For a defendant to be found negligent, the law requires a causal link between an act or omission and the harm suffered by the plaintiff. Many labels have been used to describe causal links, and certain difficulties may arise in applying those labels to the facts of a given case. In Resurfice Corp. v. Hanke, the Supreme Court of Canada has tried to clarify this complicated area.

This case involved an ice resurfacing machine at an arena in Edmonton. The machine had both a gasoline tank and a water tank. Unfortunately, Hanke mistakenly placed the water hose into the machine’s gasoline tank. Hot water pouring into the gasoline tank made it overflow, resulting in the release of vaporized gasoline into the air, leading to an explosion and fire that caused severe burns to Hanke.

Hanke sued the manufacturer and distributor of the ice resurfacing machine on the basis that it was too easy to confuse the gasoline tank and the water tank, given their appearance and proximity. Hanke’s action was dismissed at trial. On the issue of causation, the trial judge concluded that Hanke was not confused and that the design of the machine had not caused the accident.

The Alberta Court of Appeal, however, decided that the trial judge had erred by failing to consider the ‘comparative blameworthiness’ of the plaintiff and the defendants and also by not applying a ‘material contribution’ test.

The Supreme Court of Canada reversed the Alberta Court of Appeal’s decision and summarized the principles that have emerged from the cases on causation:

First, the basic test for determining causation remains the "but for" test. This applies to multi-cause injuries. The plaintiff bears the burden of showing that "but for" the negligent act or omission of each defendant, the injury would not have occurred. Having done this, contributory negligence may be apportioned, as permitted by statute.

The court said that the ‘but for’ test is intended to ensure that a plaintiff will receive compensation for negligent conduct only where a ‘substantial connection’ exists between the injury and the defendant’s conduct. Where the plaintiff’s injuries may very well be due to other factors that are not connected to the defendant, no liability will be found.

The ‘material contribution’ test only applies in special circumstances and specifically where it is impossible for the plaintiff to prove that the defendant’s negligence caused the plaintiff’s injury using the ‘but for’ test. This impossibility must be due to factors that are outside the plaintiff’s control, such as the current limits of scientific knowledge. In addition, the plaintiff must show a breach of duty that exposes the plaintiff to an unreasonable risk of injury. Causation may be relaxed only where both requirements are met.

The existence of multiple causes is not enough to relax the test:

The Court of Appeal erred in suggesting that, where there is more than one potential cause of an injury, the "material contribution" test must be used. To accept this conclusion is to do away with the "but for" test altogether, given that there is more than one potential cause in virtually all litigated cases of negligence. If the Court of Appeal’s reasons in this regard are endorsed, the only conclusion that could be drawn is that the default test for cause-in-fact is now the "material contribution" test.

Thus, it is inappropriate to focus on whether an alleged act of negligence made a material contribution to the damages. Instead, if a defendant is going to be found liable in negligence, it will generally be on the basis that ‘but for’ the defendant’s negligence, the damages would not have occurred.