Microplastics, tiny fragments of plastic less than 5mm in size, are a hidden form of pollution known to be contaminating waterways and the world's oceans - and now, scientists say, the human digestive system.
And according to a new pilot study, infants' stools contain up to 10 times higher concentrations of a microplastic called polyethylene terephthalate (PET) than adults.
Both adult and infant samples contained roughly the same average level of polycarbonate (PC) microplastics, according to the research published in Environmental Science & Technology Letters journal of the American Chemical Society.
The new study, by authors who received both US and Chinese government funding, warns that while "human exposure to microplastics and the health effects thereof are a global concern, little is known about the magnitude of exposure".
"Infants could be exposed to higher levels of microplastics through their extensive use of products such as bottles, teethers and toys," according to the researchers, but they say "larger studies are needed to corroborate these findings".
A report from Canadian researchers previously found that humans could be consuming between 39,000 and 52,000 microplastic particles a year.
It follows claims from one researcher that phthalate pollution is causing babies to be born with small penises.
The World Health Organization (WHO) has called for urgent research on the risks of microplastic to human health after confirming that tiny fragments are found in drinking water.
In its first assessment of plastic pollution, the WHO concluded that there was evidence from 50 studies that microscopic particles are found in water and could be absorbed by the human body.
The WHO reported that particles too small to be seen with the naked eye are likely to be absorbed by the human body, but "firm conclusions" on the risk "cannot yet be determined".