Scientific Uncertainty and Causation in Tort Law

Tort cases involving scientific uncertainty frequently present courts with a difficult causation issue. In the paradigmatic case, the available scientific evidence indicates that a substance might be hazardous, but does not establish that the substance is hazardous.' When presented with such evidence, courts must decide whether the plaintiff has adequately proven that her injury was tortiously caused by the substance. This causal issue potentially arises whenever we do not fully understand how a substance interacts with the body and produces an adverse health outcome. We do not, for example, adequately understand the etiology of cancer.2 To assess whether a substance may cause injuries with unknown etiology, we observe health outcomes in populations of animals exposed to large amounts of the sub- stance, study the biochemical effects of the substance on cells, organs, and embryos, and compare the substance's chemical composition to other known health hazards.3 Though informative, these studies usually cannot determine whether the substance is hazardous.

That determination typically requires a large-scale study com- paring the incidence of adverse health outcomes in groups of ex- posed and non-exposed individuals, or comparing the incidence of exposure across injured and healthy groups. These epidemiological studies are expensive, time-consuming, and require that a large number of people be exposed to the substance. Without such study, however, the hazardous properties of the substance often cannot be established with existing scientific methods. Consequently, sub- stances are often introduced into the environment before there is conclusive scientific evidence regarding their health hazards.

How should this scientific uncertainty affect the tort rights of an individual who has been exposed to such a substance and has the type of injury, such as cancer, that is plausibly attributable to the sub- stance in light of the available scientific evidence?