A new study reveals that the simple act of opening a metal bottle cap could be adding unexpected microplastics to your favorite drink—potentially making glass bottles the most contaminated of all. Researchers at France's food safety agency looked at beer, water, wine, and soft drinks packaged in different materials and found microplastics across the board, but glass bottles capped with metal stood out, harboring about 50 times more microplastics than their plastic-bottled counterparts. The culprit isn't the metal itself, but rather the polyester-based paint applied to the outer surface of the caps, per the Guardian.
It turns out, during storage and transport, thousands of caps jostle together, causing the painted surfaces to scrape and shed tiny fragments, according to the study published in the Journal of Food Composition and Analysis. Once those caps are sealed onto bottles, the debris can end up in the drink. Microplastics—minute plastic fragments originating from consumer goods and the breakdown of larger plastic items—have been linked to a host of health concerns, from neurotoxicity to increased cancer and heart attack risks.
While scientists did not assess the specific health risks posed by the particular microplastics found in these drinks, per New Atlas, the findings underscore just how widespread food and beverage contamination can be. While rinsing and drying caps eliminated the debris in lab settings, the researchers cautioned that implementing such a fix in large-scale bottling operations might not be so simple. And since the contamination happens before the product reaches store shelves, there's not much consumers can do at home to avoid it—except perhaps to skip metal-capped bottles altogether.