Scientists have analysed 10,000 marine animal autopsies to understand how plastic ingestion leads to death.

The study found seabirds face extreme risk after swallowing just 23 pieces of plastic, giving them a 90% chance of dying. Marine mammals reach similar danger at 29 pieces, while sea turtles need to ingest around 405 pieces to hit the same threshold.

The researchers were surprised by how little plastic can be dangerous – less than a soccer ball’s worth of soft plastic by volume can be fatal to a dolphin, while a seabird might die from ingesting a few pieces of rubber smaller than the size of a pea.

They say the findings could help shape global efforts to protect wildlife.

“It’s a really important reminder that plastic pollution does pose an existential threat to ocean wildlife,” said lead researcher, Dr Erin Murphy of the US-based environment group, Ocean Conservancy.

The analysis used autopsy data from seabirds, sea turtles, and marine mammals such as seals, sea lions, and dolphins, collected worldwide. Nearly half of the sea turtles studied, a third of the seabirds, and one in ten of the marine mammals had eaten plastic.

The researchers estimated the death risks from swallowing different sorts of plastic in each group of marine animals.

They found the type of plastic matters: rubber is most dangerous for seabirds; soft plastics and fishing debris pose the greatest risk to marine mammals; and both hard and soft plastics threaten turtles.

The study examined only plastics found inside the stomachs of animals. It did not assess chemical impacts or entanglement, meaning the true scale of harm is likely to be higher.

Hundreds of marine species have been found with plastic in their bodies. Birds often swallow plastic fragments, and turtles mistake plastic bags for jellyfish. Until now, however, scientists lacked precise data on how much plastic is lethal for animals of different sizes.

Dr Murphy added: “To effectively address plastic pollution, the science is clear. We need to reduce the amount of plastic we produce, improve collection and recycling, and clean up what’s already out there.”

The research is published in Proceedings of the National Academy of Sciences.