It's tires. Millions of cars and their tire wear for literally the last hundred years. How can it not be in our bodies and causing issues at this point.
Or the replacement of wool with polyester and nylon over the last 100 years
Tires are a good thing to look into but a lot of things have change drastically in the last 100 years...
I also entertain the idea that the “feminization” that appears to have occurred since the 50’s is due to (micro)plastics and PFAS being literal ligands of the estrogen receptor, and their well documented endocrine disrupting effects. I recall some study found that 100% of males tested had microplastics in their testes.
I wonder how hemp would work out as a replacement, it seems pretty promising - stable, biodegradable, likely inert, and versatile (it can be used for containers, textiles, paper, and a lot more). For degradation, (GMed) fungi or bacteria seem like a potential avenue, surely nothing could go wrong with that.
People use synthetic fabric sheets, pillow cases, not to mention mattresses. Most pillows are filled with plastic. We cover our bodies in plastic, then wash our clothes and microplastics enter the wastewater system.
Also consider the expansion of the US highway system (starting in '56), increased freight trucking, popularity of tuning / high performance cars etc. I'd imagine more tires are getting shredded into the environment than at any point before the 80's, even with improvements in tire compounds.
Take a look at a 1950s Coup de Ville, Buick Roadmaster or any other famous models from that era. All pushing 4K, sometimes 5K pounds.