In fact, it may well be the best causation opinion of the last half century; and we’re willing to bet it will be one of the most important causation opinions going forward. Dixon v. Ford Motor Company nails substantial factor causation and in the process proves that there are still plenty of judges willing to think deeply about causal inference, about uncertainty and about the limits of liability in a world of inevitable risk. Ultimately, the court held that risk is the measure of legal causation in those cases where causal inferences are made probabilistically, and therefore when a causation expert opines that a putative cause was a "substantial factor" without saying how much risk it imparted she fails to answer the question she was called to give; and so has nothing helpful to say about the matter.

Dixon is another tragic mesothelioma case in which a defendant, having contributed to the victim’s cumulative asbestos exposure something between nothing and next to nothing, was hammered by the jury. It then appealed, after taking a few judicial deductions, a $3+ million judgment. As usual the defendant complained that plaintiff’s causation expert (this time an epidemiologist) ought not to have been allowed to testify because she had "extrapolated downward" from the known segment of the asbestos/mesothelioma dose/response curve to an area in which hard data was lacking and thus could not reliably say that the defendant’s small contribution to the victim’s dose was a substantial causative factor. 1) That’s not plaintiffs’ game these days, as we said in Small Glasses; but more importantly, 2) as the court made plain in Dixon, "substantial factor" in such cases isn’t about causation, it’s about risk.

"’Substantiality’ is a legal concept and not an objective property testable by the scientific method", wrote the Dixon court. The question thus required to get at the essence of substantial factor causation in a dose/response disease case is not "was it a big cause or a little cause?" It’s not even "was it really and truly a cause?" The first question is nonsensical and the second is unanswerable. Instead, the question is: "assuming it was a cause, was the risk it imparted prospectively of such a degree as to justly warrant the imposition of liability?" Risk, said the court, is thereby the measure of (legal) causation and proving that the risk imparted was substantial is plaintiff’s burden. Showing that defendant imparted a substantial degree of risk, according to the Dixon court, is thereby what is required to bridge the "analytical gap" between "asbestos causes mesothelioma" and "plaintiff’s exposure to defendant’s asbestos caused her mesothelioma".

Accordingly, the court held that when all plaintiff’s causation expert could say about causation was that "every exposure to asbestos is a substantial contributing cause" the only thing she wound up saying about the risk given plaintiff by Ford was that it was "more than nothing." "For obvious reasons an infinitesimal change in risk cannot suffice to maintain a cause of action in tort". Thus, the opinion of plaintiff’s expert "merely implied that there was some non-zero probability that [plaintiff] was exposed to asbestos from Ford’s product, and that this resulted in some non-zero increase in her risk of contracting mesothelioma. As such, [the conclusion of plaintiff's expert] that the risk and probability of causation was ‘substantial’ provided the jury with nothing more than her subjective opinion of ‘responsibility,’ not scientific evidence of causation."

So how does a plaintiff establish a substantial risk? Not by proving the exact dose and thus exact risk incurred.The Dixon court disposed of the straw man argument that requiring plaintiff to show risk (and thereby dose) quantitatively demands an impossible degree of certainty and precision. Such a rule, wrote the court, would obviously be "folly". Rather, all plaintiff is being asked to do is to "estimate exposure and risk with reasonable scientific or medical certainty." Once such an estimate is made the jury can decide whether it is sound and if so whether the risk imparted was substantial – subject to the law’s requirement that it be more than de minimis.

That’s exactly what we’ve been arguing since a client was kind enough to let us write an amicus in Borg-Warner v. Flores. Make the plaintiff say what dose, and thereby what risk, was given. Why? The effect of plaintiff’s estimate of dose in one of the first post-Borg-Warner cases should suffice for an answer. Using their expert’s dose estimate the calculated risk of death posed by our client’s product was demonstrated to have been less than 1 in six billion; that’s equivalent to the risk of death (from cancer caused by radon) imparted by spending just fifteen minutes in a building constructed of brick or stone. Such calculations and comparative risk exercises serve to vividly demonstrate both the de minimis nature of the risks imparted and the absurdity of imposing liability for them.