In Wilcox v. Homestake Mining Company plaintiffs argued that in toxic tort cases involving an injury with multiple potential causes "causation in such cases may be proven through a substantial factor test, without regard to whether the injuries would likely have occurred in the absence of the defendant's actions." Plaintiffs further argued that a "but for" causation requirement would make recovery all but impossible in toxic tort cases. The court rejected those arguments.
All a plaintiff has to do to get to a jury is to develop testimony that that the putative cause (e.g. radiation) was more likely than not a necessary cause of her injury (e.g. bladder cancer) and if there are multiple tortious sources of that cause then she can avail herself of alternate causation approaches (e.g. Summers v. Tice) to assert liability against a specific defendant. On the other hand, it's not enough merely to show that the defendant brought plaintiff into contact with one of the known causes of her injury and it's not enough to show that the putative cause is one of the more common causes of that such injuries.
That's helpful. "But for" goes with the first legal causation question: was it more likely than not that the exposure (e.g. 5 fiber/cc yrs amphibole asbestos) was a necessary cause of plaintiff's cancer (e.g. mesothelioma). So where does "substantial factor" go? The court doesn't say. Like most courts it doesn't even say what it is.
Where should "substantial factor" go? We think it makes sense for it to go with the second legal causation question which is "was the exposure (i.e. the risk imposed) unreasonable?" The ability to estimate the ex ante risk of cancer imposed by exposure to a carcinogen allows both qualitative and quantitative assessments of the reasonableness of the defendant's conduct and informs the question of whether a particular exposure was substantial (and thus a basis for liability) or merely de minimis (and so unable to support a liability claim).
As we argued in Borg-Warner and have argued since, in a toxic tort case the ex ante risk imposed is the best measure of the reasonableness of the man (or woman). And that's what "substantial factor" causation ought to be about - determining whether there's some notion of responsibility associated with the particular risk imparted.