Causation in negligence - Material increase in risk

Learning Outcomes

This article explores material increase in risk as an exception to the standard 'but for' test for factual causation in negligence, including:

• the factual scenarios where the material increase in risk test is considered, such as multiple potential causes, sequential exposures, and scientific uncertainty;
• the relationship between the material increase in risk test, the orthodox 'but for' test, and the 'material contribution to harm' approach, and how to select the correct test in SQE1 questions;
• the differences between the McGhee/Fairchild line of authority and Bonnington/Bailey cumulative causation cases, with emphasis on exam‑relevant distinctions;
• the strict preconditions and limited scope of the Fairchild exception, including the same causal agent requirement and the role of current scientific knowledge;
• the operation of the Compensation Act 2006, s 3 in mesothelioma claims, its interaction with Barker, and its impact on joint and several liability and apportionment;
• how courts distinguish between divisible injuries (permitting apportionment) and indivisible injuries (attracting full liability), and the consequences for damages calculations;
• the concept of a material (more than de minimis) increase in risk as clarified in Sienkiewicz v Greif, and how to apply this threshold to problem questions;
• common exam pitfalls, such as confusing loss of chance with material increase in risk, or misapplying the Fairchild exception outside mesothelioma.

SQE1 Syllabus

For SQE1, you are required to understand the principles of factual causation, including the exceptions to the standard 'but for' test and to recognise situations where proving direct causation is difficult and where the concept of materially increasing the risk of harm might establish liability, with a focus on the following syllabus points:

• the standard 'but for' test for factual causation and its limitations
• the concept of 'material increase in risk' as an exception to the 'but for' test
• key case law establishing and applying the 'material increase in risk' principle (eg McGhee, Fairchild)
• the specific application of these principles in mesothelioma claims and the effect of the Compensation Act 2006
• distinguishing situations where the 'material increase in risk' test applies from those where the standard 'but for' test or the 'material contribution to harm' test is appropriate.
• the loss of chance approach (Hotson; Gregg) and why it does not replace causation
• apportionment principles for divisible injuries (Holtby) contrasted with joint and several liability under s 3 Compensation Act 2006 for mesothelioma
• the significance of scientific uncertainty and the requirement that exposures are to the same causal agent in Fairchild-type cases
• the materiality threshold: the increase in risk must be more than minimal (Sienkiewicz).

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.

1. What is the standard test for factual causation in negligence?
   1. The reasonable foreseeability test.
   2. The 'but for' test.
   3. The material increase in risk test.
   4. The Caparo three-stage test.
2. In which case was the 'material increase in risk' principle notably applied where multiple employers exposed the claimant to asbestos, leading to mesothelioma?
   1. Barnett v Chelsea and Kensington Hospital Management Committee
   2. Wilsher v Essex Area Health Authority
   3. Fairchild v Glenhaven Funeral Services Ltd
   4. Bolam v Friern Hospital Management Committee

3. True or false? The 'material increase in risk' test applies only when the claimant can prove, on the balance of probabilities, that the defendant's breach directly caused the harm.

4. The Compensation Act 2006 applies specifically to which type of claim involving material increase in risk?
   1. All industrial disease claims.
   2. Claims involving multiple tortfeasors.
   3. Mesothelioma claims caused by asbestos exposure.
   4. Claims involving clinical negligence.

Introduction

Establishing causation is a critical step in proving negligence. The claimant must demonstrate that the defendant's breach of duty actually caused the damage suffered. The primary test for factual causation is the 'but for' test. However, this test can lead to unjust outcomes in certain complex situations, particularly where there are multiple potential causes of harm or where scientific knowledge is limited, making it impossible to prove exactly which factor caused the injury. In response, the courts have developed exceptions, including the 'material increase in risk' principle. This article examines this exception, its development through key case law, and its application, particularly in the context of industrial disease claims.

A firm understanding of causation requires separating causation in fact from causation in law (remoteness). Causation in fact answers whether the defendant’s breach was a factual cause of the loss (usually by the but for test). Causation in law examines whether the loss was a reasonably foreseeable consequence of the breach. The problems discussed here arise at the factual causation stage.

Key Term: alternative causes
A factual causation situation where two or more distinct possible causes could independently explain the claimant’s injury. The claimant must prove on the balance of probabilities that the defendant’s breach was the cause, or rely on an applicable exception.

Key Term: loss of chance
A claim that a defendant’s breach decreased the prospect of a better outcome. In negligence claims for personal injury, loss of chance is generally insufficient; the claimant must establish causation on the balance of probabilities (Hotson; Gregg).

Factual Causation: The 'But For' Test and Its Limits

To establish factual causation, the claimant must typically prove, on the balance of probabilities, that 'but for' the defendant's negligent act or omission, the harm would not have occurred (Barnett v Chelsea and Kensington Hospital Management Committee [1969] 1 QB 428). If the harm would have happened anyway, regardless of the defendant's breach, then the breach is not considered a factual cause of the harm.

The but for test also governs cases where there are alternative potential causes. If a claimant cannot show that the defendant’s negligence, rather than a non-tortious cause, probably produced the harm, the claim fails. For instance, where a premature baby’s blindness could be due to five possible causes and only one is attributable to negligence, the claimant must still prove that the negligent cause was the probable cause (Wilsher v Essex Area Health Authority [1988] AC 1074).

The courts have firmly rejected replacing causation with probabilistic reductions in prospects of recovery. In Hotson v East Berkshire AHA [1987] AC 750, and later Gregg v Scott [2005] 2 AC 176, claims framed as loss of chance failed because the claimants could not show, on the balance of probabilities, that the defendants' negligence caused the injury outcome.

Problems with the 'But For' Test

The 'but for' test encounters difficulties in scenarios involving multiple potential causes, especially where scientific uncertainty makes it impossible to determine which specific cause led to the harm. For example, if a claimant is exposed to a harmful substance by multiple negligent defendants over different periods, but the resulting disease could have been caused by exposure from any single source, the 'but for' test may fail against each defendant individually. This is because it might be impossible to prove that but for any single defendant's negligence, the claimant would not have suffered the harm.

Another area of difficulty arises with cumulative causes, where multiple factors contribute to the harm. In Bonnington Castings Ltd v Wardlaw [1956] AC 613, the court developed the 'material contribution to harm' test, allowing liability where the defendant's breach materially contributed to the damage, even if it wasn't the sole cause. This differs from the 'material increase in risk' principle, which applies where the breach contributed materially to the risk of damage occurring.

Key Term: material contribution to harm
An exception to the but for test where the claimant shows that the defendant’s breach made a material contribution to the actual damage (for cumulative processes), even though other non-tortious factors also contributed.

Key Term: divisible and indivisible injuries
Divisible injuries worsen with further exposure (eg industrial deafness), permitting apportionment of damages. Indivisible injuries (eg mesothelioma) are not sensibly broken down; full liability is imposed once causation is established.

Material Increase in Risk

Where the 'but for' test is inadequate due to multiple potential causes or scientific uncertainty, the courts may apply the 'material increase in risk' test. This exception allows causation to be established if the claimant can prove that the defendant's breach of duty materially increased the risk of the harm occurring.

Key Term: Material increase in risk
An exception to the standard 'but for' test for factual causation. It applies where a defendant's breach of duty significantly increased the chance of the claimant suffering a particular injury, even if it cannot be proven that the breach was the direct cause, particularly in cases of scientific uncertainty or multiple exposures to risk factors.

Key Term: materiality (de minimis threshold)
The increase in risk or contribution must be more than minimal or trivial. A slight or de minimis increase in risk is insufficient; the claimant must prove a material (not negligible) increase (Sienkiewicz v Greif (UK) Ltd [2011] UKSC 10).

Key Term: Fairchild exception
A tailored rule for mesothelioma where defendants who negligently exposed a claimant to asbestos are liable if their breach materially increased the risk of mesothelioma, notwithstanding inability to prove which exposure caused the disease.

Key Term: joint and several liability (mesothelioma)
In mesothelioma claims, any liable defendant can be required to pay the entire damages (subject to contribution claims between defendants), due to s 3 of the Compensation Act 2006.

Development of the Principle

The principle originated significantly in McGhee v National Coal Board [1973] 1 WLR 1 (HL). The claimant, exposed to brick dust during work (non-negligent), was left without showers (negligent), prolonging exposure. Medical science could not say whether the prolonged exposure caused his dermatitis. The House of Lords held that materially increasing the risk by failing to provide washing facilities was sufficient for factual causation. Critical features were that the exposures were to the same causal agent (brick dust) and scientific uncertainty prevented pinpointing which exposure caused the disease.

Worked Example 1.1

Scenario: David worked in a brick kiln for many years. His employer failed to provide showers, meaning he cycled home covered in brick dust daily. He developed dermatitis. Medical evidence could not definitively state whether the dermatitis was caused by the dust exposure during work (non-negligent) or the prolonged exposure due to the lack of washing facilities (negligent), but confirmed the lack of washing facilities materially increased the risk.

Question: Could David establish factual causation against his employer?

Answer:
Yes. Applying the principle from McGhee, although David could not prove 'but for' the lack of showers he would not have contracted dermatitis, the employer's negligent failure to provide washing facilities materially increased the risk of him developing the condition. This was sufficient to establish factual causation.

The Fairchild Exception: Mesothelioma Cases

The application of the material increase in risk principle gained prominence in cases involving mesothelioma, a cancer caused by asbestos exposure, often resulting from contact with multiple negligent employers. The disease’s aetiology creates a special evidential difficulty: it may be triggered by a single asbestos fibre, and medical science cannot determine which exposure did so. Where multiple defendants negligently exposed a claimant to asbestos, the ordinary but for test would defeat each claim individually. To prevent injustice, the House of Lords adopted a rule of liability for any defendant whose breach materially increased the risk.

In Fairchild v Glenhaven Funeral Services Ltd [2003] 1 AC 32 (HL), the House of Lords held that where a claimant suffers mesothelioma due to negligent exposure to asbestos by multiple defendants, but cannot prove which exposure was the specific cause, causation is established against any defendant whose breach materially increased the risk of the claimant contracting the disease. The requirements emphasised by the courts include:

• the injury is mesothelioma
• each defendant breached duty by exposing the claimant to asbestos
• exposures were to the same causal agent (asbestos fibres)
• scientific uncertainty prevents identifying which exposure caused the disease
• the claimant cannot satisfy but for causation due to this uncertainty.

Worked Example 1.2

Scenario: A worker was negligently exposed to asbestos dust by Employer A for 5 years and then by Employer B for 10 years. He later developed mesothelioma, which can be caused by a single asbestos fibre. It was scientifically impossible to determine which employer's asbestos exposure caused the disease.

Question: Can the worker succeed in a claim against either Employer A or Employer B?

Answer:
Yes, against both. Following Fairchild v Glenhaven Funeral Services Ltd [2003] 1 AC 32 (HL), the House of Lords held that where a claimant suffers mesothelioma due to negligent exposure to asbestos by multiple defendants, but cannot prove which exposure was the specific cause, causation is established against any defendant whose breach materially increased the risk of the claimant contracting the disease. Both employers materially increased the risk.

The Supreme Court in Sienkiewicz v Greif (UK) Ltd [2011] UKSC 10 confirmed that material increase in risk suffices even where there is only one negligent tortfeasor and other possible background exposures are non-tortious (eg environmental asbestos). The key remains that the defendant’s breach produced more than a minimal increase in risk.

Worked Example 1.3

Scenario: The claimant worked for a single employer who negligently exposed him to asbestos dust for 12 months, in a town where there was low-level ambient asbestos from a nearby factory. He developed mesothelioma. The employer argues that ambient background exposure explains the disease.

Question: Can the claimant rely on material increase in risk to establish causation?

Answer:
Yes. In Sienkiewicz v Greif (UK) Ltd [2011] UKSC 10, the Supreme Court held that in mesothelioma cases, a defendant’s negligence which causes more than a minimal increase in risk is sufficient for causation, even where other non-tortious exposures may also exist. The claimant must prove the employer’s breach materially (not trivially) increased his risk.

Exam Warning

The Fairchild exception is specifically tailored to address the evidential difficulties in mesothelioma cases due to the nature of the disease and exposure. It represents a significant departure from the traditional 'but for' test and its application is generally restricted. Do not assume it applies broadly to all cases with multiple potential causes. Its preconditions—same causal agent, scientific uncertainty about which exposure caused the disease, and negligent exposure—are critical.

Limitations on the Material Increase in Risk Principle

The courts have been cautious about extending the McGhee/Fairchild principle beyond its specific context. In Wilsher v Essex Area Health Authority [1988] AC 1074 (HL), the claimant, a premature baby, suffered blindness. There were five potential causes, only one of which was tortious (excess oxygen administered negligently). The House of Lords refused to apply the material increase in risk principle, distinguishing it from McGhee. It held that where there are several distinct possible causes, the claimant must still prove, on the balance of probabilities, that the defendant's breach was the cause, rather than simply showing it increased the risk. The defendant's negligence was merely one possible cause among others, not a factor that contributed to a single risk agent (like the dust in McGhee).

While Fairchild applies to mesothelioma, other industrial injury contexts typically turn on material contribution to harm (Bonnington) or orthodox causation. In Bailey v Ministry of Defence [2009] 1 WLR 1052, the Court of Appeal applied material contribution to harm where cumulative factors (including negligent post-operative care) together produced the injury; this is distinct from the material increase in risk approach.

Key Term: state of scientific knowledge (causation)
The evidential context where medical science cannot identify which exposure to the same agent caused the disease. In mesothelioma, this uncertainty justifies the Fairchild exception; it does not generally apply in multi-mechanism scenarios like Wilsher.

Subsequent Developments and the Compensation Act 2006

Following Fairchild, the House of Lords revisited the issue of apportionment of damages in Barker v Corus UK Ltd [2006] 2 AC 572 (HL). The court held that liability under the Fairchild exception should be several, not joint and several, meaning each defendant would only be liable for the proportion of risk they created. This decision caused concern, particularly for claimants where one or more defendants were insolvent.

Parliament intervened by enacting the Compensation Act 2006. Section 3 of this Act specifically reverses the Barker decision only in relation to mesothelioma claims. For such claims, liability is joint and several, reverting to the position established in Fairchild. This means a claimant with mesothelioma can recover full damages from any single negligent employer who materially increased the risk, even if other exposures (including non-negligent ones or periods of self-employment) also contributed to the overall risk. The paying defendant may then seek contribution from other tortfeasors.

Apportionment remains relevant outside mesothelioma. In Holtby v Brigham & Cowan (Hull) Ltd [2000] 3 All ER 421, asbestosis (a divisible disease) was apportioned between employers by exposure time, reflecting each defendant’s contribution to the injury.

Worked Example 1.4

Scenario: Worker X suffers industrial deafness after 10 years with Employer C and 5 years with Employer D. Both negligently exposed him to excessive noise. Worker Y develops mesothelioma after 8 years with Employer E and 4 years with Employer F, both negligent.

Question: How are damages treated in each case?

Answer:
For Worker X, industrial deafness is divisible. Applying Holtby, damages are apportioned to reflect each employer’s contribution (eg by duration/exposure level). For Worker Y, mesothelioma engages the Fairchild exception and s 3 Compensation Act 2006: liability is joint and several. Y can recover full damages from either E or F; contribution between defendants is then addressed under the Civil Liability (Contribution) Act 1978.

Key Term: Civil Liability (Contribution) Act 1978
Statutory mechanism allowing a defendant who has paid damages to seek contribution from other responsible parties, assessed as just and equitable.

Revision Tip

Remember the key distinction introduced by the Compensation Act 2006. For mesothelioma claims caused by negligent asbestos exposure, liability is joint and several (Fairchild applies; s 3 Compensation Act 2006). For other conditions, even if caused by asbestos, or for situations outside the mesothelioma context where the Fairchild exception might be argued, the common law position (potentially including Barker's proportionate liability or the standard 'but for' test/material contribution to harm) would apply. Always check whether the injury is divisible and whether the exposures were to the same agent with scientific uncertainty about which exposure caused the disease.

Conclusion

While the 'but for' test remains the primary method for establishing factual causation in negligence, the 'material increase in risk' principle serves as a key exception in specific circumstances. Developed through cases like McGhee and significantly applied in Fairchild for mesothelioma claims, it addresses evidential difficulties arising from multiple potential causes or scientific uncertainty about which exposure to the same agent caused the disease. Understanding its scope, limitations (as highlighted in Wilsher), the clarification in Sienkiewicz on materiality, and the specific statutory modification for mesothelioma claims under the Compensation Act 2006 is essential for addressing complex causation issues in negligence for the SQE1 assessment. Keep distinct the separate doctrine of material contribution to harm (Bonnington; Bailey), which operates in cumulative causation contexts outside mesothelioma.

Key Point Checklist

This article has covered the following key knowledge points:

• Factual causation requires the claimant to prove the defendant's breach caused the damage, typically using the 'but for' test.
• The 'but for' test can be problematic where there are multiple potential causes or scientific uncertainty.
• The 'material increase in risk' test is an exception, allowing causation if the defendant's breach significantly increased the risk of harm (McGhee).
• This exception was notably applied in Fairchild to mesothelioma cases involving multiple negligent exposures to asbestos.
• Sienkiewicz confirmed that a single negligent tortfeasor’s more-than-minimal increase in risk suffices for mesothelioma.
• The Fairchild exception is generally limited in scope and was distinguished in cases like Wilsher (multiple distinct possible causes).
• The Compensation Act 2006, s 3, reversed Barker v Corus for mesothelioma claims only, restoring joint and several liability.
• For mesothelioma claims, liability can be established against any defendant whose breach materially increased the risk, even if other non-negligent exposures occurred.
• Outside mesothelioma, cumulative causation may engage material contribution to harm (Bonnington; Bailey) rather than material increase in risk.
• Apportionment applies to divisible injuries (eg Holtby), whereas mesothelioma engages joint and several liability due to s 3 Compensation Act 2006.
• Loss of chance is generally insufficient in personal injury negligence; causation must be proved on the balance of probabilities.

Key Terms and Concepts

• Material increase in risk
• Material contribution to harm
• Fairchild exception
• joint and several liability (mesothelioma)
• divisible and indivisible injuries
• materiality (de minimis threshold)
• alternative causes
• loss of chance
• state of scientific knowledge (causation)
• Civil Liability (Contribution) Act 1978