What is the difference between material contribution to harm and the 'but for' test?

'But for' asks whether the harm would have occurred without the breach. Material contribution to harm applies when multiple cumulative causes produce a single indivisible injury and it is impossible to say which cause did the damage, allowing liability where the breach made a real, non-trivial contribution to the harm even if it was not the sole cause.

Does material contribution to harm mean the defendant pays for everything?

Not always. Where the harm is divisible, courts may apportion liability proportionately, as in Holtby v Brigham & Cowan. Full liability tends to follow where the harm is truly indivisible and cannot be split between tortious and non-tortious causes.

Is material contribution to harm the same as the Fairchild exception?

No. Bonnington-style material contribution to harm applies where cumulative causes produce a single condition and the breach provably contributed to it. Fairchild is a narrower, policy-based exception for cases like mesothelioma where even material contribution to the disease cannot be shown, only contribution to the risk of it.

Why did the claimant lose in Wilsher if the principle in Bonnington allowed recovery?

In Wilsher there were five separate potential causes of the retinal condition, each capable of causing it independently, and only one was the defendant's breach. That is not the Bonnington scenario of cumulative causes producing one disease. Because any one independent cause might have been responsible, the defendant could not be said to have materially contributed to the harm.

Can I use material contribution to harm in a clinical negligence problem question?

Yes, Bailey v Ministry of Defence confirms the principle applies in clinical negligence. You need the same factual pattern: cumulative causes (negligent care plus a non-tortious medical condition) producing an indivisible harmful condition, with the breach making a real contribution to that condition.

Tort Law

Material Contribution to Harm: The Causation Exception Explained

When 'but for' fails, material contribution to harm may rescue a claimant's case. Here's exactly how it works.

Quill

Caselaw editorial desk · 16 June 2026

Causation breaks more negligence claims than any other element. A claimant can prove duty, breach, and damage, and still lose because they cannot show the defendant's breach actually caused their loss. The standard test, the 'but for' test, asks: but for the defendant's breach, would the claimant have suffered the same damage? If yes, the claim fails. But sometimes that test produces results so harsh they cannot stand. Material contribution to harm is the doctrine that fills some of that gap. Knowing exactly when it applies, and when it does not, is what separates a competent exam answer from a great one.

Why 'But For' Sometimes Fails

The 'but for' test works well when there is a single cause. It struggles when multiple causes are operating simultaneously, some tortious and some not, and science cannot tell you which one did the damage.

The classic scenario: a worker inhales harmful dust from two sources over many years. One source is the defendant employer's breach of duty. The other comes from background exposure or a non-tortious source. By the end of their working life they have lung disease. Medical evidence cannot say whether the tortious dust alone would have caused the disease, or whether the clean dust would have caused it anyway. Applied strictly, 'but for' fails the claimant. They lose even though the defendant unquestionably exposed them to something harmful.

That is the injustice the House of Lords addressed in Bonnington Castings Ltd v Wardlaw [1956] AC 613. It remains the foundation case for material contribution to harm, and you need to know it cold.

Bonnington Castings: The Core Rule

Mr Wardlaw worked in a dressing shop where he was exposed to silica dust from two sources: pneumatic hammers (for which the defendant was not in breach) and swing grinders (for which the defendant was in breach, having failed to maintain adequate dust extraction equipment). He developed pneumoconiosis. Medical evidence could not establish that the dust from the swing grinders alone caused his disease. The 'but for' test therefore seemed to defeat him.

The House of Lords held for the claimant. Lord Reid said a defendant is liable if their breach made a material contribution to the disease. The breach does not need to be the sole cause or even the dominant cause. It needs to have contributed to the harm in a way that is more than negligible or de minimis.

Two things to be precise about here. First, this is material contribution to the harm itself (the disease), not merely to the risk of harm. That distinction matters enormously, as we will see. Second, the principle operates because all the causes were operating cumulatively on the same injury. The tortious and non-tortious dust together produced one disease. It was scientifically impossible to separate out which dust caused which part of the lung damage.

The McGhee Detour and Why It Matters

McGhee v National Coal Board [1972] 3 All ER 1008 caused decades of confusion. Mr McGhee contracted dermatitis after working in brick kilns. The defendant was not in breach for the exposure during working hours, but was in breach for failing to provide shower facilities, meaning McGhee cycled home caked in brick dust. The medical evidence was that the additional exposure after work materially increased the risk of contracting dermatitis, but could not say it made a material contribution to the disease itself.

The House of Lords found for the claimant. Some of their Lordships suggested that materially increasing the risk of harm was equivalent to making a material contribution to it. That statement, if correct, would collapse the distinction between two different doctrines.

The House of Lords in Wilsher v Essex Area Health Authority [1988] AC 1074 made clear that McGhee did not stand for that broad proposition. Wilsher involved a premature baby who suffered a retinal condition. There were five possible causes of the condition, only one of which was the defendant's breach (excess oxygen). The House of Lords held there was no material contribution to the harm because there were multiple distinct potential causes, any one of which might have caused the injury acting alone. This is fundamentally different from Bonnington, where a single indivisible disease was caused by cumulative exposure.

Wilsher is your warning. Material contribution to harm requires a single, indivisible injury caused by cumulative sources. Where there are multiple separate potential causes, any of which might independently have caused the same damage, you cannot aggregate them and call it material contribution.

Divisible vs Indivisible Harm: The Distinction That Defines the Doctrine

This is the most important conceptual line to draw in any exam answer.

Indivisible harm is harm that cannot be parcelled up between causes. Mesothelioma is the paradigm example: you either have it or you do not. There is no such thing as a mild case caused by one exposure and a severe case caused by another. Pneumoconiosis in Bonnington is the same. One disease, multiple contributing causes.

Divisible harm is harm where the severity increases with exposure and each cause contributes a measurable portion. Noise-induced hearing loss is often treated as divisible. If three employers exposed a claimant to harmful noise over different periods, in principle you can apportion the deafness caused by each. Each defendant is liable for their share, not for the whole.

Why does this matter for material contribution to harm? Because the doctrine in Bonnington is designed specifically for indivisible harm. Where harm is divisible, courts apportion liability rather than finding full liability for the whole.

In Holtby v Brigham & Cowan (Hull) Ltd [2000] 3 All ER 421, the Court of Appeal held that where a claimant had been exposed to asbestos dust by multiple employers over many years, the defendant was only liable for the proportion of the harm it had caused. The harm (asbestosis) was treated as divisible according to duration and intensity of exposure. So even within the material contribution framework, you may find proportionate rather than full liability.

Bailey v Ministry of Defence: Extending the Principle

Bailey v Ministry of Defence [2008] EWCA Civ 883 pushed the doctrine into clinical negligence territory, and it is a case you should expect to see in problem questions.

Mrs Bailey underwent a procedure at the defendant's hospital. The defendant was negligent in her post-operative care. She also had pancreatitis (not caused by the defendant). Both the negligent care and the pancreatitis weakened her, leaving her unable to clear secretions from her airway. She aspirated vomit and suffered brain damage. The question was whether the defendant's breach caused the aspiration.

The Court of Appeal held that the breach had materially contributed to the weakness that led to the aspiration. Waller LJ accepted that where a claimant can show that the defendant's breach made a material contribution to a condition (the weakness) that itself caused the damage (the aspiration and brain injury), the 'but for' test is modified in the same way as Bonnington.

This is significant because it extends material contribution beyond industrial disease into clinical negligence. The mechanism is the same: cumulative causes, indivisible condition, impossibility of separating out contributions. The cause of action is different, the factual context is medical, but the legal test is unchanged.

Be careful not to over-read Bailey. It does not mean that any medical negligence case where causation is uncertain gets the material contribution escape hatch. The claimant must still show that the breach made a genuine, non-trivial contribution to an indivisible harmful condition.

Material Contribution to Risk: A Different Doctrine

Do not confuse material contribution to harm with material contribution to risk. They sound similar. They are not the same thing, and conflating them is a common exam error.

Material contribution to risk is the rule from Fairchild v Glenhaven Funeral Services [2002] UKHL 22 and its companion Barker v Corus UK Ltd [2006] UKHL 20. Mesothelioma is caused by a single asbestos fibre. It cannot be said to be a cumulative condition in the same way pneumoconiosis is. Science cannot identify which fibre, from which employer's workplace, triggered the disease. Strict application of Bonnington would not help because there is no evidence each defendant made a material contribution to the disease; they each contributed to the risk.

The House of Lords in Fairchild created a narrow exception: where a claimant can show that each defendant materially increased the risk of a specific disease, and the disease was caused by the same agency (asbestos) from all defendants, liability attaches. This was a policy-driven departure from orthodox causation, explicitly acknowledged as exceptional.

Barker then held that in the Fairchild context, liability should be proportionate (each defendant pays according to the share of risk they created), which Parliament then reversed by statute for mesothelioma specifically through the Compensation Act 2006 s.3, making defendants jointly and severally liable.

For your exam, keep these separate:

Material contribution to harm (Bonnington/Bailey): cumulative causes, indivisible harm, all causes producing the same damage simultaneously or cumulatively. No need for a policy exception; this is just a sensible application of causation.
Material contribution to risk (Fairchild): exceptional doctrine, mesothelioma-specific in practice, where science cannot identify which exposure caused the harm even in principle.

Applying This in a Problem Question

When you see a causation problem, run through this sequence:

Try 'but for' first. Always start here. If it works, use it. Material contribution is a fallback, not a first resort.

Identify why 'but for' fails. Is it because there are multiple simultaneous causes of a single indivisible condition? That points toward Bonnington.

Check the harm is indivisible. If the harm can be apportioned between causes (divisible harm), consider proportionate liability (Holtby) rather than full liability.

Check whether all causes operate on the same damage. If there are multiple potential independent causes, any of which could have caused the damage alone (the Wilsher scenario), material contribution to harm does not rescue the claimant.

Consider whether Fairchild applies. Only in the very specific context of mesothelioma or analogous situations where the House of Lords has said the exception applies. Do not casually extend Fairchild.

State the contribution is more than de minimis. The breach must be material, meaning real and substantial, not trivial.

A Quick Checklist for Material Contribution to Harm

Single indivisible harm? Yes.
Multiple causes, at least one tortious? Yes.
Causes operating cumulatively on the same condition? Yes.
Claimant can prove tortious cause made a real, non-trivial contribution? Yes.
'But for' fails only because of evidential difficulty in separating causes? Yes.

If you can tick all of those, material contribution to harm probably applies.

The Limits Worth Remembering

The courts have been reluctant to expand this doctrine further. Gregg v Scott [2005] UKHL 2 is a useful counterpoint. Mr Gregg had cancer. His GP negligently failed to diagnose it in time. By the time it was properly diagnosed, his statistical chance of survival had fallen below 50%. He claimed for the loss of that chance. The House of Lords refused to allow a claim for loss of a chance in personal injury cases. This was not a material contribution to harm situation because there was no harm the breach had contributed to in the Bonnington sense; the harm was his cancer, and no one suggested the GP's breach caused the cancer.

Gregg sits alongside Wilsher as a reminder that the courts treat these causation exceptions carefully. They are not a general escape from proving causation. A claimant who simply cannot establish on the balance of probabilities that the breach caused the harm, and whose facts do not fit the Bonnington pattern, will still lose.

If you want to read the primary materials on any of these cases rather than relying on secondary summaries, the Caselaw case finder is a practical way to pull up the judgments quickly and check what was actually said.