A Seattle-based law firm has filed a class action lawsuit against Pacific Market International (PMI), the parent company of Stanley, which manufactures the viral Stanley tumblers. The class action lawsuit alleges that Stanley deceived customers by not adequately disclosing that there was lead in its products. 

“As the manufacturer and designer of these products, the Stanley Defendants knew, or reasonably should have known, about this lead issue for years but chose to conceal it from the public presumably to avoid losing sales,” the suit states, per NBC News.

In January, TikTok influencers began testing the surfaces of Stanley tumblers using at-home testing kits. Many found that the tumblers contained lead, and posted their findings online. 

Stanley responded saying that while lead is used in the manufacturing process, it is covered by stainless steel and is therefore unlikely to contaminate the drinks inside, unless the barrier at the bottom of the tumbler is removed.

“At Stanley, one of the key features of our products is our vacuum insulation technology, which provides consumers with drinkware that keeps beverages at the ideal temperature. Our manufacturing process currently employs the use of an industry standard pellet to seal the vacuum insulation at the base of our products; the sealing material includes some lead. Once sealed, this area is covered with a durable stainless steel layer, making it inaccessible to consumers,” Stanley wrote in a statement on its website. 

The colorful Stanley "quencher" tumblers have become a favorite among online influencers and have been touted for their ability to keep drinks at a consistent hot or cold temperature for hours on end. In 2023, Stanley saw its revenue increase by tenfold as the product was hyped across social media.

PMI has not yet responded to TIME’s request for comment regarding the lawsuit.

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