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.
The Ruffin panel reached only the general causation opinion before affirming the summary judgment that followed the Rule 702 order. In arguing to affirm the district court’s general causation analysis, the defendant first cited to Allen v. Pennsylvania Engineering Corp., 102 F.3d 194 (5th Cir. 1996), which stated the harmful level of exposure to a chemical is a “minimal fact[] necessary to sustain the plaintiff’s burden in a toxic tort case.” Therefore, the defendant asserted, the expert had to identify a quantifiable level of exposure to a particular chemical necessary to cause the plaintiff’s prostate cancer. Having failed to do so, the defendant argued that the expert’s opinion was unreliable under Rule 702.
The Ruffin panel disagreed, taking specific issue with the contention that experts are required to define a specific quantitative threshold dose of exposure. The panel reasoned that establishing a specific threshold dose is sufficient to make general causation testimony admissible, but it is not necessary. Interpreting Allen as requiring only that there is a “harmful level of exposure to a chemical,” the panel held the existence of a “harmful level” can be established without pinpointing a quantitative threshold dose — for example, through a significant association between occupational exposure and the condition or by providing qualitative examples of exposure “generally known to cause the relevant condition.” It then found that the expert met that requirement by pointing to an increased cancer risk per incidence of lifetime oral exposure to polycyclic aromatic hydrocarbons (PAH), studies that associated exposure to PAHs and cancer, and different risks associated with occupational and non-occupational exposures.
But the panel’s view of the “harmful level” requirement did not mean the expert’s general causation opinion was admissible. In two different ways, he failed to link the plaintiff’s specific chemical exposure to the plaintiff’s specific alleged injury. First, the expert’s opinion identified only one chemical within the PAH family — benzo(a)pyrene — that allegedly increases the risk of prostate cancer. Neither he, nor the plaintiff, claimed that plaintiff was exposed to benzo(a)pyrene. Although crude oil contains PAHs in general, and although benzo(a)pyrene is among the most common of them, the expert admitted that any particular mixture of PAHs can vary. Therefore, he failed to establish that the plaintiff’s exposure to crude oil exposed him to significant amounts of the only specific chemical for which he had opined on causation. Second, although the expert opined that benzo(a)pyrene causes cancer in humans, he failed to show that it causes prostate cancer specifically. Thus, even if the plaintiff had been exposed to significant levels of benzo(a)pyrene specifically, the expert would still have failed to establish that the exposure could cause the plaintiff’s alleged injury. Because the plaintiff was left with no admissible expert testimony on general causation, the panel affirmed summary judgment against him.
Although the Ruffin panel admirably called out the expert’s failure to connect plaintiff’s specific exposure to plaintiff’s specific alleged injury, its discussion of “harmful level” may create some confusion going forward. The holding is limited by the fact that the panel did not reach the expert’s specific causation opinion. On a purely academic level, one could argue that an expert who can cite a “harmful level” has implied the existence of a threshold dose — i.e., some dose at or below the harmful level, even if that exact threshold dose is not quantifiable. The importance of knowing the threshold dose only becomes apparent when evaluating specific causation — because a plaintiff must establish that he or she was exposed to sufficient amounts of a chemical to cause injury, the plaintiff’s exposure must be compared to dosages that are known to cause the alleged injury. Thus, specific causation will usually — if not always — require a sharper focus on threshold dose than the Ruffin panel applied when dealing with the expert’s general causation opinion in isolation. In cases involving questionable specific causation opinions, practitioners would do well to show the court how a strictly qualitative “harmful level” analysis for general causation purposes will frustrate the court’s Rule 702 gatekeeping inquiry into specific causation opinions.
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