Wardlaw v Bonnington Castings Ltd [1956] UKHL 1 (01 March 1956)

A foundry worker contracted pneumoconiosis from silica dust. Some dust emanated from a non-negligent source, but some from grinders where his employer breached a statutory duty. The court held the employer liable as the 'guilty' dust materially contributed to the disease.

Facts

The appellant (the pursuer, Mr Wardlaw) was employed for eight years as a steel dresser by the respondents (the defenders, Bonnington Castings Ltd). During his employment, he was exposed to silica dust and contracted pneumoconiosis. The dust came from two sources. The primary source was pneumatic hammers used to chip sand from steel castings, for which no breach of duty by the employer was established (‘innocent dust’). A secondary source was swing grinders which were not fitted with adequate dust-extraction equipment, in breach of Regulation 1 of the Grinding of Metals (Miscellaneous Industries) Regulations, 1925 (‘guilty dust’). It was impossible to prove what proportion of the inhaled dust came from each source, and thus impossible to prove that the ‘guilty’ dust was the sole cause of the disease.

Issues

The central legal issue concerned the standard of proof for causation in circumstances with multiple potential causes. The question for the House of Lords was whether the pursuer had to prove on a balance of probabilities that the ‘guilty’ dust, resulting from the breach of statutory duty, was the whole or substantial cause of his pneumoconiosis, or whether it was sufficient to prove that it had materially contributed to the disease.

Judgment

The House of Lords unanimously allowed the appeal, finding in favour of the pursuer. The court held that the pursuer did not need to prove that the ‘guilty’ dust was the sole cause of his injury. The correct legal test was whether the defenders’ breach of duty made a material contribution to the pursuer’s condition. Lord Reid clarified the standard for a ‘material contribution’:

It appears to me that the source of his disease was the dust from both sources, and the real question is whether the dust from the swing grinders made a material contribution to the disease. What is a material contribution must be a question of degree. A contribution which comes within the exception de minimis non curat lex is not material, but I think that any contribution which does not fall within that exception must be material. I do not see how there can be something too large to be ignored but not large enough to be material.

The court reasoned that since the dust from the defective swing grinders was not negligible, it necessarily constituted a material contribution to the cumulative disease process. Lord Keith of Avonholm stated that the pursuer had successfully established the factual basis for liability:

The conclusion I reach is that on the evidence the pursuer has established that the swing grinders were a source of dust that was substantial and not merely negligible and that this dust contributed to the cause of the pursuer’s disease. I would add that it is not necessary to show that the dust from the swing grinders was the sole cause of the disease.

The court concluded that once a material contribution from a tortious source was established, the defendant was fully liable for the resulting injury, even if a non-tortious source had also contributed.

Implications

This case is a foundational authority on causation in both the English law of tort and the Scottish law of delict. It established the ‘material contribution to harm’ principle, which relaxes the strict ‘but for’ test of causation in certain situations. The decision is particularly significant for cases involving industrial diseases that develop cumulatively over time, where multiple sources contribute to the condition. It allows claimants to succeed by showing that the defendant’s breach of duty made a contribution that was more than negligible (de minimis), without having to prove it was the main or sole cause. This eases the burden of proof for claimants in scientifically complex cases.

Verdict: Appeal allowed.

Source: Wardlaw v Bonnington Castings Ltd [1956] UKHL 1 (01 March 1956)