What is the burden of proving causation in asbestos-related lawsuits?

The primary issue at trial was causation. To prove causation in an asbestos-related lawsuit, "the plaintiff must first establish some threshold exposure to the defendant's defective asbestos-containing products, and must further establish in reasonable medical probability that a particular exposure or series of exposures was a 'legal cause' of his [or her] injury, i.e., a substantial factor in bringing about the injury. . . . [T]he 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 . . . contributing to the plaintiff's or decedent's risk of developing cancer." (Rutherford v. Owens-Illinois, supra, 16 Cal.4th at pp. 982-983, italics omitted.)