For years, glass bottles have been regarded as the purer, safer alternative to plastic. In bars, restaurants, and kitchens alike, they carry an air of trust: no plastics, no leaching, no trouble.

But a new study has turned that assumption on its head.

A team of researchers from ANSES, France’s food safety agency, has found that drinks sold in glass bottles actually contain significantly more microplastics than those packaged in plastic or metal. In fact, some glass-bottled drinks contained up to 50 times more microplastic particles per liter.

The findings suggest a surprising culprit: not the bottles themselves, but the plastic-based paint coating the outside of the metal caps used to seal them.

“We expected the opposite result,” said Iseline Chaïb, the PhD student who led the research. “We then noticed that in the glass, the particles emerging from the samples were the same shape, colour and polymer composition – so therefore the same plastic – as the paint on the outside of the caps that seal the glass bottles.”

It’s Because of The Paint on Bottle Caps

Chaïb’s team analyzed over 100 drink samples sold across France, including still and sparkling water, soft drinks, iced tea, beer, and wine. The drinks came in a variety of containers—glass, plastic, cans, cartons, and cubitainers—to assess how packaging influenced contamination.

The differences were stark. Glass bottles were consistently the most contaminated container type. Some drinks in glass — including soft drinks, iced tea, lemonade, and beer — reached levels of over 100 microplastic particles per liter, with beer in small glass bottles peaking at 133 MPs/L. By contrast, the same beverages packaged in plastic bottles or cans contained far fewer microplastics — often as low as 2 to 20 particles per liter, and in some cases, even less.

Across all packaging types, soft drinks averaged 31.4 MPs/L, lemonades 45.2 MPs/L, and beers a striking 82.9 MPs/L. Water had much lower counts overall, ranging from 4.5 MPs/L in glass bottles to just 1.6 MPs/L in plastic. Surprisingly, wine had among the lowest levels of contamination, even in glass bottles. Wine bottles are typically sealed with corks or synthetic stoppers, not metal caps, possibly explaining the difference.

But what truly stood out was the consistency in the type of plastic particles found. Using advanced spectroscopic techniques, the team confirmed that the microplastics linked to glass bottles matched the polyester-based paints used on the caps.

Scratched Paint, Hidden Particles

The researchers went further. They cleaned and reused glass bottles, filling them with filtered water and sealing them with new metal caps under different conditions. When no cleaning was done, contamination was high at 287 microplastic particles per liter. When the caps were simply blown with air before sealing, the levels dropped by two-thirds. And when the caps were both blown and rinsed with alcohol and water, microplastic contamination dropped by 60%.

Scratches on the caps, the team suspects, are enough to release microscopic flakes of paint, flakes that end up inside the bottle during capping or storage.

And while this specific source of contamination hadn’t been previously identified in detail, the findings build on growing evidence that food and drink packaging—yes, even glass—is rarely pristine. Previous studies have documented contamination from plastic containers, filtration equipment, and even atmospheric dust. But this is the first to trace the trail back to something as overlooked as the paint on a bottle cap.

What This Means for Consumers

There is still no consensus on what levels of microplastic exposure are safe or dangerous for humans. While researchers have found plastic particles in human blood, lungs, and placenta, the full health implications remain uncertain.

Nevertheless, there are calls for more rigorous regulation. In March 2024, the European Commission established testing protocols for microplastics in drinking water, but these apply only to unbottled water. Beverages like soda, tea, or beer remain outside the regulatory framework.

The ANSES team stresses that their study doesn’t offer any toxicological conclusions. But it does provide a clear path for mitigation.

“Drink manufacturers could easily reduce the amount of microplastics shed by bottle caps,” the agency said.

By adopting simple cap-cleaning methods—such as air-blowing and rinsing with ethanol—beverage producers could slash contamination levels with minimal effort.

A Broader Problem with a Simple Fix

Globally, microplastic contamination is now considered a ubiquitous pollutant. Particles have been found in oceans, soil, seafood, drinking water, and even clouds. Still, packaging remains one of the most immediate and fixable contributors to ingestion.

The research team hopes their findings will spur changes across the beverage industry, especially as public awareness and regulatory scrutiny around microplastics continue to rise.

Until then, the advice is simple: don’t assume glass is always cleaner. And when it comes to capping your drink, even a little paint can go a long way.

The findings appeared in the Journal of Food Composition and Analysis.