You asked the right question. So I spent 2 hours reading this paper and the recent study on microplastics in human blood. (linked near the end of the paper)

Fair warning i am not a qualified scientist.

1st, the metrics aren't truly comparable because we aren't good at measuring microplastics yet? This paper used known lab prepared samples. Meanwhile, the blood study was very limited by the minimum level of detecting any quantity of plastic and inability to count the number or size of particles.

2nd, the blood study restricted itself to filtering out only microplastics >700nm, the chicken study used a few 1,000nm and 500nm particles for certain parts of the experiment but the most work was done with 25nm specifically because it was the most lethal to chicks in their previous study.

3rd, and possibly most important, the point of this paper was to find where the nanoplastics preferred to go in embryo development and if they had any specific potential effects. ESPECIALLY in regards to new microplastics based medication and treatment options being explored right now.

TLDR: After looking at the human blood study concentrations and how much passes through the placenta, we aren't nearly at the level used in the chicken experiment. But microplastic levels are only going to increase, even if we all stop using plastic today. Importantly, microplastic medication/treatments in development need to take extra cautions in further study.