Of course, there's about 10^23 molecules in a litre of water; 10^5 particles may not represent very much. A litre of water probably contains a few orders of magnitude more of radioactive particles.
The plastic may be insignificant.
Discussion
yeah it comes down to just how toxic they can be. maybe they just bounce around and get flushed out eventually, or stay around and disrupt things? biology is absolutely insane once you start going down that rabbit hole. So much systems complexity. Hard to wrap the mind around it all.
It'll depend on the type or plastic too. There are huge differences in chemistry between them.
Nanoplastics act as focal points for hydrophobic protein aggregation which is likely to have impacts in many chronic disease conditions—kidney dysfunction, blood/brain barrier dysfunction, digestive duct blockages, peripheral nerve neuropathy (myelin sheath inflammation), …. In addition many cell types require adhesion to regulate normal behavior and they undergo apoptosis when free floating. So streaming non digestible adhesive islands traversing your body could certainly create a permissive setting for persistent inflammation and earlier stage metastases.
Simply, don’t.