Toxic tort asks two causation questions, not one. They rest on different evidence, are proved by different experts, and fail in different ways.
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A toxic tort plaintiff must establish both general causation — that the substance is capable of causing the disease at issue — and specific causation, that it caused this particular claimant's disease given everything else known about them. The two are genuinely separate. General causation is a question about populations, answered largely with epidemiology and toxicology. Specific causation is a question about an individual, answered by a clinician reasoning from that person's exposure, history, and differential diagnosis. A matter can have overwhelming general causation evidence and still fail on specific causation, and since Daubert directed courts to examine methodology rather than credentials, exclusion of causation experts in toxic tort has become common enough to be a primary litigation strategy in its own right.
The two causation showings and the dose-response science underlying both. Each is proved by different experts and attacked in different ways.
Whether the substance is capable of causing the disease — the population question, and the evidence that answers it.
investigateThe individual question — this claimant, this exposure, this disease, and every alternative explanation.
investigateThe relationship between how much and what happens — thresholds, extrapolation, and the assumptions inside both.
investigateHow causation evidence is assembled and stress-tested before it is offered — the Institute orients on structure and evidence, and never on the conclusion.
General causation asks whether the substance is capable of causing the disease at all, in anyone, at exposure levels of the kind alleged. It is a population question, answered principally from epidemiology supported by toxicology and mechanistic evidence. Specific causation asks whether it caused this claimant's disease, and is an individual question answered by a clinician who considers the person's exposure, medical history, risk factors, and alternative explanations. Courts generally require both, and general causation is usually addressed first because failure there ends the matter regardless of the individual facts.
Because Daubert directs courts to examine the reliability of the methodology rather than the standing of the expert, and causation reasoning offers several places to go wrong. The recurring grounds are extrapolation from an accepted premise to an unfounded conclusion, failure to account adequately for obvious alternative explanations, reliance on studies at exposure levels far above those alleged, and reasoning the expert would not employ in their own professional practice. The last is the most quietly dangerous: an opinion assembled for litigation, using a method the field does not otherwise use, is vulnerable however eminent its author.
It varies by jurisdiction and by the state of the science, and it is one of the more contested questions in the field. Courts have accepted general causation supported by toxicological and mechanistic evidence where human epidemiology is absent or thin, particularly for rare diseases or novel compounds where epidemiology may never exist. Others treat human data as effectively necessary. What consistently helps is coherence — animal findings, mechanistic plausibility, and whatever human evidence exists all pointing the same way — rather than reliance on a single study type carrying the whole argument.
Some courts have treated a relative risk above 2.0 as significant for specific causation, on the reasoning that it implies the exposure more likely than not caused the individual case. The argument is influential and also widely criticised, because it applies a population statistic to an individual and ignores whether that person's exposure and susceptibility resemble the study population. It is best understood as one consideration courts weigh rather than a rule, and expert opinions that rely on it exclusively — or that ignore it entirely where the case law expects engagement — both invite challenge.
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