Category: Expert Admissibility

Defining the Limits of Lay Testimony in Complicated Products Cases

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“So when is a question too complicated for the jury?” That is the question the Third Circuit sought to answer recently in Slatowski v. Sig Sauer, Inc., ___ F. 4th ___, 2025 WL 2178533 (3d Cir. 2025), reversing a district court’s grant of summary judgment despite affirming its exclusion of the plaintiff’s causation experts. Ironically, the Third Circuit’s analysis of when an expert opinion is required is itself so nuanced that it may require expert interpretation. Upon close inspection, the Slatowski panel’s holding is not nearly as broad as the headings might suggest.

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Message Received – Delaware Follows Federal Rule of Evidence 702

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The Delaware Superior Court took the mass tort world by surprise with its May 31, 2024, refusal to exclude the plaintiffs’ experts’ causation opinions in the Zantac litigation, breaking with the federal MDL court’s prior exclusionary order and applying Delaware Rule of Evidence 702 “with a liberal thrust favoring admission.” The ruling was so unexpected that some outlets questioned whether Delaware courts would become a new preferred venue for mass tort plaintiffs. But, in a rare move, the Delaware Supreme Court granted the defendants’ request for interlocutory appeal — despite the Superior Court’s refusal to certify the order — and has now restored order, reversing the Superior Court’s decision and remanding for further proceedings. In re Zantac (Ranitidine) Litig., — A.3d —, 2025 WL 1903760 (Del. July 10, 2025).

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Double Take: Fifth Circuit’s Dual BELO Rulings Show Both General and Specific Causation Are Essential

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We previously blogged about a decision in the In re Deepwater Horizon BELO litigation – Ruffin v. BP Exploration & Production, Inc. – in which the Fifth Circuit affirmed summary judgment for defendants in an alleged chemical exposure case based solely on exclusion of the plaintiff’s expert testimony on general causation without reaching the experts’ specific causation opinions.   As it turns out, in another BELO case argued before the same panel on the same day, the panel analyzed the plaintiff’s experts’ specific causation opinions and declined to reach their general causation opinions. Williams v. BP Exploration & Production, — F.4th —, 2025 WL 1904153 (5th Cir. July 10, 2025). Notwithstanding the concerns we previously expressed about the Ruffin panel’s approach to general causation in isolation from specific causation, the Ruffin and Williams decisions provide a potent couplet illustrating that general causation and specific causation are two distinct steps in the analysis. Both must be proven in order for a plaintiff to make a prima facie case.

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Fifth Circuit Clouds Threshold Dose Analysis in Ruffin v. BP

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Plaintiffs in toxic tort cases must prove both general and specific causation, generally through the testimony of experts. Experts must establish that a specific chemical exposure can (and did) cause the specific injury at issue. In order to make that showing, the plaintiff’s exposure must at least have exceeded the minimum harmful level of the chemical — the “threshold dose.” As the Eleventh Circuit made clear last year in its handling of In re Deepwater Horizon BELO litigation (which we discussed here), threshold dose is a concept that straddles general and specific causation. A more recent BELO case, Ruffin v. BP Exploration & Production, Inc., — F.4th —, 2025 WL 1367185 (5th Cir. May 12, 2025), shows how isolating an expert’s general causation opinion from its implications on specific causation can cloud the analysis.

In Ruffin, the plaintiff worked for five months as a clean-up worker following the Deepwater Horizon oil spill. He was diagnosed with prostate cancer five years later and sued the defendant, claiming he was exposed to chemicals that caused his cancer. He described two instances of exposure: one when oil splashed onto his face while travelling by boat and another when he fell in the water. The plaintiff had a known genetic risk for prostate cancer, but his expert claimed the oil exposures were a “second and necessary hit to initiate his prostate cancer.” The defendant moved to exclude the plaintiff’s causation expert, a genetic and molecular epidemiologist, under Federal Rule of Evidence 702. The district court had excluded both the expert’s general and specific causation opinions as unreliable, largely for failure to specify a threshold dose, and then granted summary judgment for want of admissible expert evidence.

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Foundation, Not Façade — The Fifth Circuit Affirms the Proper Basis Requirement for Admissibility of Expert Opinions in Newsome v. International Paper Co.

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In a toxic tort case, plaintiffs must establish general causation. If a substance is incapable of causing the type of injury plaintiff claims, then it certainly didn’t cause theirs. Under Texas law, toxic tort plaintiffs must prove general causation either by “direct, scientifically reliable proof,” or by “indirect” epidemiological evidence. Merrell Dow Pharms., Inc. v. Havner, 953 S.W.2d 706, 714-15 (Tex. 1977). In Newsome v. International Paper Company, plaintiff attempted to bypass this foundational requirement, and neither the district court, nor the Fifth Circuit was fooled. WL 5117195 (5th Cir. Dec. 16, 2024).

In Newsome, plaintiff was a truck driver for a company that supplied International Paper with sodium hydrosulfide (NaHS). Under certain conditions, NaHS releases hydrogen sulfide (H2S), an invisible gas with a characteristic rotten-egg odor. During a delivery in January 2019, plaintiff alleged he “smelt something” then “came to” on the ground. He presented to an urgent care clinic the following day but was diagnosed with only a rash. He did not visit a doctor again for four months. Then, more than a year later, plaintiff sued International Paper claiming “a host of life-threatening injuries” related to his alleged exposure to H2S.

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