Although an injured worker need not prove that an employer was responsible for causing an injury or disease, WCB benefits are only payable where a worker's injuries or disease arose out of and occurred in the workplace. In other words, a successful WCB claim involves proving a causal relationship between the injury or disease and the worker's job duties or work environment.

While causation can be obvious or uncontroversial in some circumstances, causation is often at the centre of a WCB decision denying coverage to an injured worker. Medical history, employment duties, witness statements, and other evidence are critical in determining whether an injury or disease is related to work and, therefore, compensable by the WCB.

Where there is uncertainty about whether a worker's injury or disease was caused in the course of employment, the WCB may order a medical examination or seek the opinion of a medical consultant. Although unfavourable examination results or expert opinions are a significant barrier to a WCB claim, it may be possible to seek further examinations or opinions to bolster an injured worker's claim.

In assessing causation, the WCB will apply the "but-for" test, which is the same causation test used in personal injury cases proceeding through the courts. This involves the hypothetical determination of the likely position that the worker would be in had the workplace accident or exposure been avoided. If the worker would have been in a better position but for accident or exposure, then the worker's injury or disease is causally related to the job and, therefore, compensable under the WCA.

Where the evidence for and against a worker's claim is roughly equal, the WCB is required to decide the claim in favour of the worker.