Plastic rain is the new acid rain
arstechnica.com
arstechnica.com
Of course it still sucks if you want to drink the water or breathe the air and there are all these polymers in them.
Sure, the sand has more density so usually settles sooner, but apart from that both are effectively biologically inert.
What's key is defining the magnitude of the harm of each, and just stating 'X million microparticles' doesn't do it.