Causation in toxic tort cases is almost always a difficult issue to prove. It is made even harder when the evidentiary standards for proving it are unclear.
The basic evidentiary rule for causation in toxic tort cases is that there must be proof of 2 things: 1) scientifically-accepted proof that the contaminant of concern can cause the injury alleged by plaintiff, and 2) the exposure (“dose”) of the Plaintiff to the contaminant was sufficient to cause the harm. Fairly simple rule to state, but it can be difficult to apply.
The problem, of course, is that humans are not homogeneous, and our bodies do not always react the same way to every exposure. Two people can stand in the same hayfield and one can have an allergic reaction while the other blissfully enjoys the sun’s warmth. Similarly, the same person can stand in the same field one day with no problems but experience a reaction in the same field (i.e, under the same conditions) the very next day.
But we can’t create a legal system of liability based upon “any exposure” that may injure someone, regardless of the risk. The Texas Supreme Court addressed this recently in the asbestos case linked above. There is clear scientific consensus that asbestos can cause the mesothelioma alleged by the plaintiff; so that was not at issue. On the exposure prong, however, the Court reasoned that an “any exposure” standard undermines the requirement of proving causation by a preponderance of the evidence standard. Instead, the Court relied on its prior holdings that require proof 1) of the approximate exposure dose and 2) that the dose was a “substantial factor” in causing the injury. In other words, simply proving the fact of exposure “should not end the inquiry and result in automatic liability.” Instead, the Court held “the plaintiff should be required to establish more than a doubling of the risk attributable to the defendant’s product…,” (this would usually be based on epidemiological studies).
Toxic tort cases rarely fall into neat little evidentiary boxes, and the factual and expert issues are usually enough to keep the legal armies fully engaged, but it is made somewhat easier when the battlefield has been mapped and we know by which standards to load our cannons.