## Facts

- The claimant, a worker (C), developed pneumoconiosis, a lung disease caused by inhaling silica dust during employment at Bonnington Castings Ltd.
- C was exposed to silica dust from two sources: unavoidable dust generated by pneumatic hammers (non-negligent), and dust from swing grinders, attributable to the employer’s negligent maintenance.
- The disease resulted from a gradual accumulation of silica particles in the lungs.
- Only part of the exposure was due to the employer’s negligence.
- The dispute centered on whether the employer was liable given that the negligent exposure was only a portion of the total dust inhaled.

## Issues

1. Whether the employer could be held liable for negligence when the claimant’s exposure to harm arose from both negligent and non-negligent sources.
2. Whether the defendant’s actions needed to be the sole or primary cause of harm, or if a material contribution sufficed to establish causation.
3. Whether application of the traditional “but for” test or the “material contribution test” was appropriate in the context of cumulative causes.

## Decision

- The House of Lords held the employer liable, finding that their negligence made a material (more than negligible) contribution to C’s pneumoconiosis.
- It was not necessary to determine that the negligent exposure was the probable or main cause of harm.
- The court departed from the strict “but for” test, holding that a significant contribution was sufficient where injury resulted from cumulative causes.
- Full compensation was awarded to the claimant because the employer’s breach materially contributed to the injury.

## Legal Principles

- The “material contribution test” applies where injury arises from cumulative exposures, and the defendant’s tortious conduct is a more than negligible factor.
- The test differs from the “material increase in risk” approach, which applies where individual contributions to an indivisible injury cannot be identified.
- The material contribution test is appropriate for divisible injuries (e.g., pneumoconiosis) where additional exposure increases severity, but not for indivisible injuries (e.g., lung cancer).
- Proportional damages may now apply in similar factual contexts due to later case law development.
- Employers are responsible for ensuring that avoidable harmful exposures in the workplace are minimized, even where some level of exposure is unavoidable.

## Conclusion

Bonnington Castings Ltd v Wardlaw established that an employer can be held liable if their negligence materially contributes to a claimant’s divisible injury, even where non-negligent factors are also present. This case created a key distinction between cumulative causation and increased risk, clarifying the legal framework for causation in occupational disease claims and delineating the proper use and limits of the material contribution test.


Bonnington Castings Ltd v Wardlaw [1956] AC 613