Bottle-fed babies are consuming millions of microplastic particles in their milk, new research shows.
The research published in the journal Nature Food shows the high heat used to sterilise bottles made of polypropylene - or PP - significantly increased the release of microplastics.
Co-author of the study, Professor John Boland, at Trinity College in Ireland, told Morning Report the team were surprised by the large quantities found while preparing new bottles of formula using WHO guidelines.
"What we found is you have at least a million microplastics and in fact many trillion nano-plastics actually."
Particle shedding accelerated at higher temperatures, and shaking bottles also increased their release, he said
"But even if you reduce the temperature of the water down to room temperature, it turns out you get at least a hundred thousand or several hundred thousand microplastics."
It was not yet known whether ingesting microplastics was harmful to children, Professor Boland noted.
However, he recommended using glass baby bottles, or preparing formula in another non-plastic container and allowing it to cool before putting it in the bottle to feed a baby.
Researchers estimate almost 69 percent of bottles are made of polypropylene plastic, including most of those sold in New Zealand.
Philips Avent - one of the only baby bottle companies to supply full product specifications on its website - does have products made of polypropylene.
An Australian-based spokesperson said the company was aware of the research but unable to comment at this stage.
A director of Newborn Brands New Zealand, Jock Schoeller, whose company distributes the popular Tommee Tippee products, said he was also waiting on guidance from the UK-based company.
He confirmed the company also had products made of polypropylene.
However, he pointed out it was not yet known whether microplastics had any effect on human health.
"What I can say is, if people are concerned, they can switch to glass [bottles], and more people are already doing so."