In all honesty, you're probably right to not waste (a lot of) energy worrying about it. The best you can do is not use many plastics and advocate for better policies to reduce, reuse, and recycle the plastics are out there, as well as cleanup programs for the huge problems like the macroplastics in the ocean currently.

As for "the environment is full of tons of stuff", microplastics are pretty new to the biosphere, so who knows what we should suspect? Personally I think microplastics are bad at all scales, and it'll just get worse. They weaken individuals, species, and whole ecosystems right as we're delivering them a big punch in the gut with habitat destruction, overfishing, and climate change. Are they what's going to kill the world? Nah, maybe only #4 on the list. Only!

If we treat microplastics as a symptom of business-as-usual, then they are a marker, and we can think of them as the canary in the coal mine. Entropically, we dug up a bunch of dead stuff and either burned it into the air or refined it into plastic, which we are now mixing into the biosphere at all scales. What other effluents are we dispersing that will have unwanted effects?

Structurally, we concentrate exotic materials and then disperse them. Nothing like that has ever happened at global scale to life before. The economic changes to achieve a system that doesn't make this its default trend are almost unthinkable. That's what I see as the real #4 (or whatever) on the list.

No, maybe #220, if they're on the list at all. An nth-order effect.

It is by no means unreasonable or unscientific to suspect whether plastics in your stool, the air and water around is a special threat. Animal studies have shown cause for concern. Sensationalist? Perhaps.

I've used this information to try to eke out my single use plastic use. (Use of single use plastic? single plastic use?)

Here's a study:http://www.fao.org/3/a-i7677e.pdf

The cause for concern?

"Adverse effects of microplastics ingestion have only been observed in aquatic organisms under laboratory conditions, usually at very high exposure concentrations that exceed present environmental concentrations by several orders of magnitude"

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