Many mothers decide to bottle-feed their babies instead of breastfeeding them. But now an experiment reveals that when bottled milk is prepared, significant amounts of microplastics are released from the plastic of the vial. Per liter, the polypropylene of a baby bottle can release up to 16 million microplastic particles and billions of nanoparticles to the liquid. The hotter the water being filled in, the higher the release of these plastic particles, the researchers report. They estimate that an infant in Europe and North America absorbs an average of more than two million microplastic particles per day.

Plastics have long been ubiquitous in our everyday lives, because they are versatile and affordable. But the consequences are vast amounts of plastic waste and microplastics that enter the environment and pollute oceans, waters, soils, and even the air. Studies also show that the plastic particles, which are between a few microns and five millimetres small, are already found in our food and in beverages. It is estimated that each person absorbs an average of around 100,000 such particles per year – via drinking water, breathing air, but also the consumption of fish, seafood or honey. Researchers have also detected microplastics in the human stool. The widely used plastic polypropylene accounted for the largest proportion of the particles in the feces. So far, however, there is no comprehensive overview of the mechanisms behind humans’ plastic intake.

Up to 16 million microplastic particles per litre

Dunzhu Li of Trinity College’s AMBER Research Centre in Dublin and his team are now contributing to this. They investigated whether and how much microplastics are released when using bottled for infant feeding. “These baby vials are mostly made of polypropylene and are regularly exposed to hot water and intense shaking when preparing bottled milk,” the researchers explain. For example, the World Health Organization (WHO) recommends first rinsing the vials with boiling water and then stirring the milk powder with hot water of at least 70 degrees to keep germ intake as low as possible. Based on this recipe, Li and his team first filled ten common baby bottles made of polypropylene with boiling, then 70 degrees hot water, and then determined the amount of plastic particles in the water using laser spectroscopy and scanning force microscopy.

The analyses revealed a strong release of microplastics in all vials. The concentrations determined ranged from 1.3 to 16.2 million microplastic particles per litre. This is significantly more than the usual values of up to 1000 particles per litre for drinking water and therefore points to the polypropylene of the vials as the source of contamination, the researchers write. The high values remained similarly high even after three weeks of regular use of the vials. As Li and his team explain, their values are probably still greatly underestimated. Nanoparticles below 0.8 microns were not detected by their filters. However, in complementary analyses of samples of baby vial water, they found trillions of around 100 nanometers of small nanoparticles per liter. In total, almost two thirds of the recorded particles were smaller than 20 microns.

The scientists then calculated what this release of microplastics means for infants worldwide. The basis was the contamination identified in their experiment, the average daily milk intake of an infant, the respective market share of polypropylene baby bottles and the proportion of mothers who do not breastfeed their babies but feed them by vial. “We found that an infant absorbs an average of 1.58 million plastic particles per day through bottle feeding on a global average,” Li and his colleagues report. However, the amount varies greatly from region to region: in the countries of Asia and Africa, where more mothers breastfeed their children, the contamination is lowest, at just over 500,000 to just under 900,000 particles per day. In Europe, North America and Oceania, on the other hand, the highest number was 2.1 to 2.6 million microplastic particles per infant per day.


According to the researchers, one of the main reasons for the high contamination of the baby vial contents is the high temperature to which the plastic is exposed when preparing bottled milk. “The sterilization recommended by the WHO is increasing the release of the microplastic,” Li and his team said. On the other hand, if the baby milk is stirred at lower temperatures, the load decreases, as additional tests have shown. The scientists therefore recommend to cool the vials first after sterilizing them using boiling water and then rinse them three times with cold, boiled water. The milk should also be stirred with the 70 degree hot water in a non-plastic container and then put into the vial. The researchers advise against reheating bottled milk that has already been stirred in the vial.

“We have to accept that plastics permeate our modern lives and that they release microplastics and nanoplastics when used on a daily occurrence,” says Li. However, the researchers stress, the sheer amount of microplastic particles does not yet tell us whether and how harmful this is to the health of children and adults. “The last thing we want is to cause unnecessary panic among parents,” says Li’s colleague John Boland. “Because so far we simply don’t have enough information about the potential effects of microplastics on the health of young children.” Hanns Moshammer, an environmental physician from the Medical University of Vienna, who was not involved in the study, sees it in a similar way: “There is still a need for research. There is probably a grain size where absorption of microplastics from the intestine is relevant or possible,” he says. “However, in the case of a healthy infant, I would not assume a particularly relevant intake according to current knowledge.”

Elena Mars

Born in London, England and raised in Orlando, FL, Elena graduated from the University of Central Florida with a bachelors' degree in Health Sciences. She later received her masters' in Creative Writing  from Drexel University. She writes part-time for the Scientific Origin and focuses mostly on health related issues.