There's a wide array of different plastics and plastic additives which turn into microplastics. Some of those might be totally harmless to us, but some may not.

For example, BPAs have been shown to cause issues such as gynocomastia (man boobs) by estrogen mimicking compounds found in plastic.

They've also been linked to obesity, cardiovascular disease, reproductive disorder, and breast cancer

Plastics are pretty much forever and although we've pretty much moved away from some of the more harmful plastics and additives, they can still be floating around in the environment.