"I don't want to alarm people," Sanjay Mohanty, the lead researcher behind the new study which has not yet been peer-reviewed, told AFP.

There is no evidence directly showing that microplastics are harmful to human health, said Mohanty of the University of California, Los Angeles (UCLA).

The pilot study instead sought to illustrate yet another little-researched way that these mostly invisible plastic pieces enter our bodies -- chewing gum.

Lisa Lowe, a PhD student at UCLA, chewed seven pieces each of 10 brands of gum, before the researchers then ran a chemical analysis on her saliva.

They found that a gram (0.04 ounces) of gum released an average of 100 microplastic fragments, though some shed more than 600. The average weight of a stick of gum is around 1.5 grams.

People who chew around 180 pieces of gum a year could be ingesting roughly 30,000 microplastics, the researchers said.

This pales in comparison to the many other ways that humans ingest microplastics, Mohanty emphasised.

For example, other researchers estimated last year that a litre (34 fluid ounces) of water in a plastic bottle contained an average of 240,000 microplastics.