https://www.wired.com/story/for-the-love-of-god-stop-microwaving-plastic/
A study of baby-food containers shows that microwaving plastic releases millions upon millions of polymer bits.
A big issue is that we're still quite limited when it comes to analytical methods for quantifying and classifying microplastics. I've seen a method from ASTM from like 2020 referenced once or twice, but the most telling one is that EPA doesn't have one for drinking water yet. I know PFAS for example seems like a recent hot topic, but Method 537 dates back to to 2009 and UCMR3 (even if Method 533 is much more recent). Until we get a consensus on what exactly microplastic is and isn't and a consistent way to put a number on it, we're not really generating high quality data.