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Why Bonnington Castings Ltd v Wardlaw is important

In Bonnington Castings Ltd v Wardlaw, the House of Lords held the defendant was liable to the full extent for the claimant’s harm where their negligence was one of a number of sources of the damage but materially contributed to the injury.


Wardlaw worked in the defendant’s dressing shop for eight years. The machines in the shop produced dust, part of which contained small particles of silica. One machine used was a pneumatic hammer. At the time Wardlaw worked in the factory, there was no known way of removing dust produced from pneumatic hammers. Similarly, there was no known mask or respirator which would have protected the workers from inhaling the dust.

With regards to the other machines, a dust extraction system could effectively remove the dust from the air. However, it was common for the extraction system to become blocked causing dust to escape into the atmosphere.

Wardlaw contracted the disease pneumoconiosis by inhaling air containing minute particles of silica, forcing him to stop working. It was impossible to show whether this was caused by dust from the hammer or dust escaping from own hammer, or from using the factory’s hammer.


Wardlaw brought a claim in the tort of negligence against Bonnington Castings Ltd.


It was accepted that Bonnington Castings did not fail to take reasonable care if Wardlaw was exposed to dust from the pneumatic hammer. This was because there was no way to stop the claimant being exposed to dust from the hammer. However, where the dust extraction system choked leading to dust entering the atmosphere from the swing grinders, this was a breach of Bonnington Castings’ duty of care.

The difficulty was it could not be shown whether dust from the pneumatic hammer or the swing grinders caused the claimant’s lung condition.


The House of Lords unanimously held that Bonnington Castings Ltd materially contributed to the harm. This was sufficient for the purposes of causation in the tort of negligence, and they were held liable for the entire loss.


The main judgement of the House was given by Lord Reid. He rejected that the onus was on the defendant to show the breach did not cause the claimant a loss. Instead, Wardlaw had to show that Bonnington Castings’ breach of duty (letting dust from the swing grinders escape into the air) caused his loss. Lord Reid said:

the employee must in all cases prove his case by the ordinary standard of proof in civil actions: he must make it appear at least that on a balance of probabilities the breach of duty caused or materially contributed to his injury.

Examining the medical evidence, Lord Reid found that the lung condition developed through gradual exposure over time. Therefore, where a person is exposed from two sources, the condition is in some way attributable to both sources. In these circumstances, the correct question was whether the dust from the swing grinders had “materially contributed” to the injury. In Lord Reid’s words:

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 materially contributed 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.

Based on the workers’ evidence, the HL held that dust from the swing grinder did materially contribute to the damage. Morevoer, Bonnington Castings was held liable for the entire loss of earnings. This is regardless of the fact that more Wardlaw was exposed to more dust from the pneumatic hammer.

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