In Rutherford v. Owens-Illinois, Inc. (1997) 16 Cal.4th 953, the California Supreme Court held that a plaintiff suing for asbestos-related latent injuries must first establish exposure to the defendant's defective asbestos-containing products, and then "must further establish in reasonable medical probability that a particular exposure or series of exposures was a 'legal cause' of her injury, i.e., a substantial factor in bringing about the injury." (Rutherford, supra, 16 Cal.4th at p. 982; see id. at pp. 957-958.) In a lawsuit for damages caused by an asbestos-related cancer, "the plaintiff need not prove that fibers from the defendant's product were the ones, or among the ones, that actually began the process of malignant cellular growth. Instead, the plaintiff may meet the burden of proving that exposure to defendant's product was a substantial factor causing the illness by showing that in reasonable medical probability it was a substantial factor contributing to the plaintiff's or decedent's risk of developing cancer." (Id. at pp. 982-983.) The court elaborated on the test, describing the reasonable medical probability of the exposure being "a substantial factor in contributing to the aggregate dose of asbestos the plaintiff or decedent inhaled or ingested, and hence to the risk of developing asbestos-related cancer . . . ." (Id. at pp. 976-977.) Rutherford cautions that the term "substantial factor" evades precise definition, and that it is "'neither possible nor desirable to reduce the term to any lower terms.'" (Rutherford, supra, 16 Cal.4th at p. 969, quoting Prosser & Keeton on Torts (5th ed. 1984) 41, p. 267.) Rutherford articulated a partial, negative definition: that a force which plays only a theoretical or infinitesimal role in bringing about an injury or illness is not a substantial factor. (Rutherford, supra, at p. 969) Consequently, the substantial factor standard "is a relatively broad one, requiring only that the contribution of the individual cause be more than negligible or theoretical." (Rutherford, supra, at p. 978.) At "a level of abstraction somewhere between the historical question of exposure and the unknown biology of carcinogenesis," Rutherford poses this question as defining the issue whether a particular plaintiff's exposure is the legal cause of her disease: "Taking into account the length, frequency, proximity and intensity of exposure, the peculiar properties of the individual product, any other potential causes to which the disease could be attributed . . . , and perhaps other factors affecting the assessment of comparative risk, should inhalation of fibers from the particular product be deemed a 'substantial factor' in causing the cancer? " (Rutherford, supra, 16 Cal.4th at p. 975.)