Had his employer provided shower facilities, the coal dust could have been washed off before cycling, reducing the risk of contracting dermatitis.
Due to the limits of scientific knowledge, it was impossible to rule out the possibility that he hadn't contracted dermatitis during the non-wrongful exposure to brick dust while working in the kiln.
The House of Lords held that the risk of harm had been materially increased by the prolonged exposure to the dust, as resulting from the defendant's negligent failure to provide the appropriate washing facilities.
Lord Reid stated: "The effect of such abrasion of the skin is cumulative in the sense that the longer a subject is exposed to injury the greater the chance of his developing dermatitis: it is for that reason that immediate washing is well recognised as a proper precaution."
[1] The judgement was subsequently questioned by the House of Lords in Barker v Corus, wherein the defendant's material contribution to risk was instead constructed as their causation of a lost chance for the plaintiff to avoid the harm.