Mold and Bacteria Inside Squeeze Bath Toys: What the Research Actually Found

What the research actually measured

In 2018, a team from Eawag (the Swiss Federal Institute of Aquatic Science and Technology), ETH Zurich, and the University of Illinois published the first controlled study of what grows inside hollow bath toys, in the journal npj Biofilms and Microbiomes. The design was deliberate: new toys were split into two groups over 11 weeks, one bathed in clean tap water and the other in water spiked with the kind of soil real bath water actually contains — soap, body fluids, and bacteria.

Every toy in both groups developed a biofilm — a slimy microbial layer — on its interior surface. The researchers measured between 5 million and 75 million microbial cells per square centimeter inside the toys. Fungal species were found in nearly 60% of the toys exposed to dirty-water conditions, and potentially pathogenic bacteria, including Pseudomonas aeruginosa and Legionella species, turned up in about 80% of all the toys tested — including some in the clean-water group.

That last point is worth sitting with: even toys bathed only in clean tap water still grew a biofilm and, in a majority of cases, hosted potentially pathogenic bacteria. The dirty-water group grew more of it, and grew fungi more often, but the clean water alone wasn't a fix.

Why the hole is the actual problem

The study also looked at what was driving the growth, and the answer is mostly structural rather than about any one household's cleaning habits.

A hollow toy with a single small hole lets water in easily but drains poorly. What's left behind is a warm, dark, low-oxygen, nutrient-rich cavity — skin cells, soap residue, and bath water all sitting somewhere air can't easily dry it out. That combination is close to ideal for biofilm growth, and it's largely independent of how often the toy gets rinsed, because the interior surface is difficult to reach with a rinse in the first place.

The plastic itself also mattered. The researchers found that softer, more flexible polymers released more carbon compounds into the water than harder plastics did, and that additional carbon fed more microbial growth. So a toy's material composition isn't irrelevant — but shape is the dominant factor. A solid toy, or one with no enclosed cavity at all, has nowhere for the biofilm described in this study to establish itself, regardless of what it's made from.

A separate question: what the toy is made of

Bath squirties and other hollow squeeze toys are often made of PVC (polyvinyl chloride), which is rigid on its own — it has to be softened with plasticizers to become squeezable, and phthalates are the plasticizer historically used for that job. The U.S. has restricted several specific phthalates in children's toys and child care articles since a Consumer Product Safety Commission rule finalized in 2017 took effect in 2018, capping eight named phthalates at more than 0.1% concentration.

That's a real, separate improvement, and it addresses a different question than the one the 2018 biofilm study answers. A toy can be phthalate-compliant and still be exactly the shape that traps water and grows a biofilm — the chemical rule governs what the plastic contains, not whether the toy has a hole that never fully drains. Our TPR explainer covers why TPR and silicone, which don't need plasticizers to stay soft, are generally an easier material to trust — but material alone doesn't solve the biofilm problem if the toy is still hollow with a poorly-draining hole.

What to actually do about toys already in the tub

Throwing out every bath toy in the house isn't necessary, and it isn't what the research supports either — the study found biofilm growth is close to universal for this toy shape, not a sign that any specific toy or household did something wrong. A few things do help:

• Air-dry toys fully between uses, somewhere with airflow rather than sealed in a mesh bag while still wet — a closed bag holds in the same moisture the study found the biofilm needs.
• Rinse with clean water, not the bath water itself, after each use, and periodically hand-wash with soap or run dishwasher-safe toys through a cycle.
• Check what you can't see. For a hollow toy, hold it up to a light or squeeze it gently over a sink; if anything discolored or cloudy comes out, that toy is a candidate to retire rather than keep cleaning.
• When replacing toys, favor solid construction or a wide, fully-drainable opening over a toy whose only opening is a narrow squirt hole — that single design choice is what the research points to as the actual driver of interior contamination, more than the plastic type or how diligently any one household rinses.

See the toys hub for more on how SafeMama evaluates play materials generally, including the TPR-versus-PVC distinction referenced above.

The other soft play material parents ask about for the same reason — hands in it, then in a mouth — is play dough. Our play dough showdown covers what Hasbro actually discloses about Play-Doh, and the retail alternatives that publish a full ingredient list.