Sample size isn't the problem. The problem is we don't have a control group to compare against. Ideally, you'd compare people with micro plastics to people without them, but those people don't really exist anymore.

https://hsc.unm.edu/news/2025/02/hsc-newsroom-post-microplastics-human-brains.html

... brain tissue from people who had been diagnosed with dementia had up to 10 times as much plastic in their brains as everyone else, Campen said. But while there is a clear correlation, the study design cannot show whether higher levels of plastic in the brain caused the dementia symptoms – they may simply accumulate more due to the disease process itself, he said.