Is expert evidence required to prove a link? Or can circumstantial evidence suffice?
In a case released last week, the highest court in Canada offered guidance on how causation may be inferred from the evidence. The case is British Columbia (Workers’ Compensation Appeal Tribunal) v. Fraser Health Authority 2016 SCC 25, and at issue was whether seven technicians at a single hospital laboratory, who each developed breast cancer, were entitled to WCB compensation on the basis that the cancer was an occupational disease. The WCB tribunal had decided that the women were entitled, then a higher court had reversed that decision – finding the evidence did not prove the causal link.
Proving causation under the WCB legislation is less difficult than at common law, where the standard of proof is on a balance of probabilities (i.e. more likely than not). The Supreme Court’s guidance on causation, however, is of general applicability:
 All that said, the central problem in the handling of causation in the courts below arose not in their failure to have appropriate regard to the less stringent standard of proof required by s. 250(4), but from their fundamental misapprehension of how causation — irrespective of the standard of proof — may be inferred from evidence.
 The presence or absence of opinion evidence from an expert positing (or refuting) a causal link is not, therefore, determinative of causation (e.g. Snell, at pp. 330 and 335). It is open to a trier of fact to consider, as this Tribunal considered, other evidence in determining whether it supported an inference that the workers’ breast cancers were caused by their employment. This goes to the chambers judge’s reliance upon the Court of Appeal’s decisions in Sam and Moore and to Goepel J.A.’s statement that there must be “positive evidence” linking their breast cancers to workplace conditions. Howsoever “positive evidence” was intended to be understood in those decisions, it should not obscure the fact that causation can be inferred — even in the face of inconclusive or contrary expert evidence — from other evidence, including merely circumstantial evidence. This does not mean that evidence of relevant historical exposures followed by a statistically significant cluster of cases will, on its own, always suffice to support a finding that a worker’s breast cancer was caused by an occupational disease. It does mean, however, that it may suffice. Whether or not it does so depends on how the trier of fact, in the exercise of his or her own judgment, chooses to weigh the evidence. And, I reiterate: Subject to the applicable standard of review, that task of weighing evidence rests with the trier of fact — in this case, with the Tribunal.