Lead
"BPA-free" became a standard label around 2012, and most parents absorbed the message. Then, in 2020, a study in Nature Food quantified the microplastic particles released from polypropylene (PP) bottles during formula preparation — and the material question moved into new territory.
More information does not always make decisions easier. Glass or polypropylene? PPSU? Does nipple shape affect oral development? This article works through what the peer-reviewed literature and regulatory record actually establish: what is known, what is uncertain, and what practical choices follow from that.
The microplastics question: what the 2020 study measured
Li and colleagues (2020) tested ten commercially available PP baby bottles using the WHO-recommended formula preparation procedure — dissolving formula in water at 70°C or above. Microplastic particle counts from the bottles reached up to 16.2 million particles per liter of prepared formula, with estimated infant daily exposure ranging from hundreds of thousands to several million particles depending on the region [1].
Two caveats are essential. First, the study quantified release — it did not measure health outcomes. As of this writing, no longitudinal study has established what this level of microplastic ingestion does to infant health [1]. Li and colleagues make this explicit in the paper. Second, the study identified a practical mitigation that does not require discarding all PP equipment: prepare formula in a stainless-steel or glass container, then transfer to the PP bottle. This substantially reduces the time the formula spends in contact with PP at high temperature and cuts measured particle release sharply [1]. This is not a call to abandon PP bottles entirely — it is a handling adjustment.
From BPA to BPS: the substitution problem
Bisphenol A (BPA) was phased out of infant bottles after endocrine-disruption concerns accumulated in the research literature. The European Union banned BPA in bottles in 2011; the US Food and Drug Administration confirmed market withdrawal in 2012. In Japan, industry self-regulation has achieved the same result, and polycarbonate (which contains BPA) bottles are effectively absent from the current market.
The problem is what replaced BPA. Bisphenol S (BPS) and related structural analogs spread as "BPA-free" alternatives. Rochester and Bolden's 2015 systematic review in Environmental Health Perspectives found that BPS and bisphenol F display hormonal activity similar to BPA in animal models; whether the human health implications are equivalent remains under active investigation [3].
The practical consequence: a "BPA-free" label does not exclude BPS. For parents trying to minimize bisphenol exposure, material composition beyond the BPA question needs to be verified.
Material-by-material comparison
Polypropylene (PP): The most widely used material. Lightweight, inexpensive, impact-resistant, and BPA-free. The downside, as detailed above, is elevated microplastic release during high-temperature preparation [1].
Polyphenylsulfone (PPSU): a high-performance plastic used in medical equipment, valued for heat resistance and chemical stability under repeated sterilization: A medical-grade engineering plastic. Free of BPA, BPS, and phthalates. Thermal stability up to 180°C; repeated high-temperature sterilization cycles do not compromise its chemical integrity — this was confirmed in a materials study by Eckardt and colleagues (2018), which found no migration of polymer-related substances from PPSU bottles at levels of toxicological concern [4]. PPSU bottles are heavier and more expensive than PP.
Glass: The lowest concern from a chemical-migration standpoint. The practical trade-offs — weight and breakability — are real, though glass is more manageable during the newborn phase when the infant is held for every feed and the bottle never leaves the caregiver's hands.
Silicone bodies: A minority of brands use food-grade silicone for the bottle body. Silicone has good thermal stability and flexibility, but long-term migration data for silicone-bodied infant bottles are limited in the published literature.
Nipple shape and oral development
Bottle nipple selection involves more than material: shape and flow-hole type affect the infant's sucking mechanics.
The oral mechanics of breastfeeding have been documented in detail by Donna Geddes and colleagues using ultrasound imaging. The infant applies both compression (tongue peristalsis: a rolling, wave-like motion of the tongue from front to back used to express milk against breast tissue) and negative pressure to extract milk — a coordinated dual mechanism [5]. Bottle nipples vary in how closely they replicate this coordination.
Flow-hole design matters. Round-hole nipples set flow rate by hole diameter; flow is essentially fixed. Slit (cross-cut) nipples function as a one-way valve and vary flow in response to the infant's suction pressure. How this difference affects sucking pattern and fatigue has been studied in preterm populations, but large RCTs in typically developing infants are limited [see Translator notes].
The concern sometimes called nipple confusion: the worry that introducing a bottle teaches a different sucking pattern, causing the baby to struggle with or reject the breast — that bottle feeding will cause an infant to reject the breast — is common in clinical practice but its physiological mechanism remains debated. Breastfeeding and bottle feeding require different oral configurations and suction pressures, which may contribute to transition difficulty. At the same time, breastfeeding is regularly maintained alongside bottle use when adequate lactation support is available. The deterministic framing — "once you introduce a bottle, you lose breastfeeding" — is not supported by the evidence [5].
For practical nipple selection, observing the actual feed is more informative than choosing by shape or brand alone: if a feed takes 15 to 20 minutes and the infant finishes without signs of strain or gas, the flow rate is likely appropriate for that infant's developmental stage. That benchmark holds across nipple types.
Putting it into practice
If the goal is reducing microplastic exposure: prepare formula outside a PP bottle (stainless steel or glass vessel), then transfer [1]. Alternatively, switch to PPSU or glass bottles.
If the goal is minimizing bisphenol exposure: choose PPSU or glass. "BPA-free" labeling alone is insufficient because it leaves BPS status unaddressed [3].
If bottle choice is primarily driven by durability and budget: PP remains the most available option. The mitigation for its main documented concern is a simple change to preparation protocol rather than requiring a full equipment swap.
When navigating the combination of bottle feeding and breastfeeding, a lactation consultant is the appropriate resource for individualized guidance — more so than nipple selection decisions.
Summary
PP bottles release microplastics in measurable quantities during high-temperature formula preparation; this is an established quantitative finding, but health effects at these exposures remain unknown [1]. BPS, which replaced BPA in many "BPA-free" products, carries similar endocrine-disruption concerns in the current literature [3]. PPSU and glass are the materials with the smallest current chemical safety questions [4]. Nipple shape and oral development is an area where research continues to develop, and flow-rate appropriateness — observable in the feed itself — is the most actionable criterion at hand.
References
- Li D, Shi Y, Yang L, et al. Microplastic release from the degradation of polypropylene feeding bottles during infant formula preparation. Nat Food. 2020;1(11):746–754. doi:10.1038/s43016-020-00171-y. PMID: 37128027.
- European Commission. Directive 2011/8/EU amending Directive 2002/72/EC as regards the restriction of use of Bisphenol A in plastic infant feeding bottles. Off J Eur Union. 2011;L 26:11–14.
- Rochester JR, Bolden AL. Bisphenol S and F: A Systematic Review and Comparison of the Hormonal Activity of Bisphenol A Substitutes. Environ Health Perspect. 2015;123(7):643–650. doi:10.1289/ehp.1408989. PMID: 25775505.
- Eckardt M, Kubicka M, Schrenk D, et al. Polyphenylsulfone (PPSU) for baby bottles: a comprehensive assessment on polymer-related non-intentionally added substances (NIAS). Food Addit Contam Part A. 2018;35(6):1115–1127. doi:10.1080/19440049.2018.1448963. PMID: 29537947.
- Geddes DT, Gridneva Z, Perrella SL, et al. 25 Years of Research in Human Lactation: From Discovery to Translation. Nutrients. 2021;13(9):3071. doi:10.3390/nu13093071. PMID: 34578950.