Asbestos defendants—especially those in take-home exposure cases—should note the Ninth Circuit’s recent opinion in Stephens v. Union Pacific Railroad Company. There, the court affirmed that plaintiffs cannot prove substantial-factor causation without establishing “substantial exposure to the relevant asbestos for a substantial period of time.” While the decision’s reach might be limited—it is, after all, a federal-court interpretation of Idaho law—it provides a sound roadmap for raising evidentiary challenges in substantial-factor asbestos cases.

Stephens involved alleged take-home exposure from an Idaho Union Pacific roundhouse. William Stephens’s father had worked at the facility when Stephens was a child, and Stephens testified that his father’s work clothes had been dusty when returning home. He also testified that he had visited his father at work on a few occasions and had seen men removing and replacing insulation on steam engines. Union Pacific admitted that it probably used asbestos-containing insulation on steam engines at that time, but disputed that steam engines had ever been serviced at Stephens’s father’s workplace. The court recognized that whether Stephens had take-home exposure was “close question” on this evidence, but bypassed that issue.

Instead, the court focused on whether Stephens had sufficient evidence to establish that take-home exposure to Union Pacific asbestos was a substantial factor in causing his mesothelioma. The court wrote that the “liberal” substantial-factor standard “is not without limit.” Instead, the court noted that substantial-factor causation require asbestos plaintiffs to demonstrate “substantial exposure to the relevant asbestos for a substantial period of time.” “Minimal exposure,” the court wrote, is “insufficient” to establish that a defendant’s asbestos was a substantial factor in an injury.

The Ninth Circuit ruled that Stephens’s evidence did not meet that standard. The court reasoned that Stephens had no evidence that his father frequently worked around asbestos, and therefore no evidence that the dust on his father’s clothes contained asbestos. The court observed that Stephens offered no evidence about his father’s duties at the Union Pacific facility, and none that his father worked with or around asbestos. Without evidence that his father worked around asbestos, Stephens could not establish that his father regularly brought asbestos dust home on his clothes. While Stephens’s experts testified that he would have had significant exposure from his father’s clothes, and that that level of exposure would have been a substantial factor in his mesothelioma, the court rejected both opinions because there was insufficient evidence that Stephens’s father had actually worked around asbestos.

While Stephens may directly control only Idaho-law cases in the Ninth Circuit, it nevertheless provides a sound roadmap for defendants in any jurisdiction in cases with questionable or sporadic evidence of exposure.