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Comparing two competing theories on causation

Ayannuga & Ors v One Shot Products Ltd [2022] EWHC 590


On the evening of 1 January 2015, two men tried to unblock a kitchen sink using One Shot drain cleaner. Within minutes, one was dead and the other had suffered a catastrophic brain injury as a result of hydrogen sulphide poisoning.

Mrs Justice Yip DBE was called upon to decide which of two apparently improbable explanations for the accident – each depending on “incredible coincidence” and “terrible timing” – was more likely to be right. There was no dispute that the damage was caused by the inhalation of hydrogen sulphide, and therefore there were two essential issues to resolve:

  1. Did the use of One Shot materially contribute to the formation of the gas?
  2. If so, did that mean that in the circumstances of the case One Shot was defective for the purpose of the Consumer Protection Act 1987?

Held, dismissing the claimants’ case, that the produced was not deemed to be defective under the Act and it was not the cause of the disastrous events. While acknowledging that the outcome was likely to be bitterly disappointing for the men’s families, Yip J found that the use of One Shot that evening was mere coincidence.

Background facts

The accident occurred in the fourth floor flat where Mr and Mrs Ayannuga and their three children lived. The kitchen sink became blocked over the Christmas and New Year period. Mrs Ayannuga tried to unblock it with a plunger. When that was unsuccessful, they called in a friend of the family, Mr Gbadegeshin, who did some plumbing and general handywork.

Mr Gbadegeshin brought a bottle of One Shot with him, an instant drain cleaner which is 91% sulphuric acid. It has been produced for 30 years and in that time over 16 million bottles have been sold. Although a licence is now required to buy it, at the time, it was readily available to consumers.

It was uncontentious that Mr Gbadegeshin’s work on the plumbing included pouring One Shot down the plughole and removing a piece of waste pipe. While working in the kitchen, Mr Gbadegeshin was overcome by hydrogen sulphide gas, collapsed and died. He was pronounced dead at the scene. Mr Ayannuga went to his aid. He too was overcome and was rendered unconscious. He remains in a persistent vegetative state. Mrs Ayannuga saw her husband and their friend lying unconscious in the kitchen, came in to help and also lost consciousness. Her son, then just 12 years old, spoke to the emergency services and then led his two younger slings to safety.

At the inquest into Mr Gbadegeshin’s death, the Assistant Coroner recorded that the source of the gas was the use of One Shot, but stated that she could not be satisfied of how the use of the product  resulted in the release of the gas:

Indeed there is a complete lack of evidence regarding the latter. There is an evidential gap in this case which I must not seek to fill with speculation.

It was this evidential gap which Yip J had to try to resolve at trial.

Harry Lambert, instructed by Leigh Day, represented the family at the Inquest.

The issues at trial

The claim was brought solely under the Consumer Protection Act 1987 (“the Act”). Section 2(1) of the Act imposes strict liability on the producer of a product for any damage “caused wholly or partly by a defect in a product.” Section 3(1) provides that there is a defect in a product for the Act’s purposes if “the safety of the product is not such as persons generally are entitled to expect.

As set out above, the two key issues were (1) whether the use of One Shot materially contributed to the formation of the gas, and if so, (2) whether One Shot was defective within the meaning of the Act.

The defendant advanced a positive case that there was an alternative cause, namely the escape of sewer gas which was unrelated to the use of One Shot.

Yip J was invited by the defendant to follow the approach adopted by Andrews J in Gee v DePuy International Limited [2018] EWHC 1208, and address whether or not there was a defect first, before turning to the question of a causal connection between defect and damage.

However, in light of there being a fundamental factual dispute as to whether the product in question played any part in the release of the gas which caused the damage, Yip J decided to deal with factual causation first. It was for the claimant to prove its case on the balance of probabilities. Yip J pointed to Graves v Brouwer [2015] EWCA Civ 595 at [24] to [3] for a useful recent summary of the relevant principles, and stated the right approach at paragraph 24:

I must approach the issue of causation, applying common sense and looking at the whole evidential picture. The expert evidence forms part of that evidential picture, but it is just a part and I must have regard to all the evidence in the case. I should also bear in mind any gaps in what is known, and the reasons for those gaps. I note that, at first sight, both sides’ explanations appear improbable. It is always possible that there is an unknown explanation, but the experts have given anxious consideration to what else might have caused the gas and can suggest nothing. I should consider each side’s theory and test it against the evidence. In doing so, I will bear in mind that I am not bound to find one way or another, although the reality in this case may be that analysis of the competing explanations will lead to the answer. Ultimately, having analysed the evidence, I must (as the Court of Appeal in Graves suggest) stand back and ask myself whether I am satisfied that the claimants’ explanation is more likely than not to be right.” (emphasis added)

Did the use of One Shot contribute to the formation of the gas?

The claimants’ case was opened on the basis that that the sulphuric acid in One Shot had reacted with a product containing lime sulphur to produce hydrogen sulphide. Lime sulphur is readily available to consumers, but it has limited uses; it can be found in pet shampoos, products for whitening the dead wood of bonsai trees, and in plant pesticides. It was readily accepted that there was a significant lacuna in the evidence in that there was no explanation as to how or why a product containing lime sulphur was in the flat and/or put down the sink.

The defendant’s theory was that the use of One Shot shortly before Mr Gbadegeshin’s collapse was entirely coincidental and not in any way causative of the accident. As well as using One Shot, Mr Gbadegeshin removed a piece of pipework. In simple terms, the defendant’s case was that this allowed sewer gas from a foul drain system to enter the kitchen. Hydrogen sulphide gas represents an occupational hazard for sewer workers. Although not unheard of, it is extremely rare, and requires a confluence of certain conditions, for it to escape into domestic settings.

Yip J was invited to “compare and contrast” the claimants’ theory with the sewer gas theory. In relation to the former, she weighed up the following considerations (in summary):

  • There was no evidence of the purchase or use of any product containing lime sulphur by the Ayannuga family [39].
  • The theory that lime sulphur might have been poured down the plughole by someone trying to clear the blockage actively contradicted Mrs Ayannuga’s evidence that she and her husband had not used any products to try and unblock the sink [42].
  • It was not “in any way realistic to suggest” that Mr Ayannuga might have bought it in error, thinking it was a drain cleaner [43].
  • The evidence “positively [pointed] away” from Mr Gbadegeshin having done so, since he was a cleaning supervisor and experience at carrying out plumbing tasks [44].

Yip J considered that this explanation required backwards reasoning and overlooking clear evidence. Turning to the defendant’s theory:

  • Although extremely rare, research by counsel for the defendant turned up two reports in scientific journals of hydrogen sulphide poisoning in domestic contexts where there was evidence of attempt to unblock drains.
  • CCTV inspection of the drainage system at Boulter House found it was “not in a satisfactory structural condition” [55].
  • Since the event occurred on a cold winter’s night, the warmer hydrogen sulphide might have been driven up to the fourth floor by convection [64].
  • The defendant’s expert accepted that an incredible coincidence was required for the pipe to be removed at exactly the moment that conditions were right for a concentrated accumulation of hydrogen sulphide to rise up into it [66].

Yip J found this theory “scientifically sound and … consistent with what is thought to have occurred in other tragic and rare circumstances”, albeit incredibly unlucky and not something that would usually happen [73]. As such, the use of One Shot was coincidental. It did not materially contribute to Mr Gbadegeshin’s death or the injuries sustained by any of the claimants.

If so, was One Shot defective?

The finding that One Shot was not the cause of the hydrogen sulphide was sufficient to dispose of the claim, and therefore Yip J dealt with the issue of whether the product was defective quickly.

The company’s Managing Director, Mr Steven Mounce, was criticised in the judgment for displaying a distinct lack of empathy and for having “rejected any possibility that his company’s product might be unsafe without adequate investigation” [12]. However, given the strong unlikelihood of One Shot coming into contact with lime sulphur in normal use, together with the clear hazard markings and the warning not to use it where other chemicals are present, Yip J concluded that the product was not unsafe within the meaning of the Act.


Personal injury litigation provides numerous examples of accidents occurring because of a combination of inherently unlikely events. Here, both explanations for the accident which befell the Ayannuga household were strikingly improbable, and the question was which was less unlikely than the other.

The approach in Gee – namely, that “proof of a causal connection between defect and damage cannot rationally, or even conceptually, be attempted without ascertainment of whether there is a defect, and, if so what that defect might be” – was acknowledged to be correct. But this does not mean that courts are bound to consider the question of whether there was a defect first, before turning to causality. In some circumstances, as here, there will be more logic in adopting the opposite approach.

The judgment can be found here.

Alethea Redfern, Pupil Barrister.


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