Microplastics in baby bottles: what the research actually shows

In 2020, a team at Trinity College Dublin filled brand-new plastic baby bottles with hot water, shook them and counted what came out. Their headline: polypropylene bottles released up to 16.2 million microplastic particles per litre.

Parents panicked, reasonably. Bottles are supposed to be the boring, safe part. So here’s what the study measured, why another lab couldn’t reproduce it, what we know (and mostly don’t) about harm to babies, and how to cut exposure without breaking the one formula rule that really can’t bend.

What the 2020 study did

The researchers bought ten polypropylene (PP) bottle products that together made up about 69% of the global market in their sales data. They prepared each one the way manufacturers and the WHO tell parents to: sterilise in hot water, let it dry, add 70°C water, shake. In the lab that meant a 5-minute soak at 95°C, then 70°C water shaken for 60 seconds at 180 rpm (Supplementary Note 2). They then filtered the cooled water and counted particles at least 0.8 micrometres across using Raman spectroscopy, which identifies plastics by their chemical fingerprint.

The results, from the paper’s supplementary tables:

Every bottle shed particles. Bottle bodies released between about 1.3 million and 16.2 million particles per litre at 70°C, depending on the product. Accessories made of PP shed too, mostly less.
Heat drove it hard. One leading bottle released about 0.6 million particles per litre with room-temperature water and about 54.7 million per litre with 95°C water (Supplementary Table 2). The other two products tested showed the same upward trend.
Sterilisation raised release further, according to the paper’s abstract.
It didn’t stop. Over a 21-day test of daily sterilising and five preparations a day, release went up and down rather than fading, ranging from about 0.6 million to 14.5 million particles per litre across the test days (Supplementary Table 3).
Formula made no difference. Water, tap water and real formula gave statistically similar counts, so formula didn’t coat the plastic and protect it (Supplementary Note 5).

The particles were mostly thin flakes 0.8 to 5 micrometres across, far thinner than a hair. In a separate test on one product, the team estimated about 7 trillion nanoparticles per litre (Supplementary Note 8). That figure deserves extra caution: it comes from a single bottle product, the particles were counted under a microscope rather than chemically identified as polypropylene, and it rests on subtracting a background count.

Two side findings matter for every kitchen. Plastic kettles released about 11 million particles per litre in a single boil, and microwaving water in a PP lunchbox released roughly ten times as much as pouring the same hot water in (Supplementary Note 2.1). We cover kettles in our kitchen swap guide.

Where the “millions a day” estimate comes from

The study’s most-quoted number is its global estimate: infants up to 12 months old swallowing between 14,600 and 4.55 million particles per day, depending on the region.

That is a model, not a measurement in babies. It multiplies the lab release counts (averaged over the 21-day test) by:

each bottle product’s market share, mined from e-commerce sales data in mid-2019,
national breastfeeding rates, and
daily feed volumes taken from formula brands’ instructions, for example 630 mL a day at 12 months.

Because the breastfeeding rate is part of the formula, the per-capita figures are averages across all babies in a country, breastfed ones included. In the supplementary tables, a 12-month-old works out at about 2.5 million particles a day in the US and 3.1 million in the UK (Supplementary Table 8). Two caveats cut in opposite directions. The authors call their counts a lower bound, since they analysed small areas of each filter and missed anything smaller than 0.8 micrometres. But the counts also assume real-world bottles behave like brand-new lab bottles shaken by machine. Particle counts also aren’t mass: millions of flakes this thin weigh very little, and nobody yet knows whether count or mass matters more for health.

The critics: a lab that found almost nothing

In 2022, a German team tested seven baby bottles using the same method and got a very different answer. Only one bottle produced microplastics above the detection limit, and those particles weren’t made of the same material as the bottle, which points to contamination.

Their explanation is the interesting part. When they filtered the water while still hot (70°C) instead of after cooling, as the Dublin team did, the particle counts changed markedly. Chemical analysis suggested that additives such as fatty acid esters, used to help release bottles from their moulds, can leach into hot water and then come out of solution as it cools, producing particles that can be mistaken for microplastics.

The WHO’s 2022 review of microplastics and health cited this critique, noting concern about the lack of standardized methods for testing packaging, while also concluding that heating plastic containers appears to increase particle release.

So where does that leave you? The direction of the finding (hotter water, more stuff coming off the bottle) is consistent with other studies of heated plastics. The exact numbers are disputed and haven’t been settled by independent replication. We’d treat “millions per litre” as plausible but unproven, and we’d note that “it might be precipitated additives instead” isn’t exactly comforting either.

What we know about the health effects in babies

Short version: not much, and nothing from studies of actual infant health outcomes.

The WHO concluded in 2022 that available data on microplastic toxicity are insufficient for a quantitative assessment of risk to humans. Even the Dublin researchers said they didn’t want to unduly alarm parents given how little is known about effects on infant health.

What exists is lab work:

Cell studies. Particles shed from PP baby bottles triggered oxidative stress and inflammatory signals in human intestinal cells grown in a dish, more so for particles from boiling-water disinfection and microwaving. A different team found particles released by microwaving plastic food containers killed about 77% of human kidney cells in culture, but at 1,000 micrograms per millilitre, a very high dose. Neither study tells us what happens inside a baby.
Human exposure. A small Chinese pilot study of 18 mother-infant pairs found microplastics in infant stool and formula samples, and its questionnaire flagged feeding bottles as one possible source. That’s a hint, not a measurement of how much comes from bottles.

Babies are worth extra caution anyway. They drink a lot relative to their body weight, and their guts, immune systems and hormone systems are still developing. That’s a reason to reduce exposure where it’s easy. It isn’t evidence of harm, and we won’t pretend it is. For the wider picture on plastics in the body, see what microplastics inside you actually means.

The formula safety rule that doesn’t change

Powdered formula is not sterile. It can carry bacteria such as Cronobacter sakazakii (formerly Enterobacter sakazakii), which can cause serious illness and death in infants. The WHO’s guidance is to make up formula with water no less than 70°C, which dramatically reduces that risk, and never to use a microwave. The NHS says to let boiled water cool for no more than 30 minutes so it stays at least 70°C.

Do not lower the water temperature to protect a plastic bottle. Bacteria are a known, serious, immediate risk. Microplastics are an uncertain one. The fix is to change what touches the hot water, not how hot it is.

The researchers’ own suggested steps

Alongside the paper, Trinity College Dublin published the team’s suggested preparation steps for parents using plastic bottles. In summary:

Sterilise the bottle following WHO guidance and let it cool.
Boil water in a non-plastic kettle or pan (glass or stainless steel).
Rinse the sterilised bottle at least three times with sterilised (boiled) water cooled to room temperature.
Make up the formula in a non-plastic container with water of at least 70°C, cool it to room temperature, then pour it into the bottle.
Don’t reheat prepared formula in plastic containers, and avoid microwaves.
Don’t shake formula vigorously in the bottle.
Don’t clean plastic bottles with ultrasonic (sonication) cleaners.

These keep the 70°C rule intact: the hot step just happens in glass or steel. Note that these are the researchers’ proposals. Neither the WHO’s 2007 guidance nor the NHS page includes them, and they haven’t been tested for how much they actually reduce what a baby swallows. If you follow them, the hygiene rules still apply to the extra equipment: the mixing container and anything you stir with must be cleaned and sterilised like the bottle, and the rinse water must be boiled, not straight from the tap (the NHS gives the same instruction for rinsing bottles after cold-water sterilising). Cool the feed quickly, check it on your wrist, and follow the usual time limits for prepared formula.

Glass and stainless steel bottles

The simpler route is a bottle that isn’t plastic. Glass bottles are common: in a German survey of 226 bottles on the market, 28% were glass and 65% plastic. Stainless steel bottles exist too. Glass or steel can’t shed polypropylene, and with them you can follow the NHS method as written.

Two honest caveats. “Not plastic” doesn’t mean “sheds nothing”: the Dublin team noted that some glassware and stainless steel kettles released non-plastic particles of their own (Supplementary Note 2.1), though nobody has shown those are a health concern. And glass can break, so check for chips.

The teat problem

Almost every bottle, glass or not, ends in a teat, and 85% of teats in the German survey were silicone. Silicone isn’t a conventional plastic, but it is a synthetic polymer and it isn’t inert. A 2022 study found that steam-disinfecting silicone teats released micro- and nanoparticles, estimating a baby could ingest more than 0.66 million of them by age one. The authors note the health risks are unknown.

That’s why our standards never call a product “plastic-free” if silicone touches what you eat or drink. A glass bottle with a silicone teat is a big cut in plastic contact, not zero. Steel bottles also often have plastic collars or lids, so check what the hot liquid actually touches.

What to do

If your baby is formula-fed, these are the low-cost steps, in rough order of payoff:

Keep the 70°C rule. Always. It’s about bacteria, and it’s not optional.
Boil water in glass or steel, not a plastic kettle.
Never microwave formula or water in a plastic bottle or container. Health services already say no microwaving for scalding reasons; the plastic is a bonus reason.
Consider glass or stainless steel bottles, which let you follow standard guidance with no extra steps.
If you use plastic bottles, the researchers’ method (mix in a sterilised glass or steel container, rinse the bottle with cooled boiled water, transfer) is a reasonable precaution. Swirl rather than shake hard.
Replace damaged bottles and teats as the manufacturer advises. (In the 21-day test, release fluctuated rather than dropping steadily, so don’t assume a well-used plastic bottle has stopped shedding.)

None of this is proven to improve a baby’s health, because nobody has studied that yet. These are cheap ways to reduce exposure to something we don’t understand well, while keeping the safety rules we do understand. The bigger fix is upstream: bottle makers and regulators should be testing what heated baby products shed, not leaving it to parents with a thermometer and a glass jug.