In fact, KN95 masks offered on Amazon and through other retailers are being sold without authorization for use in health care settings from the Food and Drug Administration, which last July revoked its emergency use authorization for imported masks that lack approval from the Centers for Disease Control and Prevention — a category that includes all KN95s from China.

They include brands like Boncare, which is produced by a company that has repeatedly failed federal testing standards; Yotu, whose manufacturer has also failed European Union testing; and ChiSip, an Amazon top seller whose manufacturer, Chengde Technology, was cited by the C.D.C. for falsely claiming approval by federal regulators.

All but a handful of the 50 best-selling KN95 masks on Amazon are plagued by similar problems, according to an analysis of sales data published by the marketing analytics firm Jungle Scout. Last month, companies that make or sell masks of dubious quality racked up almost $34 million in sales.

“It’s really the Wild West out there with so many bad actors ripping people off,” said Anne Miller, executive director of Project N95, a nonprofit that connects people to bona fide personal protective equipment.