We aren't really looking. In the most well known case we were able to identify they were killing salmon because the salmon were dieing and worked back from that, not because some study led there first.
https://www.ehn.org/toxic-tire-chemicals-threaten-salmon-as-...
Isn't [bad thing is happening] let's work backwards and find [difficult to find cause] a really solid approach?
That is a case of a specific chemical in tires, not microplastics generally, or even rubber tire particles generally.