As man's technology has grown exponentially, so has the number of industrial byproducts which are introduced into the nation's environment, frequently with little or no research done prior to production to discover possible deleterious effects. Even where attempts have been made to analyze and predict hazards, advances in scientific knowledge often expose years later the inadequacy of the prior research methods and the inaccuracy of conclusions derived from them. For example, carcinogenic substances often require a ten to twenty year latency period to reveal their effects; only recently did scientists discover that vinyl chloride, used for year in the manufacture of plastics and as aerosol propellant, is linked to a fatal liver cancer, angiosarcoma.

Environmentalists contend that although plaintiffs usually have the burden of proof, in suits challenging defendants' activities as serious public health hazards, where the probability of harm can not be determined with absolute certainty, public policy dictates that, upon a prima facie showing by plaintiffs, defendants should be held to show the safety of their products. Proponents of this view argue that such a modification of the usual burden rules is in keeping with a traditional justification for shifting the burden of proof: that the greater burden should be on the party with the greatest access to information.