What is the test for a tortfeasor to be held liable if they aggravate a non-tortious cause?

The plaintiffs cite Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458, for the proposition that tortfeasors will be liable if they aggravate a non‑tortious cause. Anticipating that the defendants would argue that their illnesses arose from the initial exposure, before the clean‑up, the plaintiffs submit that "even if there might have been some health effect lingering from the initial exposure, [it] was significantly exacerbated, if not solely caused, by returning to the low levels of contamination remaining after the clean‑up."