It is not clear whether the presence of plastic inside the mammals can be linked to their death. Study co-author Rob Deaville told HuffPost that other factors may have killed them, such as live stranding, infectious diseases or starvation. Deaville noted that only a few plastic fibers were found inside each animal.

Released Thursday by the University of Exeter, the study examined the digestive tracts of 50 sea mammals across 10 different species that were discovered on beaches around Great Britain. Researchers found microplastics, which are smaller than 5 mm in length, inside all of the animals. Eighty-four percent of the particles were fibers, while only 16 percent were fragments of larger plastic items. Nylon, which made up 60 percent of the fibers found, is used in many types of garments, including yoga pants, stockings and socks.

Researchers studying plastic debris discovered inside the bodies of dozens of dead whales, dolphins and seals say the majority of the items found were not plastic bags or drinking straws, but synthetic fibers from clothing.

In general, plastic’s effects on marine animal welfare is not well studied, and it seems the bigger the animal, the less we know. A 2016 study from Lund University found that fish that ate zooplankton containing microplastics experienced a change in their predatory behavior and a decreased appetite. The Exeter report’s authors affirm that more research is needed to understand the health implications of plastic particles on living organisms that ingest them.

In a statement, Louise Edge, head of Greenpeace U.K.’s ocean plastics campaign, called the findings “ominous,” adding that the study “shows the scale of plastic pollution in our seas.” Greenpeace collaborated with researchers on this study.

But the study author is wary of characterizing the findings as dark. “From my perspective, I’m not sure I would necessarily describe them as ominous — although we’ve found evidence of microplastics in all animals examined in this study and this may be cause for potential concern, the levels are low and we don’t yet know what effect, if any, these particles may be having on the individuals,” Deaville said.

The results of this study come as governments and big businesses face increased calls to reduce production and distribution of plastics. Approximately 12 million tons of plastic are dumped into the ocean on an annual basis. Public conversation has focused mostly around single-use packaging, such as plastic bottles, straws, bags and wrappings, for instance, which enter waterways via a number of sources, such as mismanaged garbage collection networks, or as litter that gets blown in by wind or washed in by rain. Sea turtles and seabirds have been found to be particularly vulnerable to marine trash.

Researchers have also cautioned that an even bigger source of plastic pollution is clothing made from synthetic materials such as polyester, nylon or acrylic. Garments made from plastic-based fibers shed in the washing machine; the particles released can be 100 times thinner than a human hair, small enough to slip through filtration systems at wastewater treatment plants. The fibers then make their way into the ocean, where they become part of the food chain. One 2016 study found that acrylic fabric was the biggest culprit of shedding.

Synthetic fibers are used in approximately 60 percent of the fabrics used for clothing around the world, Vox reported. “Estimates vary, but it’s possible that a single load of laundry could release hundreds of thousands of fibers from our clothes into the water supply,” the outlet wrote.

“We know very little about the impacts of microfibers on the health of nonhuman animals and people,” journalist Mary Catherine O’Connor recently wrote for Ensia, a nonprofit media site that focuses on solutions for the environment. “But what we do know suggests a need for additional research.”

“We now need to move beyond the demonstration of presence of these particles and try to collectively learn more about what impact, if any, they may be having on the health of the individual animals and of the wider population,” said Deaville.

This story is part of a series on plastic waste, funded by SC Johnson. All content is editorially independent, with no influence or input from the company.