Oregon's high court ruled unanimously Thursday that smokers must show actual harm to make a negligence claim against cigarette manufacturers — not just the possibility they will be harmed.

The lawsuit, brought by Patricia Lowe on behalf of about 400,000 Oregonians, argued the tobacco companies were negligent because they "knew or should have known that their cigarettes contained toxic and hazardous substances likely to cause lung cancer."

Lowe argued the industry should pay for tests to detect lung tumors at their earliest and most treatable stage.

The court ruled instead that Oregon law has long recognized that "a threat of future physical harm is not sufficient" grounds for a legal claim.

James Coon, who represents Lowe, said the ruling shows the law is trailing behind science.

"Certain toxic products put people at risk for future injury," Coon said, but "medical monitoring is a concept that ancient common law has trouble dealing with, and the court in this case applied old common law concepts without flexing them in any way."

Carl Tobias, a University of Richmond law professor who specializes in torts, or damage claims, agreed.

"It doesn't fit in the box of traditional tort law," Tobias said. "Tort law by definition is after the fact. It aims primarily to compensate for past harm — not to prevent future harm."

But Tobias noted that Justice Martha Lee Walters, in a concurring opinion, left open the possibility the law could change "when science and medicine are able to identify harm before it becomes manifest."

Such techniques may be coming soon, said Thomas Glynn, the American Cancer Society's cancer science and trends director.

"We're probably about two years away before we can say whether we can detect lesions early enough to know what the effect will be," Glynn said.

Ben Zipursky, a Fordham University School of Law professor who specializes in product liability, said it was ironic the ruling came from the same court that recently affirmed a nearly $80 million punitive damages award against tobacco giant Philip Morris after it was struck down by the U.S. Supreme Court.

"This is the very court that has most aggressively ruled against Philip Morris," Zipursky said.

The ruling was similar to those in state courts around the nation in similar cases, despite a move toward loosening the definition of actual harm, he said.

Philip Morris and R.J. Reynolds Tobacco Co., two of the five companies named in the lawsuit, welcomed the ruling in a statement released Thursday.

The other companies were Brown & Williamson Tobacco Corp., Lorillard Tobacco Co. and Liggett Group Inc.