In a basement laboratory at the University of New Mexico, Marcus Garcia rummaged through a bin full of plastic waste. He picked past bottles, chunks of fishing net, a toothbrush, a cup with a Pokemon character and a G.I. Joe.

“Yes!” he exclaimed, holding up a discarded pipette tip. “Found it.”

Dr. Garcia, a postdoctoral fellow in pharmaceutical sciences, discovered the pipette tip last summer with colleagues on a remote Hawaii beach. It was miraculously intact though it had most likely been degraded for years by sun, ozone and the ocean. How poignant, he thought. It was an object he and thousands of other scientists used every day. And there it was, washed up on a beach along with hundreds of pounds of other plastic waste they were now cleaning up and collecting for research.

Dr. Garcia is part of a leading lab, run by toxicologist Matthew Campen, that is studying how tiny particles known as microplastics accumulate in our bodies. The researchers’ most recent paper, published in February in Nature Medicine, generated a string of alarmed headlines and buzz in the scientific community: They found that human brain samples from 2024 had nearly 50 percent more microplastics than brain samples from 2016.

“This stuff is increasing in our world exponentially,” Dr. Campen said. As it piles up in the environment, it is piling up in us, too.

Some of the researchers’ other findings have also prompted widespread concern. In the study, the brains of people with dementia had far more microplastics than the brains of people without it. In papers last year, the researchers showed that microplastics were present in human testes and placentas. Other scientists have also documented them in blood, semen, breast milk and even a baby’s first stool.

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