Plastic contamination is rampant in bottled water. That was the unsettling conclusion of a study published last year in Frontiers in Chemistry that analyzed samples taken from 259 bottled waters sold in several countries and found that 93% of them contained “microplastic” synthetic polymer particles.

Many of those particles weren’t all that small. “Some were definitely visible without a magnifying glass or microscope,” says Sherri Mason, author of the study and a sustainability researcher at Penn State Erie, The Behrend College.

The 11 bottled water brands tested in Mason’s study are among the most popular and widely available in the U.S. and around the world. Samples from the brands tested varied in plastic concentrations, and the average across brands was 325 microplastic particles per liter of bottled water, researchers found. Nestlé Pure Life had the largest average concentration of plastic particles out of all the brands tested; one sample from the brand was found to contain more than 10,000 microplastic particles per liter.

Mason’s findings generated headlines and a World Health Organization announcement that the group plans to investigate the safety of bottled water. (The results of that review should be published later this year, according to a WHO spokesperson.) But Mason says the problem of microplastic contamination is far bigger than bottled H2O. “These plastic particles are in our air, in our water and in our soil,” she says.

Last month, a study published in Nature Geoscience found that microplastic particles were blowing through the air of the verdant Pyrenees Mountains in France. Another study published this year found microplastic contamination in U.S. groundwater. “Every time and everywhere we look for plastics in a scientific context, we find them,” says Phoebe Stapleton, an assistant professor of pharmacology and toxicology at Rutgers University.

That includes in people. A small 2018 study analyzed stool samples taken from people in Finland, Japan, Italy, Russia and other countries. Every sample contained microplastics.

“We know that humans are exposed to these particles,” Stapleton says. “We know they get into our body through ingestion and inhalation, and depending on their size, we know they usurp the natural physiological barriers.” This means some of these plastic particles are small enough to pass through the body’s protective tissues and into the bloodstream and organs, she explains.

There’s also evidence in animals and lab tissues that suggests females who are pregnant may pass these microplastics on to their unborn offspring. “Preliminary [rodent] studies from our group, and published studies from others, indicate that after maternal exposure, these particles have the propensity to cross the placental barrier and enter the fetal compartment, depositing in fetal organs,” Stapleton says.

What’s not clear, though, is how this plastic exposure affects human health. “Unfortunately, we do not currently know the toxicological outcomes of these exposures,” she says. The notion that plastics are accumulating in our bodies “is uncomfortable and scary,” she says. “But the studies to prove [negative effects] need to be done.”

Other researchers say we know enough already to deem these plastic exposures a threat to human health. “In animal models and in epidemiological studies in humans, we have a correlation between plastic exposures and known health hazards,” says Frederick vom Saal, a distinguished professor emeritus of biological sciences at the University of Missouri.

He says there’s evidence that plastics and the chemical pollutants that bind to them have toxic effects. “They’re implicated in the obesity epidemic and in other metabolic diseases such as diabetes and heart disease, as well as cancer and reproductive problems and neural problems like attention deficit disorder,” he says. “If you look at the trendlines of non-communicable diseases around the world, you see there is a correlation between exposure to these [plastic] pollutants.”

While correlation is not causation, he says, direct cause-and-effect data will be hard to come by. It would be unethical to purposely expose pregnant women to specific plastic particles in order to observe the biological effects. This means the research on microplastics and health will likely always be correlational in nature or taken from animal and lab models, he says.

Based on the existing data, vom Saal says we know enough to recognize that we should change how we interact with—and dispose of—plastics. “A lot of this is a consequence of dumping literally billions of pounds of plastic into the environment,” he says.

A 2017 study found that 79% of all the plastic humans have produced has ended up either in landfills or in nature. In 2010 alone, up to 12 million metric tons were dumped into the world’s oceans, the study found.

Ironically, the volume and variety of plastic-related exposures is another of the major challenges researchers face when attempting to show that these pollutants could be making people sick. “We’re all exposed to so many chemicals every day that if you’re 30 and you develop some rare form of cancer, no one’s ever going to be able to connect that to something you were exposed to,” Mason says. “Making that connection is basically impossible.”

More of Mason’s research has found plastic contamination in tap water, beer and sea salt. While all this suggests that microplastic exposure is unavoidable, Mason says focusing on bottled water is worthwhile for two reasons.

For starters, she says most of the particles her study found in plastic water bottles turned out to be fragments of polypropylene, which is the type of plastic used to make bottled water caps. “This seemed to suggest that it was the act of bottling the water that was contributing most of the plastic,” she says. At the particle sizes she and her colleagues were able to detect and measure, there was “about twice as much” plastic in bottled water compared to tap water or beer, she explains.

“Bottled water is marketed as though it’s cleaner than tap, but numerous studies show it’s definitely not cleaner,” Mason says. “Based on all the data we have, you’re going to be drinking significantly less plastic from tap water out of a glass than if you go and buy bottled water.”

A statement from Nestlé Waters North America included assurances of their water products’ quality and safety. Said Nestlé: “So far, our testing has not detected micro-plastics in our plastic water bottles beyond trace level. It is not possible at this stage to determine exactly where such traces originate from. We have been sharing our expertise and we are collaborating with the scientific community to advance understanding on the topic.”

Another reason to focus on bottled water, Mason says, is that its popularity is a major contributor to the world’s plastic pollution problem. By some estimates, Americans buy 50 million plastic bottles of water annually.

“Forgoing bottled water and plastic bags and plastic straws is a basic thing we could all be doing that can dramatically affect how much plastic ends up in the environment,” she says.

Reducing how much bottled water we drink would also save U.S. consumers billions. “If we took what we spend on bottled water just in the U.S. and we used that instead on water infrastructure,” Mason says, “every person on this planet could have access to clean water three times over.”

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