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
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11100893/
and the study referenced there