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
From my understanding of the study they used an unverified method on a small part of the brain to get the amount of plastic in the entire brain. Which can be misleading because if you only look at the parts of the brain that are microplastics, the brain is 100% plastic. There hasn't been widespread replication of the results yet either nor has the paper been peer reviewed.
I'm sceptical especially with the microplastic doomer narrative going around. As far as I know the danger of microplastics on the human body is yet to be determined. But it's definitely alarming I just don't think it's responsible to be saying there's a credit card in your brain.