Introduction
In the law of negligence, a claimant must establish that the defendant owed them a duty of care, that the defendant breached that duty, and that this breach caused the claimant to suffer damage. This causal link is a fundamental element of the claim and is traditionally broken down into two components: factual causation and legal causation. Factual causation is the initial inquiry, concerned with establishing a physical connection between the defendant’s breach of duty and the claimant’s harm. Legal causation, or remoteness, acts as a subsequent filter to determine whether the law ought to attribute responsibility for that harm to the defendant. This essay will focus on the principles of factual causation in English law. It will begin by explaining the primary test, the ‘but for’ test, before examining the exceptions and alternative approaches developed by the courts in cases where the application of this test would lead to an unjust result. Through an analysis of key cases, this essay will show that while the ‘but for’ test remains the starting point, the judiciary has demonstrated a willingness to adapt the rules of causation to deal with evidential uncertainty.
The 'But For' Test
The standard test for establishing factual causation is the 'but for' test. The question the court must ask is: 'but for the defendant's negligent act or omission, would the claimant have suffered the harm?' (Horsey and Rackley, 2021). If the claimant would have suffered the harm regardless of the defendant's negligence, then factual causation is not established, and the claim will fail. The burden of proof is on the claimant to establish on the balance of probabilities (i.e., that it is more likely than not) that the defendant’s breach caused their loss.
The foundational authority for this principle is Barnett v Chelsea & Kensington Hospital Management Committee [1969] 1 QB 428. In this case, three night-watchmen went to the defendant’s hospital emergency department after becoming ill from drinking tea which, unbeknownst to them, had been poisoned with arsenic. The doctor on duty, who was also feeling unwell, negligently refused to see or examine the men and sent them home, advising them to see their own doctors. One of the men, Mr Barnett, died several hours later from arsenic poisoning. His widow brought a claim against the hospital for negligence. The court found that the hospital had breached its duty of care by failing to examine Mr Barnett. However, the claim ultimately failed on the issue of causation. Medical evidence showed that even if Mr Barnett had been examined and admitted for treatment, it was highly probable that he would have died anyway, as the poison was already too far advanced in his system and there was no effective antidote. Applying the 'but for' test, the court concluded that the doctor’s negligence was not a cause of the death. But for the breach, Mr Barnett would still have died. This case provides a clear illustration of the uncompromising nature of the 'but for' test; even where negligence is proven, a claim cannot succeed without a direct causal link to the harm suffered.
Material Contribution to Injury
The 'but for' test can become problematic in situations where there are multiple potential causes of a claimant's harm, and it is scientifically difficult to prove which one was the operative cause. This is particularly true in cases of industrial disease where a condition may develop over a long period due to cumulative exposure to harmful substances from different sources. In these circumstances, the courts have developed an alternative approach, relaxing the strictness of the 'but for' test.
In Bonnington Castings Ltd v Wardlaw [1956] AC 613, the claimant contracted pneumoconiosis after being exposed to silica dust during his employment with the defendant. The dust came from two sources: a pneumatic hammer, for which the employer was not negligent, and swing grinders, where the employer had negligently failed to maintain the dust-extraction equipment. The claimant was unable to prove on the balance of probabilities that 'but for' the negligent dust from the grinders, he would not have contracted the disease. The disease was cumulative, caused by the build-up of all the inhaled dust. The House of Lords held that the claimant did not need to prove that the negligent dust was the sole or even the main cause of his illness. It was sufficient for him to show that the defendant’s breach of duty had made a 'material contribution' to the injury. As the dust from the grinders was not negligible and had contributed to the claimant's condition, the defendant was held fully liable for the resulting disease. This decision established that where a breach of duty can be shown to have materially contributed to an indivisible injury, causation can be established without satisfying the traditional 'but for' test.
Material Increase in Risk
A further relaxation of the causation rules occurred in cases where it was not even possible to establish a material contribution to the injury itself. In certain situations, the courts have been prepared to find causation established if the claimant can prove that the defendant’s negligence materially increased the risk of the injury occurring.
The leading early case is McGhee v National Coal Board [1973] 1 WLR 1. The claimant worked in a brick kiln and was exposed to brick dust. This exposure was not, in itself, a breach of duty. However, the employer was in breach of duty for failing to provide washing facilities, meaning the claimant had to cycle home covered in the dust. He subsequently contracted dermatitis. The medical evidence was unable to determine whether the dermatitis was caused by the initial non-negligent exposure during the working day or the extended, negligent exposure from the lack of showers. It was impossible to prove on the balance of probabilities that 'but for' the lack of showers, the claimant would have avoided the disease. However, the House of Lords held that it was sufficient for the claimant to show that the defendant’s breach had materially increased the risk of him contracting the disease. Lord Wilberforce stated that in such a situation, it was sensible to infer that the breach which had materially increased the risk had, in fact, caused the harm.
This principle was controversial and for many years was thought to be confined to its own facts. However, it was resurrected and applied in the landmark case of Fairchild v Glenhaven Funeral Services Ltd [2002] UKHL 22. This case concerned several claimants who had contracted mesothelioma, a fatal lung cancer caused by exposure to asbestos. A key feature of the disease is that it can be triggered by the inhalation of a single asbestos fibre, but it is scientifically impossible to determine which fibre from which exposure caused the illness. The claimants had all been exposed to asbestos dust while working for different employers at different times. They were therefore unable to satisfy the 'but for' test against any single employer. The House of Lords, in a unanimous decision, held that the claimants could succeed. They created a special exception to normal causation principles based on the reasoning in McGhee. Where a claimant had been negligently exposed to asbestos by multiple defendants and had contracted mesothelioma, each defendant who had made a material contribution to the risk of the claimant contracting the disease would be held liable. The court acknowledged this was a departure from traditional principles, motivated by considerations of policy and justice to ensure the victims of an industrial disease were not left without a remedy due to scientific uncertainty.
The scope of the Fairchild exception was later considered in Barker v Corus UK Ltd [2006] UKHL 20, where the House of Lords held that liability in such cases should be apportioned between the different defendants according to the share of the risk each had created. This decision was viewed as unfavourable to claimants, who would have to sue every responsible party to recover full compensation. In a swift response, Parliament effectively reversed the ruling in relation to mesothelioma claims by passing section 3 of the Compensation Act 2006, which restores the principle of joint and several liability. This means a claimant can recover their full damages from any single negligent employer, who can then seek a contribution from others.
Conclusion
The cases on factual causation demonstrate a clear doctrinal journey. The law begins with the simple, logical, and often strict 'but for' test, as exemplified in Barnett. This test remains the default position and serves to filter out claims where no factual link exists between the breach and the harm. However, the courts have recognised its limitations, particularly in the context of scientific uncertainty and cumulative or multiple potential causes of harm. In response, a more flexible approach has been developed. The 'material contribution to injury' test from Bonnington Castings allows for liability where a defendant's breach has added to the harm, even if it cannot be proven to be the sole cause. More significantly, the 'material increase in risk' approach, developed in McGhee and confirmed in Fairchild, represents a major judicial policy decision to depart from traditional causation rules to achieve practical justice for claimants in specific, difficult circumstances. While these developments have been described as creating complexity and exceptions, they illustrate the common law's capacity to adapt and respond to challenging factual scenarios, ensuring that the rules of causation do not operate as an unjust barrier to compensation for those harmed by negligence.
References
Barker v Corus UK Ltd [2006] UKHL 20.
Barnett v Chelsea & Kensington Hospital Management Committee [1969] 1 QB 428.
Bonnington Castings Ltd v Wardlaw [1956] AC 613.
Compensation Act 2006.
Fairchild v Glenhaven Funeral Services Ltd [2002] UKHL 22.
Horsey, K. and Rackley, E. (2021) Tort Law. 7th edn. Oxford University Press.
McGhee v National Coal Board [1973] 1 WLR 1.
