Learning Outcomes
This article examines the rules that govern factual causation in negligence when the claimant’s injury may have more than one cause. It explains when the orthodox ‘but for’ test is applied and why it often fails in multi‑cause or scientifically uncertain scenarios. It distinguishes, with case‑driven clarity, between material contribution to harm (for cumulative injuries) and material increase in risk (a tightly confined exception for single‑trigger diseases such as mesothelioma). It sets out how courts treat divisible and indivisible injuries, including apportionment across defendants, joint and several liability, and contribution claims between tortfeasors. It also analyses consecutive causes and intervening events, assessing when a later occurrence breaks the chain of causation or merely adds further damage. Embedded worked examples connect doctrine to realistic SQE1‑style fact patterns and expose common exam traps, such as confusing increase in risk with loss of chance or misidentifying when Fairchild‑type reasoning applies. The section emphasises precise use of terminology and authorities so candidates can construct clear, well‑structured problem‑question answers on multiple causes, mesothelioma claims, and overlapping tortious and non‑tortious factors.
SQE1 Syllabus
For SQE1, you are required to understand factual causation in negligence where a claimant’s injury may have multiple causes, with a focus on the following syllabus points:
- The 'but for' test for factual causation and the civil burden of proof (balance of probabilities).
- Difficulties with multiple potential causes and scientific uncertainty.
- Material contribution to harm in cumulative injury cases (e.g. industrial disease and clinical negligence).
- Material increase in risk as a narrow exception (e.g. mesothelioma), and its limits.
- Divisible versus indivisible injuries; apportionment and joint and several liability.
- Consecutive causes and intervening events (novus actus interveniens), including Baker v Willoughby and Jobling v Associated Dairies.
- Mesothelioma: Fairchild v Glenhaven Funeral Services, Barker v Corus, Compensation Act 2006 s 3, and the Mesothelioma Act 2014 fund.
- Performance Cars v Abraham and the treatment of successive property damage.
- Contribution between tortfeasors under the Civil Liability (Contribution) Act 1978.
Test Your Knowledge
Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.
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In negligence, what is the standard test for establishing factual causation?
- The 'reasonable foreseeability' test.
- The 'material contribution' test.
- The 'but for' test.
- The 'balance of probabilities' test.
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In which landmark case was the principle that a defendant could be liable for materially increasing the risk of harm established, particularly where scientific certainty on the precise cause was lacking?
- Barnett v Chelsea & Kensington Hospital Management Committee
- Bonnington Castings Ltd v Wardlaw
- McGhee v National Coal Board
- Wilsher v Essex Area Health Authority
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The 'material contribution to harm' test is most applicable in which scenario?
- Where there is only one potential cause of the claimant's harm.
- Where the claimant's harm is caused by exposure to a single, identifiable source of danger attributable to the defendant.
- Where the claimant's harm results from the cumulative effect of multiple factors, at least one of which is due to the defendant's breach.
- Where the defendant's breach only creates a risk of harm, but causation cannot be definitively proven.
Introduction
Establishing causation is the essential third element in negligence, linking breach of duty to damage. In factual causation the question is whether, as a matter of fact, the defendant’s breach caused the claimant’s harm. The orthodox approach is to ask the ‘but for’ question: would the harm have occurred but for the breach? In straightforward cases that question suffices. In complex cases—where harm may be cumulative, where there are multiple potential causal agents, or where science cannot isolate the causal mechanism—the ‘but for’ test may defeat meritorious claims or produce arbitrary outcomes. To avoid injustice, the courts have developed principled alternatives: material contribution to harm and material increase in risk. They have also created solutions for consecutive events, and for splitting responsibility between multiple defendants depending on whether the injury is divisible or not. These rules operate within the civil standard of proof and reflect both doctrinal coherence and policy choices about fair allocation of loss.
Key Term: Factual Causation
The requirement that the claimant must prove, on the balance of probabilities, that the defendant’s breach was a factual cause of the damage suffered. The starting point is the ‘but for’ test.
Factual Causation: The 'But For' Test
The core inquiry is: but for the defendant’s breach of duty, would the claimant have suffered the harm? If the answer is no, the breach is a factual cause. If yes, the claim fails for lack of causation.
Barnett v Chelsea & Kensington Hospital Management Committee illustrates the point. A doctor negligently failed to examine a patient who later died of arsenic poisoning. Medical evidence showed the patient would have died anyway. The breach did not cause the death on the ‘but for’ analysis.
Two features of the ‘but for’ test matter in practice:
- The standard of proof is the balance of probabilities. The claimant must show that their case is more likely than not true. If competing causes each have a similar probability and only one is negligent, the claimant may fail.
- The civil courts do not compensate for a mere loss of a chance in clinical negligence claims. In Hotson v East Berkshire AHA and Gregg v Scott, claims failed where the chance of avoiding harm (or improved survival) was below 50% and the claimant could not prove that the defendant’s breach caused the actual injury on the balance of probabilities.
Limitations of the 'But For' Test
The ‘but for’ test strains in cases with:
- Cumulative injuries where harm builds over time from multiple sources.
- Several distinct potential causes (some negligent, some not) where medical science cannot attribute the particular injury to one cause.
- Diseases with single‑hit triggers where only one fibre or agent may initiate the condition, but its source cannot be proven.
Consider a worker exposed to harmful dust from two sources: ‘guilty dust’ due to the employer’s breach and ‘innocent dust’ arising from the process. Pneumoconiosis is cumulative. It may be impossible to prove that but for the guilty dust, the disease would not have occurred. Strict application of ‘but for’ risks an unjust result.
Multiple Causes: The Material Contribution Exception
To bridge the gap where ‘but for’ fails in cumulative injury cases, the courts allow causation to be established if the defendant’s breach made a material (i.e. more than negligible) contribution to the harm actually suffered.
Material Contribution to Harm
This test applies where the injury is cumulative and the overall ‘dose’ or load causes or aggravates harm. If a defendant’s breach adds a non‑trivial amount to that cumulative process, causation is satisfied—even if the claimant cannot prove the injury would have been avoided but for the breach.
Key Term: Material Contribution to Harm
In cumulative injury cases, if the defendant’s breach made a contribution that is more than de minimis to the damage actually suffered, factual causation is established without requiring ‘but for’ proof.
The leading case is Bonnington Castings Ltd v Wardlaw, where pneumoconiosis resulted from inhaling ‘innocent’ and ‘guilty’ dust. Even though the claimant could not prove ‘but for’ causation, the employer was liable because the breach materially contributed to the total dust inhaled.
Bailey v Ministry of Defence demonstrates the same principle outside industrial disease. There, negligent post‑operative care left the claimant severely weakened; when she aspirated vomit and suffered brain damage, the court found that the negligent weakening made a material contribution to the final harm, even though precise causal apportionment was impossible.
Key Term: Divisible Injury
An injury whose severity can be attributed proportionately to different causes over time (e.g. noise‑induced deafness or asbestosis), allowing apportionment of damages by contribution.Key Term: Indivisible Injury
A single injury where it is not practicable to break down the damage by cause (e.g. a broken bone, or a single episode of brain damage). Each tortfeasor who materially contributes is liable in full to the claimant, subject to contribution between defendants.
Holtby v Brigham & Cowan (Hull) Ltd shows how divisible injuries are treated. The claimant’s asbestosis progressed over multiple employments; damages were apportioned according to the period and degree of exposure attributable to each negligent employer.
Worked Example 1.1
A factory worker develops pneumoconiosis after years of inhaling dust. Some dust comes from a faulty machine due to the employer's negligence ('guilty dust'), while other dust arises from the process ('innocent dust'). Medical evidence indicates the disease is cumulative, caused by total dust inhaled, but cannot isolate the effect of 'guilty' dust alone. Can factual causation be established against the employer?
Answer:
Yes, likely. Applying Bonnington Castings, the claimant need only show that the employer’s breach made a more than trivial contribution to the total dust inhaled and thus to the disease’s development or severity. ‘But for’ proof is not required in cumulative injury cases.
Worked Example 1.2
A patient is negligently managed post‑operatively and is left markedly weakened. She later aspirates vomit and suffers brain damage. Experts cannot quantify precisely how the negligent care contributed to the aspiration event. Does material contribution apply?
Answer:
Yes. Following Bailey v Ministry of Defence, if negligent care materially contributed to the claimant’s weakened condition and that weakness was part of the cumulative causes of the eventual harm, factual causation is satisfied.
Material Increase in Risk
A further refinements addresses cases of scientific uncertainty where harm could be caused by a single exposure and the agent cannot be traced to a particular defendant. In such cases the courts may treat materially increasing the risk of the injury as sufficient for causation.
Key Term: Material Increase in Risk
Where science cannot pinpoint which of several exposures caused a single‑trigger disease, a defendant who materially increased the risk of the claimant contracting that disease may be held causative.
McGhee v National Coal Board illustrates the principle. Lack of washing facilities extended the claimant’s exposure to brick dust; although it could not be proven that the extended exposure caused the dermatitis, the failure materially increased the risk of the disease.
The rule was applied in Fairchild v Glenhaven Funeral Services Ltd to mesothelioma. Exposure to asbestos fibres from multiple employers made it scientifically impossible to identify which exposure initiated the disease. Each employer who materially increased the risk was held liable.
Sienkiewicz v Greif (UK) Ltd confirmed that the Fairchild exception applies even where only one defendant exposed the claimant to asbestos and the main competing exposure was environmental. The question is whether the defendant’s breach materially increased the risk.
Importantly, Parliament adjusted damages for mesothelioma in response to Barker v Corus, which had apportioned damages by contribution. Section 3 of the Compensation Act 2006 now provides for joint and several liability in mesothelioma claims: any responsible defendant can be required to pay 100% of the damages, leaving contribution to be worked out between defendants. Due to tracing difficulties, the Mesothelioma Act 2014 also created a fund of last resort for eligible sufferers exposed after 2012 whose employers or insurers cannot be found.
Worked Example 1.3
A worker develops mesothelioma after negligent asbestos exposure during several employments. It is scientifically impossible to show which fibre caused the disease. One of the employers is insolvent. Can the claimant recover full damages against a solvent employer?
Answer:
Yes. Under Fairchild and the Compensation Act 2006 s 3, each employer who materially increased the risk is liable on a joint and several basis, so the claimant can recover 100% from any solvent defendant. Contribution claims are then dealt with between defendants.
Exam Warning
Do not confuse:
- Material contribution to harm (Bonnington; Bailey) for cumulative injuries where the defendant’s breach adds to the total load causing harm.
- Material increase in risk (McGhee; Fairchild; Sienkiewicz) for single‑trigger conditions where science cannot identify the causal fibre or agent.
Nor should material increase in risk be conflated with loss of chance. Loss of chance claims in clinical negligence (Hotson; Gregg) fail where the claimant cannot prove, on the balance of probabilities, that the breach caused the actual injury.
Scope of the Material Increase in Risk Test
The courts have confined McGhee/Fairchild to situations of genuine scientific uncertainty about the causal mechanism, most notably mesothelioma (and, in limited modern authority, certain asbestos‑related lung cancer claims with proportionate damages). In Wilsher v Essex Area Health Authority a premature baby’s blindness had five possible distinct causes, only one negligent. The House of Lords declined to apply increase‑in‑risk reasoning because there were multiple independent potential causes, rather than a single causal agent whose risk was increased by the breach. The claimant failed for lack of proof on the balance of probabilities that the negligent oxygen caused the harm.
In short:
- Use material contribution where multiple factors cumulatively cause harm.
- Use material increase in risk only where a single agent could cause the harm and science cannot identify which exposure did so.
Multiple Causes: Consecutive Events
Cases often involve an initial injury and a later event affecting the same interest. The key questions are whether the later event is tortious or non‑tortious and whether it breaks the chain of causation.
Key Term: Novus actus interveniens
A new intervening act breaking the chain of causation between the defendant’s breach and the claimant’s loss. If established, the defendant is not liable for damage occurring after the intervening act.
Baker v Willoughby involved an initial negligent leg injury and a later armed robbery leading to amputation. The first defendant remained liable for the ongoing effects of the initial injury; the subsequent tort did not extinguish the disability caused by the first tort, and the robber’s liability was limited to additional damage.
Jobling v Associated Dairies Ltd concerned an initial back injury followed by a naturally occurring spinal disease unrelated to the tort. The onset of the disease was a vicissitude of life that superseded the original tort; the employer’s liability for future loss ended at the point the disease would have disabled the claimant anyway.
Performance Cars v Abraham deals with successive property damage where the loss already existed. A second negligent driver was not liable for the cost of a respray that the Rolls‑Royce already required due to an earlier collision; the second incident did not cause that particular loss.
Worked Example 1.4
Driver A negligently injures Claimant's leg, causing permanent disability and loss of earnings. Three years later, before the negligence claim is settled, Claimant is diagnosed with an unrelated spinal disease which would also have rendered them unable to work from that point onwards. Is Driver A liable for loss of earnings after the onset of the spinal disease?
Answer:
No. Under Jobling v Associated Dairies, the disease is a supervening non‑tortious event that breaks the chain for future loss. Liability for loss of earnings ceases when the disease would have disabled the claimant in any event.
Worked Example 1.5
A luxury car is involved in two rear‑end collisions separated by days. The first collision damages the paintwork so the car needs a respray. The second collision also damages the rear bumper. Can the owner recover the cost of the respray from the second driver?
Answer:
No, not for the respray need. Performance Cars v Abraham holds that the second driver is not liable for a loss that already existed at the time of the second collision. The second driver is liable for any additional damage (e.g. to the bumper) caused by the second collision.
Multiple Tortfeasors and Indivisible Injuries
Where two or more defendants contribute to a single indivisible injury, each is liable to the claimant in full if their negligence materially contributed to the harm. The Civil Liability (Contribution) Act 1978 then governs apportionment of responsibility between defendants according to what is just and equitable.
Key Term: Joint and several liability
Liability to the claimant for the entirety of the damages where multiple defendants have caused the same damage. The claimant can recover 100% from any one defendant; contribution between defendants is resolved separately.
In Fitzgerald v Lane a pedestrian was struck successively by two negligent drivers. The injuries were indivisible; causation was established against each driver. The claimant could recover in full, with apportionment between drivers subsequently determined.
Worked Example 1.6
A pedestrian is struck first by negligent driver A, then immediately afterwards by negligent driver B. The pedestrian suffers severe injuries, but it is impossible to determine which specific injury was caused by which impact. Are both drivers liable?
Answer:
Yes. Fitzgerald v Lane confirms that where negligent acts contribute to a single indivisible injury, each tortfeasor is liable to the claimant in full, subject to apportionment between them under the Civil Liability (Contribution) Act 1978.
Worked Example 1.7
A worker develops asbestosis (divisible) after periods of exposure across three employers. The medical evidence allows estimation of the contribution by length and intensity of exposure. How will damages be assessed?
Answer:
Damages are apportioned. Following Holtby, a divisible injury is split according to each defendant’s material contribution. The claimant must sue all liable employers to recover in full.
Revision Tip
When faced with multiple causes:
- Identify the injury type. Cumulative? Use material contribution to harm. Single‑trigger with scientific uncertainty? Consider material increase in risk in line with Fairchild.
- Determine if the injury is divisible or indivisible. Divisible injuries permit apportionment of damages against multiple defendants; indivisible injuries attract joint and several liability to the claimant.
- Consider whether a subsequent event breaks the chain (novus actus) or merely adds further damage. Distinguish tortious later events (Baker) from natural vicissitudes (Jobling), and pre‑existing losses (Performance Cars).
Key Point Checklist
This article has covered the following key knowledge points:
- Factual causation usually turns on the ‘but for’ test; the claimant must prove causation on the balance of probabilities.
- The ‘but for’ test can fail in cumulative injury and single‑trigger disease cases with multiple exposures.
- Material contribution to harm allows causation to be established where a breach adds a non‑trivial amount to cumulative injury (Bonnington; Bailey).
- Material increase in risk is a narrow exception where a single agent may trigger harm and science cannot identify which exposure caused it (McGhee; Fairchild; Sienkiewicz). It is confined largely to mesothelioma.
- Wilsher distinguishes increase‑in‑risk cases with multiple independent potential causes; the claimant must prove the negligent cause on the balance of probabilities.
- Mesothelioma claims benefit from joint and several liability under the Compensation Act 2006 s 3. Barker’s apportionment was reversed for mesothelioma. The Mesothelioma Act 2014 provides a fund of last resort for qualifying cases post‑2012.
- Divisible injuries (e.g. asbestosis, industrial deafness) are apportioned between tortfeasors by contribution; indivisible injuries attract joint and several liability to the claimant with contribution between defendants governed by the Civil Liability (Contribution) Act 1978.
- Consecutive events: non‑tortious vicissitudes (Jobling) can break the chain for future loss; subsequent torts (Baker) do not extinguish the first tortfeasor’s liability for their original damage. Pre‑existing losses cannot be re‑attributed to later defendants (Performance Cars).
- Do not confuse material increase in risk (a causation rule in narrow circumstances) with loss of chance (which generally fails in clinical negligence).
Key Terms and Concepts
- Factual Causation
- Material Contribution to Harm
- Material Increase in Risk
- Divisible Injury
- Indivisible Injury
- Novus actus interveniens
- Joint and several liability