If a plaintiff's cancer was caused by one molecule or one fiber damaging a single cell's DNA causing it to become malignant, and plaintiff was exposed to billions of molecules or fibers from multiple sources, how could he possibly prove that ,"but for" any single source of exposure or subset of exposures, he would not have developed his cancer? As stated, this is the Summers v. Tice problem traditionally resulting in burden shifting rather than dismissal so long as the conduct of each defendant was tortious.
Similarly, if plaintiff's cancer was caused by the cumulative effect of any one of several subsets of exposures from multiple sources, e.g. if three widgets were sufficient to cause cancer and plaintiff was exposed to one widget from each of A, B, C and D, she could never prove that "but for" A's widget she wouldn't have gotten cancer since the widgets of B, C and D were sufficient to have caused it; and the same would be true if she sued B, C or D. Again, the traditional answer to plaintiff's problem, assuming each defendant's conduct was tortious, has been to shift the burden of proof on causation. See Landers v. Texas Salt Water Disposal Co.
Suddenly the 5th Court of Appeals has in essence held that Borg-Warner v. Flores puts the burden of proof of causation back on plaintiff; doing so in a case in which the defendant stipulated so-called "general causation". See Georgia-Pacific v. Bostic. There's essentially no discussion of the rationale for imposing a "but for" causation proof burden on a toxic tort plaintiff nor is the impact of this monumental shift in Texas law even discussed so maybe the court didn't intend such a result. Indeed it did proceed to discuss substantial factor causation even after concluding that a "but for" burden was borne by a mesothelioma plaintiff with the result being that plaintiff's evidence was held insufficient to "provide quantitative evidence of [plaintiff's] exposure to asbestos fibers from [defendant's product] or to establish [plaintiff's] exposure was in amounts sufficient to increase his risk of developing mesothelioma."
So if you establish an increase in risk you've established substantial factor causation and that's the same as"but for" causation?! This case has a lot of lawyers scratching their heads.
It's time for a clearly articulated definition of "substantial factor" and it's time to get rid of the confusing and, in cancer cases, nonsensical "general causation" v. "specific causation" dichotomy. More on that after I get home.