Ingredient glossary
Depends on contextPolypropylene (PP)
What it is
Polypropylene is a lightweight, durable, low-cost plastic that is the most common material for baby-bottle bodies in the US. It also shows up in glass and stainless systems as screw rings, collars, caps, stoppers, vent parts and internal reservoir tubes, so a bottle sold as glass or steel can still have polypropylene in its fluid path.
Why it's flagged
A 2020 study in Nature Food (Li and colleagues) tested polypropylene infant feeding bottles under formula-preparation and sterilization conditions and measured microplastic release as high as 16,200,000 particles per litre, which sterilization and high-temperature water significantly increased. It also modelled infant intake of 14,600 to 4,550,000 particles per person per day across 48 regions. The study measured particles released, not illness.
How concerned to be
The study is evidence of exposure potential and not of harm, and the health significance of infant ingestion at those measured levels has not been established. We treat it as a reason to prefer a short fluid path with as little polypropylene as practical, not as a finding that any bottle is unsafe. It never overrides the CDC's hot-water guidance for babies under 2 months, born prematurely or with weakened immune systems.
What it's called on a label
Look for any of these on an ingredient list: PP, Polypropene, Plastic #5, Resin code 5, Polypropylene, PP #5, 5 PP.
Sources
- Microplastic release from the degradation of polypropylene feeding bottles during infant formula preparation, Nature Food (2020)
- Infant Formula Preparation and Storage, U.S. Centers for Disease Control and Prevention (2026)