First, Stanley cups were all over social media because so many people wanted them. Now, the oversize tumblers are back in the spotlight over fears they may contain lead.

Videos on social media sites such as TikTok show people breaking out a lead testing kit and trying it on their Stanley cups and other travel flasks.

It’s true: There is some lead sealed within the base of some brands of travel drinking cups — including the wildly popular brand Stanley.

Lead is used as part of the tumbler’s vacuum insulation and is covered by a stainless steel layer that protects consumers from lead exposure, according to the Stanley cup manufacturer, Pacific Market International.

“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,” a Stanley spokesperson said in an emailed statement. “Once sealed, this area is covered with a durable stainless steel layer, making it inaccessible to consumers.”

“Rest assured that no lead is present on the surface of any Stanley product that comes into contact with the consumer nor the contents of the product,” the statement said.

But people on social media are nervous, considering the dangers of any exposure to lead. The toxic metal can lead to kidney problems, anemia, reproductive issues and developmental problems, according to the US Centers for Disease Control and Prevention. And Stanley cups are everywhere.

The tumblers, which come in a rainbow of colors and hold up to 40 ounces of water while still fitting in a car cup holder, have become both a status symbol and a wellness essential for those who ascribe to the “clean girl” beauty trends of hydrated, natural skin.

Stanley cups are in the hands of beauty bloggers, the cars of parents at school pickup, and even on the Christmas lists of many kids this past year. Some parents have reported their children were bullied by peers if they came to school with any other brand of cup.

Some caution is warranted, said Jane Houlihan, research director for Healthy Babies, Bright Futures, an alliance of nonprofits, scientists and donors with a stated mission of reducing babies’ exposures to neurotoxic chemicals.

“If the cup stays intact, there’s likely no lead exposure risk for consumers. But if that bottom seal comes off, all bets are off,” Houlihan said.

“Lead is so toxic you just can’t take chances with it,” she said in an email. “If a company has to rely on their product remaining perfectly intact in order for it to be safe, that company has a basic material safety problem that they are passing on to their customers.”

If the base cap of a Stanley cup does come off and exposes the seal, which is rare, the cup is eligible for replacement under the lifetime warranty, according to a statement from Stanley.

Not just Stanley cups

It’s not just the Stanley brand that utilizes lead. Other brands have been subject to recall over their use of the material, according to the US Consumer Product Safety Commission.

In November, the federal agency announced the recall of about 84,000 Tiblue Stainless Steel Children’s Cups and 3,600 Klickpick Home Children’s Cup Sets sold on Amazon due to “an accessible solder bead with levels of lead that exceed the federal lead content ban.”

Earlier that month, some 1,600 PandaEar and 200 Laoion stainless steel children’s cups sold on Amazon were recalled for the same reasons, according to the CPSC website.

And in July of last year, around 346,000 Cupkin stainless steel children’s cups were recalled by the manufacturer because they contained levels of lead that exceeded the federal lead content ban of 100 parts per million. (For water, the action level is 15 parts per billion, according to the US Environmental Protection Agency.)

“In general CPSC has strict regulations on lead in the content, especially for children’s products. We have issued several recalls for lead in children’s products,” said CPSC Press Secretary Patty Davis.

“And we encourage consumers to report if they’re concerned about a safety hazard with a product to tell us about it at our website, which is http://www.saferproducts.gov/,” Davis said.

Why lead?

Why would lead be found in a drinking cup at all?

“Using lead to solder is a very old manufacturing practice, which had been quite common,” said Olga Naidenko, vice president of science investigations at the Washington, DC-based Environmental Working Group, a nonprofit, nonpartisan organization “dedicated to protecting human health and the environment.”

“In insulated cups, lead was used in solder that seals the vacuum between the inner and the outer layer of a cup,” she said via email. “Many manufacturers are now stating that they are using alternatives to lead for sealing vacuum in those types of cups.”

Any potential lead exposure is unacceptable, and companies should move to make these products without lead, Naidenko added.

No safe levels of lead

There is no safe level of lead for children, according to the US Consumer Product and Safety Commission. “Even low levels of lead in blood have been shown to affect learning, ability to pay attention, and academic achievement,” the CDC said.

Lead “bio-accumulates” in the body, which means it stays and builds up over time, so ongoing exposure, even at extremely low levels, can become toxic. Children, especially fetuses and infants are the most vulnerable, says the Environmental Protection Agency, because it takes very little lead exposure to damage a child compared with an adult.

“A dose of lead that would have little effect on an adult can have a significant effect on a child. In children, low levels of exposure have been linked to damage to the central and peripheral nervous system, learning disabilities, shorter stature, impaired hearing, and impaired formation and function of blood cells,” the EPA states on its website.

The main source of lead exposure in the United States comes from inhaling dust or eating particles contaminated by paint chips. That’s because lead was a common additive in house paint, gasoline and many other materials for years before its toxicity was known.

Between 10% and 20% of our exposure to lead comes from contaminated water, according to experts. It’s even worse for the youngest and most vulnerable: Babies can get between 40% and 60% of their exposure to lead by drinking formula mixed with contaminated water.

Many experts suggest that parents get their child’s lead level tested at ages 1 and 2, and possibly more often, depending on the area of the country. The test is easily done by a pediatrician, or at a local state, county or city department of health.