In Milward v. Rust-Oleum Corp., No. 13-2132, 2016 U.S. App. LEXIS 7470 (1st Cir. April 25, 2016), plaintiffs sued numerous defendants in the United States District Court for the District of Massachusetts for negligence, alleging exposure to defendants' benzene-containing products caused the plaintiff husband's acute promyelocytic leukemia ("APL") ( see October 2013 Foley Hoag Product Liability Update). On the eve of trial, the lone remaining defendant moved to exclude plaintiff's specific causation expert and for summary judgment. The expert had opined that: (i) there is no safe level of benzene exposure; (ii) regardless of plaintiff's dose, certain epidemiologic studies established that an individual's relative risk of developing APL increased when exposed to certain amounts of benzene; (iii) using differential diagnosis, some more common risk factors associated with APL could be ruled out, and since benzene was in general a "potential cause" of APL, she could rule out idiopathic APL, i.e., APL of unknown origin, and thereby rule in benzene as the "only significant potential cause." 

The district court excluded the expert's opinion under Federal Rule of Evidence 702 and Daubert v. Merrell Dow Pharms., 509 U.S. 579, 597 (1993) (holding proponent of expert testimony must show it to be relevant, reliable and helpful to the fact-finder). and thus granted summary judgment. The court noted that in deposition testimony the expert disavowed any intention or ability to analyze epidemiologic studies that appeared to conflict with her hypothesis that there was no safe level of benzene exposure, and without such analysis it was not possible to determine if her methodology was reliable. In addition, the expert's ruling in of benzene as a potential cause to rule out idiopathic origin and hence opine that benzene was the actual cause was impermissibly circular.  

On appeal, the United States Court of Appeals for the First Circuit affirmed. Regarding the epidemiologic studies, plaintiff argued that studies showing an increased risk of APL above a certain level of exposure and other studies detecting no increased risk at such levels did not actually conflict because the latter studies did not affirmatively conclude there was no relationship between benzene and APL. Plaintiff also argued the expert did not disavow any willingness to consider the other studies, and her opinion was in any event reliable because it was based on reliable epidemiologic evidence.  

The appellate court rejected these arguments. The court noted that reliance on studies showing no evidence of increased risk would necessarily lead to a different opinion than reliance on studies that showed such risk. Because the expert's opinion was based on the scientific literature, for her opinion to be reliable she needed to explain why she had relied on some studies but not others. At deposition, however, she had clearly testified she could not opine as to which studies could be relied upon and which should be discounted.  

As for the expert's differential diagnosis, the court agreed her reasoning was circular. Plaintiff's only argument in this respect was essentially to assert that differential diagnosis is a reliable methodology, which defendant had not disputed. Here, however, the expert provided no scientifically reliable method to rule out idiopathic APL, particularly given the frequency of idiopathic cases. The court further held the expert had no reliable method even to rule in benzene as a potential cause of plaintiff's disease because this conclusion was based on the same unreliable methodology by which the expert concluded there was no safe level of benzene exposure without considering the other studies.

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