I don't know about the oceans proper or the rest of the "western US" that we seek to address in our parent article, but I was specifically told the biggest source of microplastics in Califonia's coastal waster was car tires.

https://www.cnn.com/2019/10/04/us/microplastic-pollution-car...

And that synthetic fibers were only #2.

Ocean pollution, especially in fishing areas is especially detrimental due to biomagnification. Unfortunately the more I learn about micro-plastics, the more bleak my outlook gets.

This is true. It's also true that far more than 99% of bullets fired in wars don't hit people.

.. and then people spend the next century slowly cleaning up toxic lead and leftover explosives, like the "iron harvest" of WW1 battlefields.

Military small arms ammunition must be jacketed, per the Hague Convention.