Up to 50 million Americans who have smoked "light" cigarettes believing them less dangerous to their health than regular cigarettes are eligible to participate in a class-action lawsuit certified yesterday.

U.S. District Court Judge Jack Weinstein ruled that plaintiffs' attorneys had presented "substantial evidence" to show that eight cigarette makers defrauded U.S. consumers of billions of dollars by using bogus health claims to inflate the prices of low-tar cigarettes.

At issue, Weinstein said, "is not whether a fraud case can be proven, but whether damages can be" calculated for smokers who allegedly were unaware of a fact the tobacco companies have known since the late 1960s: Consumers of low-tar cigarettes smoke more intensively to compensate for their nicotine addictions, putting them at elevated health risks.

Plaintiffs' attorney Michael Hausfeld, who built the smokers' legal claims around a federal racketeering statute, said he will prove damages of $26 billion to $200 billion through statistical models that estimate how much Americans allegedly were overcharged for "light" cigarettes and how much faster they would have quit smoking had not "light" cigarettes been marketed so effectively since the early 1970s.

Unlike state and federal tobacco lawsuits that centered on medical damages, the lawsuit known as Schwab v. Philip Morris states an economic fraud claim. One of the plaintiffs' expert witnesses, Jeffrey Harris, a practicing physician as well as a professor of economics at the Massachusetts Institute of Technology, estimates "light" cigarettes lose up to 92 percent of their market value once their health dangers are exposed.

Defendants, including Philip Morris USA, owned by Altria Group, and Reynolds American's R.J. Reynolds Tobacco unit, said they would appeal the judge's ruling. Tobacco lawyers contend the four-year statute of limitations for racketeering claims significantly reduces their financial exposure. They also contend they should not be required to retry issues of fact and law already decided in other courts, including the Justice Department's recently concluded racketeering lawsuit against the tobacco companies in Washington, D.C.

In the Justice Department case, the same defendants were ordered to remove such descriptions as "light" and "ultra light" from their cigarette packaging but did not have to pay $14 billion sought by government prosecutors to counter their marketing efforts.

Altria stock fell 6.4 percent yesterday to $77.06, and Reynolds American's shares dropped 3.7 percent to $59.75 after the class-action certification was announced. But Citigroup analyst Bonnie Herzog advised the company's clients: "We think the selloff of all the stocks is overdone, and investors should be buyers."

In an investors' note, Herzog expressed confidence that a federal appeals court would reverse Weinstein's ruling and "decertify" the class of "light" cigarette smokers, "given the past success the tobacco industry has experienced" in contesting the judge's decisions, including one previous class-action lawsuit.