I was under the impression that polyethylene and polypropylene microplastics don't actual harm humans until they get out into the environment, absorb nasty substances, then re-release them when entering our digestive tract.
If you eat those plastics they just go through you (for the most part). The reason why we all have microplastics permanently stuck in our bodies is because we breathed them in. Not because we ate them.
It's an important distinction, I think. Because a tiny microplastic thread that stuck inside a can of tuna can be 1000% worse than a tiny chunk of polypropylene or polyethylene that has only come in contact with your own food.
Also, polypropylene microplastics fall—they don't float in the air so easily like polyethylene. So there's a great big difference when it comes to the type of plastic bag (as far as this lawsuit goes... In terms of harm). I'm also not certain that a frozen or microwaved polyethylene or polypropylene bag would emit microplastics into the air at all. I couldn't find any studies about it.
For reference, normally—to get polyethylene airborne in the form of microplastics—it needs to be processed into threads. Those threads then break apart into tiny strands that can float on the wind (because they're so light). The type of microplastic you'll get from a polyethylene bag won't be a tiny thread like that. It'll be a literal chunk (will look like a rough rock under a microscope).
Having said all that, there are some studies indicating the that microplastics in our guts can change the makeup of our microbiome (temporarily—while the plastic is there). This can cause a swelling response which is bad. So there's that 🤷
I say all this because it annoys TF out of me when articles talk about "microplastics" like they're all the same. They're not, damnit! Some aren't even a problem! PHA is harmless, for example and PLA breaks down within 3 years or so which is fast enough that it doesn't really matter (because we don't use it that much anyway so the rate at which is accumulates in any given spot is unlikely to be a serious problem).
Don't even get me started on colorants! 😤