The average brain weighs 1300-1400 grams. If 0.5% of that is plastic by weight, your brain contains 6.5 - 7 grams of plastic. The average plastic credit card weighs 5 grams.
https://www.theguardian.com/environment/article/2024/aug/21/microplastics-brain-pollution-health
That first paragraph is fair enough.
I'd disagree that the microplastic doomer narrative is incorrect, the plastic in the brain is just the newest tidbit. It's already in our kldneys, testicles, placentas, blood, semen, livers and lungs. Even if this study is wrong (which I don't think it is, because its on trend with plastic contamination growth in the rest of our bodies) about how much plastic there is specifically in brains, there's still easily a credit cards worth in our bodies.
Microplastics are killing worms, it is contaminating soil, it is hurting crops, it is killing fish, and studied effects of plastic on other mammals are bad, to the point it seems pointless to confirm it hurts humans in specific ways before doing something about it.