A mother’s pictures on social media highlight the health hazards that can occur when mold grows inside the lids on children’s drinking cups.

The images sent a shock wave through social media.

Photos of a child’s sippy cup with thick black mold growing inside it were posted in late February by a parent in Canada to the Facebook page of the cup’s manufacturer.

The images set off a whirlwind of parental indignation.

Since that first post, the manufacturer, Tommee Tippee North America, has replied to the concerns on social media. There was also an apology added on the company web site along with a way for parents to get replacement parts.

To see dark mold living inside the cup that a toddler drinks from every day is an unnerving experience for any parent.

With all that parents have to stay on top of in regard to toddlers — adequate nutrition, proper sleep — is the potential for mold to grow inside a sippy cup yet another thing to worry about?

Dr. Claire McCarthy, a pediatrician at Boston Children’s Hospital and assistant professor of pediatrics at Harvard Medical School, told Healthline via email that mold growing inside a sippy cup is likely quite common.

“Almost by definition, because the way sippy cups work, fluids can get trapped in whatever is being used as the mechanism — and when fluids get trapped and left there, mold can grow,” McCarthy said.

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Mold needs three elements to survive: moisture, warmth, and food.

In the case of the Tommee Tippee cups, it appears the mechanism, or stopper, in question isn’t a solid piece of plastic. Rather, it’s two pieces fused together. The small, hollow center of the stopper can trap liquid and eventually cause mold to grow.

But even with a solid stopper, it’s important to keep cups clean.

Consider a day in the life of a sippy cup. It goes from the house, to the diaper bag, to the car. It also goes along to day care and is taken on errands or to the park.

All the while, liquid is sloshed about and warmed to the right temperature by the little hands carrying the cup around. When the bacteria in your child’s mouth make contact with the sipper, that perfect breeding ground sets the stage for mold to grow.

McCarthy said that while it’s upsetting to think about your child drinking from a sippy cup laced with mold, it’s important for parents to know that not all molds are toxic.

“But some of them are literally toxic [because] they make toxins,” she said. “Some can cause illnesses if breathed in, and some people can be allergic to them.”

The hard part, McCarthy said, is that “most children aren’t going to have symptoms of mold ingestion,” so it can be challenging to determine any health issues.

“If the child is having new, unusual symptoms or an unexplained rash, then it is worthwhile calling the doctor,” she said. “Otherwise, taking a deep breath and relaxing is a good idea — as is cleaning or throwing out the cup.”

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Since the initial post, the Tommee Tippee North America Facebook page has been hit with more than 300 comments. They include dozens of photos of valves pried open that show mold in plain sight.

The post has also been shared more than 45,000 times, according to published reports.

Jessica Rushton, a mother of three, is one of the parents who complained on the Tommee Tippee Facebook page. Her youngest child uses the Tommee Tippee cups and months ago the child started to complain about a funny taste.

“Every time she drank from one of the cups she told me, ‘mommy yucky,’ [and] I didn’t really understand because she typically drinks the same few drinks, and I hadn’t changed anything in that sense,” she said. “I washed the cups and valves after every use, and every couple days I would boil the lids and valves to try and ensure they would be as clean and germ free as possible.”

Rushton said she even used a small scrub brush to clean the cups.

“I thought I had a good system going, till I got curious and used a knife to snap one of the valves open. That’s when I started putting two and two together.”

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Healthline requested an interview with the manufacturer’s parent company, Mayborn Group, but did not hear back from them.

The company’s online statement does take responsibility for the issue.

“We want everyone to be happy with our products and we always want to exceed expectations. We’ve therefore listened to our customers, and will be launching a new [s]ippee cup in the next few months. In the meantime, we are making the transparent, see-though values shown here immediately available to parents that have concerns,” the statement said.

The company also posted a video that shows how to clean the lids.

Rushton said it took her a week to finally get through to someone on the hotline that Tommee Tippee set up. She was promised new valves within 10 business days, but it has been three weeks and nothing has arrived.

“I think their eagerness to commit to sending replacement valves or cups would be great, if there was actually follow through,” she said. “I have yet to meet a single person who has spoken to somebody at their hotline that has actually received anything, anything at all. So this is frustrating.”

It’s unclear how many replacement valves have been requested and how many have shipped.

While this moldy cup problem may be blamed on faulty design, McCarthy said it’s a good opportunity to remind parents to not have sippy cups sitting around for children to drink from at will.

“It’s important to remember that children shouldn’t be sipping from sippy cups all day anyway,” she said. “Doing so can lead to cavities and weight gain. It’s fine to use a sippy cup at a meal, and perhaps fill one with water to take to the park, but otherwise, they aren’t needed.”