They found some brains have 1% of plastic in them by weight. 1% of the human brain by weight is 14g. A solid 14g plastic ball is 1.2 inches in diameter which is about the size of a golf ball.
You'd only be off by a factor of 3 instead of 4, then.
In any case, comparing it to a solid ball isn't so useful. Even the idea of "density" for something that is dispersed as small particles isn't too useful; proportion of mass is really the only useful metric.
This is not correct. A golf ball is closer to 1.7 inches in diameter, which is nearly three times the size of a 1.2 inch diameter ball in terms of volume.
