A federal appeals court is weighing whether to certify a class action that could engulf more than 330 million people—99% of the United States population—who were exposed to toxic chemicals made by 3M, DuPont and others.

“The class certified here is truly extraordinary,” former Solicitor General Paul Clement, of Washington, D.C.’s Clement & Murphy, told the U.S. Court of Appeals for the Sixth Circuit last month during oral arguments.