OPINION
This is an appeal from a judgment in the superior court affirming a decision by the Alaska Workers’ Compensation Board. The Board denied Paul Grainger disability benefits for a heart condition he developed while working for the City of Ketchikan as a powerhouse operator. We
*977
are reviewing the case for the second time.
See Grainger v. Ketchikan,
The Board found that Grainger had submitted sufficient evidence to establish a preliminary link between his employment and his heart condition. Therefore, the Board applied the statutory presumption of compensability to his claim. AS 23.30.-120(a);
see Burgess Constr. Co. v. Smallwood,
We hold that Ketchikan failed to rebut the presumption of compensability as a matter of law. Once the presumption arises, an employer can overcome it by presenting substantial evidence
1
that either (1) provides an alternative explanation which, if accepted, would exclude work related factors as a substantial cause of the disability; or (2) directly eliminates any reasonable possibility that employment was a factor in causing the disability.
Fireman’s Fund,
The issue here is whether Ketchikan has presented substantial evidence that either provides an alternative explanation for the cause of Grainger’s heart condition or eliminates any reasonable possibility that work related stress was a substantial factor in aggravating or accelerating the disease.
2
See Thornton,
In answering this question, it must be remembered that no single factor can be isolated as the “cause” of Grainger’s disease, arteriosclerosis. Both of the physicians who testified agreed that several risk factors often operate together to precipitate or accelerate the development of the disease. Thus, it is not a sufficient “alternative explanation” for Ketchikan merely to point to Grainger’s obesity and lack of exercise as precipitating factors in the development of his disease. While these factors were likely to have contributed to the onset of Grainger’s condition, there is no evidence that they were the exclusive causes or that work related stress was not another causal factor. 3
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In
Grainger I,
we remanded with instructions that the Board consider the facts of this case in light of our decisions in
Wade v. Anchorage School Dist.,
In finding that the City of Ketchikan successfully rebutted the presumption of compensability, the Board relied on the testimony of Grainger’s co-workers. The Board found that the co-workers’ testimony “effectively rebutted” Grainger’s testimony regarding the sources of his stress at work. The Board has the “sole power to determine the credibility of witnesses.” AS 23.30.122. However, even granting that the co-workers’ accounts of employee interaction and job stress are more reliable than Grainger’s own account, the record does not support the Board’s conclusion that their testimony rebutted Grainger’s testimony. Nor does the record substantiate the claim that Grainger experienced no work related stress. 4
Not one witness testified that Grainger did not suffer stress because of his work. When cross-examined by Grainger’s attorney, the co-workers and supervisors stated that they did not know whether he experienced work related stress and acknowledged that his work could have been stressful to him. At best, this testimony supports the conclusion that Grainger was subjected to no greater stress than an average powerhouse operator in the normal course of his duties. Given our decisions in Fox and Wade, it would make little sense to conclude that Ketchikan can successfully rebut the presumption of compensability merely on a showing that Grainger experienced no more stressful work conditions than an average employee in his profession.
The Board also relied on the testimony of Grainger’s treating physicians, Dr. Salness and Dr. Stewart. The Board’s reliance on Dr. Stewart’s medical testimony was surprising as well as misguided. 5 As we pointed out in Grainger I:
Dr. Stewart did not testify that job related stress was or was not a “substantial factor” in bringing about the final result. Dr. Stewart’s testimony was that there was insufficient evidence or documentation in his records regarding environmental or physical or emotional stress [to conclude that work related stress played *979 a “unique or primary” role in the development of Mr. Grainger’s disease].
Grainger I,
We hold that the Board erred in concluding that Ketchikan presented substantial evidence rebutting the presumption of compensability. We will reverse a Board decision when we “ ‘cannot conscientiously find that the evidence supporting that decision is substantial.’ ”
Delaney v. Alaska Airlines,
REVERSED and REMANDED.
MATTHEWS and MOORE, JJ., dissent, for the reasons expressed in Justice Matthews’ dissent in
Grainger v. Alaska Workers’ Compensation Bd,
Notes
. We have defined substantial evidence as “ 'such relevant evidence as a reasonable mind might accept as adequate to support a conclusion.’”
Fireman’s Fund Am. Ins. Cos. v. Gomes,
.
We examine Ketchikan’s evidence standing alone without weighing it against the evidence Grainger presented to establish a causal connection between stress and his heart condition.
Wolfer,
.Indeed, one physician testified that all three factors were likely "components" in the development of Mr. Grainger’s heart disease. There is no requirement in our law that work related factors be the
unique
or
primary
cause of a compensable disability. In
Thornton,
. On the contrary, the testimony of Grainger’s supervisors and his fellow powerhouse operators establishes at a minimum that: (1) Grainger had "personality conflicts” with his supervisor; (2) his chest pains began at work after he had worked several overtime shifts; (3) he received numerous calls at his station from customers angry over power outages; (4) as the operator at the Ketchikan powerhouse, Grainger was "in charge of the whole [electrical power] system”; (5) major power outages are stressful for the powerhouse operator; and (6) the operator's performance during these outages was subject to close inspection and was often criticized by superiors.
All of these features of Grainger’s job could be considered stress producing. This testimony cannot be reasonably relied on to conclude, as the Board apparently concluded, that Grainger experienced no work related stress.
. We should point out that we did not dispute Dr. Stewart’s credibility or the weight to be accorded his testimony in
Grainger I.
Rather we suggested that his testimony failed to provide substantial evidence to support the Board's conclusion that work related stress was not a factor in the development of Grainger’s disease.
Grainger I,
