How Much Plastic Kills a Sea Turtle?

The ocean is full of plastic – more than 171 trillion pieces, according to scientists, and the number is growing. Animals get tangled in plastics or swallow them, the chemicals released from these items are often toxic, and once the plastic is inside a creature, it can stay inside, potentially blocking its airways or intestines.

A binding global treaty regulating the manufacturing and disposal of plastic could help change the situation. Even though the last round of negotiations on such a treaty ended without results in August 2025, that hasn’t stopped scientists from thinking about how it might work. A treaty will benefit from figures, backed by evidence, on how much plastic is deadly to marine life, and that’s why a team of researchers publishes in the journal Proceedings of the National Academy of Sciences examined the results of more than 10,000 animal necropsies of sea turtles, seabirds and marine mammals. They found that while the definition of a lethal amount of plastic can vary between creatures, the numbers are often shockingly small.

Drawing on reports of more than a century of animal deaths, the researchers determined both how much plastic the creatures ingested and whether that plastic was the cause of their deaths. Then they calculated how much plastic was linked to a 50% chance of death, and how much was linked to a 90% chance of death, says Erin Murphy, head of ocean plastics research at the Ocean Conservancy and co-author of the study. They also looked at different types of plastics to determine which ones are most dangerous for different animals.

“I was surprised by some of the thresholds we found,” says Murphy. According to the team’s calculations, if a bird the size of an Atlantic puffin had eaten just under 3 pieces of plastic sugar, it had a 90% chance of dying from plastic exposure. For a loggerhead sea turtle, the quantity would be about as large as a pair of baseballs.

Learn more: What happens to the plastic in single-serve coffee pods?

Rubber materials are particularly dangerous for birds, the team found. “For seabirds, just six pieces of rubber, like a balloon, each smaller than the size of a pea on average, are 90 percent likely to cause death,” says Murphy. (Although rubber may be a natural product, much of the rubber used today is synthetic and could be regulated by a plastics treaty.)

The analysis focused on situations in which plastic was clearly responsible for the animal’s death – where it blocked its airways, ruptured its stomach or caused its intestines to tear, for example. It’s likely that plastic causes other, less obvious problems for marine creatures, such as feeling full even if they haven’t eaten enough to survive.

“It’s a sad thing and hard to imagine, but it also reminds us that everyone can be part of the solution,” Murphy says. “And if we’re serious about solving this problem, the science is clear. We need to reduce the amount of plastic we produce, improve collection and recycling, and clean up what’s already there, and that’s really something everyone can get involved in.”