A central tenet of toxicology is that “the dose makes the poison.” Every chemical is toxic if enough of it is consumed, and every chemical has some dose – even if miniscule – at which it poses no significant risk. A chemical must be given in sufficient amount – something exceeding the “threshold dose” – before it will cause effects. This has obvious implications for toxic tort litigation, where a plaintiff who alleges injury from exposure to a toxic chemical must prove at minimum that he or she was exposed to enough of the chemical to produce the alleged injury. This poses a problem for plaintiffs who have been exposed only to very small doses of the chemical at issue. What is a plaintiff to do when their exposure falls below the threshold dose? One approach that generally does not work is to reject the very concept of a threshold dose altogether.

In Pinares v. Raytheon Technologies Corporation, 2023 WL 2661521 (11th Cir. Mar. 28, 2023), Plaintiff alleged that she had developed kidney cancer after chemical compounds from the defendant’s facility made their way into the groundwater near her home. Plaintiffs relied on three experts to prove causation – a toxicologist to establish general causation and two physicians to establish specific causation. The district court excluded Plaintiffs’ toxicology expert, holding that the expert had not conducted a reliable dose-response assessment. The district court then also excluded each of Plaintiffs’ specific causation experts, noting that they had not performed an independent dose-response assessment of their own and therefore relied on the toxicology expert’s deficient opinion. Plaintiffs could not establish causation without expert opinion, and the district court therefore granted summary judgment.

Plaintiffs appealed, but the result was no different in the Eleventh Circuit. The toxicologist purported to conduct a dose-response assessment, but he had not specified any doses of the chemicals at issue that were “too much.” One need not wonder why; the expert conceded that Plaintiff had been exposed only to an amount that was “generally recognized by toxicologists” as “low,” and he acknowledged that the amount was well within the jurisdiction’s “safe drinking water standard.” And, he had not cited any authority suggesting that the amount could cause cancer. In short, Plaintiff had received less than the threshold dose. So, the expert denied the very concept of a threshold dose and opined that “every dose greater than zero” would implicate “some human health risk.” This reasoning not only rejected one of the elementary principles of the toxicologist’s field, but also contravened well-established Eleventh Circuit law holding that the “any amount is dangerous” approach “clearly contradicts the principles of reliable methodology.” Pinares, 2023 WL 2661521 at *4 (quoting McClain v. Metabolife Int’l, Inc., 401 F.3d 1233, 1243 (11th Cir. 2005)).

Plaintiffs’ expert nevertheless attempted to support his opinion by referencing the so-called “one-hit theory” of causation – i.e., that any dose of a carcinogenic compound can cause an irreversible genetic change that will lead to cancer. But, as noted in a case cited by the Pinares panel (Wills v. Amerada Hess Corp., 379 F.3d 32, 49 (2d Cir. 2004)), that theory is not only “controversial” at best, but fails to satisfy any of Daubert’s reliability factors. Because Plaintiff’s expert was unable to explain why it was appropriate to ignore dosage, his opinion was unreliable and properly excluded.

Having excluded Plaintiffs’ general causation expert, the Eleventh Circuit made short work of Plaintiffs’ specific causation experts. Neither expert had performed his own dose-response calculation, and they could not rely on the inadmissible assessment by Plaintiffs’ toxicology expert. Plaintiffs argued that the specific causation experts had conducted a differential diagnosis, but the Eleventh Circuit held that this did not substitute for reliably showing that Plaintiff had been exposed to enough of the chemicals at issue to cause her cancer. Having affirmed the exclusion of Plaintiffs’ causation experts, the Eleventh Circuit then affirmed summary judgment due to Plaintiffs’ lack of expert testimony supporting causation.

As Judge Posner famously wrote in the oft-quoted Rosen v. Ciba-Geigy Corp., 78 F.3d 316, 319 (7th Cir. 1996), “the courtroom is not the place for scientific guesswork, even of the inspired sort. Law lags science; it does not lead it.” To be sure, there are some notable examples throughout history of visionary scientists making discoveries that undermined a foundational assumption of his or her field. But a litigation expert who disagrees with a well-established basic principle of the field should not pass muster under Federal Rule of Evidence 702. And, as reinforced in Pinares, a toxicology expert who cannot say how much of a chemical it takes to cause the claimed injury should not be allowed to testify that the plaintiff was exposed to enough to cause the injury.