But, as with any early science, there are caveats. For one, these tiny particles are extremely difficult to measure. And nobody has yet repeated the research to see if the results hold up. The big question is whether everything they’re measuring is actually plastic — or if some of it is lipids, which can look similar chemically but occur naturally in the body.

“The estimates they have for how much is in the brain seem high,” Dr. Woodruff said. But even if they are, she said, “that wouldn’t negate the findings that they’re seeing more plastics over time. And that actually is very consistent with what we know about the production of plastic.”