitted, contrary to Court of Appeal level authority and had wrongly concluded that dicta in two cases in the Court of Appeal to the effect that the “material contribution” test had no application to cases of “indivisible” injury (such as PD) were contrary to House of Lords/Supreme Court case law.

The judge, it was argued, had a twofold task in relation to the issue of causation. First, the judge had to decide whether the claimant had established "generic causation", that is that exposure to TCE could cause PD. If "generic causation" could be established then the judge had to consider whether the particular exposures identified in this case as being in breach of duty did in fact cause the claimant's PD – "individual causation". The defendant submitted that the claim should have failed at the "generic causation" stage but even had this succeeded the evidence did not support a finding that "individual causation" could be established.

In summary, it was argued that the trial judge had made four distinct errors, two of law and two of fact, namely:

  1. He was wrong to use the material contribution test at all in an indivisible injury case such as this;
  2. Even if such test was available that did not remove the need for C to prove that TCE was a cause of his PD;
  3. On the evidence before the judge there was insufficient to show generic causation;
  4. In any event the evidence was insufficient to show individual causation.

The defendants argued that the overall risk factors from TCE related to very high or acute one off exposure and that it was widely used without special precaution. The primary allegation of breach of duty was in respect of lifting heavier items out of the TCE tank that was used to degrease contaminated metal components, cleaning of the tank and use of neat TCE on components. The evidence before the judge had been that there would have been long periods over the course of the claimant's employment where there would have exposure to TCE well below the recommended limits of 100 ppm over an 8 hour working day and a short term limit (from 1984) of 150ppm In addition, it was argued that there had been errors in relation to the estimation of the likely levels of exposure.

The defendants contended that and that the trial judge had not adequately dealt with the issue of whether the claimant's admitted exposure had been tortious or not. In addition, the judge had erroneously relied upon the material contribution test for establishing causation as the test had no application to situations involving indivisible injuries and even if a material contribution test was applicable it was necessary for a claimant to prove that the tortious exposure to TCE for which the employer was responsible had been a "but for" cause of his Parkinson's or that there had been a doubling of the risk

The claimant maintained his argument that the judge was entitled to make the findings that he did. He argued that while there was some inconsistency in the case law, the judge was entitled to make a finding that material contribution to damages applied to indivisible injuries. It was, the claimant argued, a clear case of very high levels of exposure to TCE which were more than de minimis and on that basis generic and individual causation were established. It was important, it was argued, for the court to distinguish between scientific knowledge to support causation and the legal test for causation. Further, it was asserted that in light of some of the judge's findings on exposure levels, any evidential shortcomings should be exercised in the claimant's favour.

The decision of the Court of Appeal

The Court of Appeal held that as far as the material contribution and "but for" causation issues were concerned, the material contribution test as set out in Bonnington Castings Ltd v Wardlaw [1956] A.C. 613, [1956] 3 WLUK 6, applied to indivisible diseases and where the principle applied it was not necessary for the claimant to prove that the injury would not have happened but for the tortious exposure for which the defendant was responsible.

On the question of the levels of exposure, the Court of Appeal noted that no specific findings in relation to the levels and duration of exposure had been made by the trial judge and while mathematical precision could not reasonably be expected, the levels and frequently of exposure would affect the likelihood of the exposure having contributed to a mechanism causing PD. The appeal court said that "the extreme generality of the judge's findings about the duration and levels of exposure has a knock-on effect when considering what findings were open to the judge when he came to the issues of general and individual causation." 

The judge, while describing the levels of exposure in general terms, had made no attempt to identify the extent to which any exposure had not been tortious because it had not fallen outside the accepted safe limits, or what the incremental quantity and effect of the tortious exposures could have been

With regard to the question of general causation the evidence at trial established nothing more than that TCE was a risk factor for PD and that there was a "plausible mechanism by which TCE could contribute to the development of Parkinson's disease". The proof of generic causation was absent and the evidence had not justified a finding of generic causation.

As to individual causation, in the light of the conclusions that had reached about generic causation the claimant/respondent could not establish that the tortious exposure to TCE had caused or materially contributed to his PD unless there were features of his case, not reflected in the generic evidence, that compelled a finding of causation and no such features existed in this case.

In the light of its findings, the Court of Appeal allowed the appeal.


In overturning the erroneous first instance decision, the Court of Appeal has provided much needed clarification as to the applicability of the material contribution test while providing a clear restatement that in any claim, irrespective of the material contribution test, it is necessary for a claimant to establish both generic and individual causation.

In the future, in the light of the views expressed by the Court of Appeal, in indivisible claims where there are multiple possible and/or competing causes, establishing causation will be more problematic for those asserting it.

While the case of Wilsher v Essex Area Health Authority [1988] AC 1074 in which situations where a number of different factors could have caused the condition in question were considered, was not addressed by the Court of Appeal it is submitted that the test set out in that case is likely to remain the test where cases involve competing causes. This was largely due to the court not needing to consider this as the claimant had not met the generic & individual causation threshold. However, Wilsher is still likely be a major consideration where there are a number of competing causes.

The Court of Appeal did not address the question of apportionment and the question of whether apportionment should be attempted on indivisible cases where there are other tortfeasors or causes remains to be determined. However, there is a clear indication that this may need to be determined at a later date by the court. Logically, an apportionment exercise would still be possible.


This is an important decision which provides a good deal of clarity on how the courts should approach causation on cases of indivisible conditions. It is clear that the burden of proof remains with the claimant to establish both generic and individual causation, rather than their exposure was more than de minimis and that tortious exposure must be shown to have played a relevant causative role.

Our Disease Team deals with cases like this on a regular basis. For more information or advice, please contact one of our experts.