Researchers in Ireland did a study over a period of 21 days, and they found that the bottles made from polypropylene (commonly used plastic for food containers) released between 1.3 and 16.2 million plastic microparticles per liter. They also followed official guidelines from the World Health Organization on sterilization and formula preparation conditions.

They then used this data to model the potential global infant exposure to microplastics from bottle feeding, based on the national average rates of breastfeeding. Results are alarming with an estimate of 1.6 million plastic microparticles being ingested by bottle fed babies for the first 12 months of their lives. 

Potential health risks
Published in the Nature Food journal, it said that sterilization and exposure to high water temperatures had the biggest effect on microplastic release, going from 0.6 million particles per litre on average at 25C to 55 million/litre at 95C. The authors told AFP that the aim of the research was "not to worry parents" about the potential health risks of bottle microplastics. "We have communicated, as strongly as we can, that we do not know the potential health risks of infant ingestions of microplastics," said the team, from Trinity College Dublin. "This is an area of research we are now actively pursuing."
Even more alarming is that there’s a possibility that babies in developed nations such as North America and Europe ingest a higher number of microplastic particles—2.3 million and 2.6 million, respectively.

Prevention tips
The levels could be lowered by taking a few additional steps, including rinsing bottles with cold sterilised water and preparing formula milk in a non-plastic container before filling the bottle.
Fay Couceiro, Senior Research Fellow in Biogeochemistry of the University of Portsmouth, said Monday's research highlighted the "urgency for studies on microplastic impacts on human health.” She said that it was important not to be "alarmist" when it came to bottle feeding, which many parents prefer for a variety of reasons.
"The risks from not sterilising bottles or using hot water are well understood and very real, and these known risks of disease must outweigh that of microplastic production until their health risks are understood," said Couceiro, who was not involved in the study.