[RedDevil]

Polyurethane tires look very promising; see Will Polyurethane Replace Rubber for Tires? - Tom Dwyer Automotive and http://www.thomasnet.com/articles/pl...silicone-tires

Polyurethane tires will rub off loads of plastic microparticles. But - those would be polyurethane microparticles. And that matters.

As polyurethane is quite inert. So much so that it is the first choice for baby toys, pacifiers, etc.
I was looking for possible health hazards from PU microparticles but found hardly anything.
Now why is that?

This study Microplastics in the Marine Environment: A Review of the Methods Used for Identification and Quantification - ResearchGate grouped the results of other studies, listing what plastic types have been identified how often in those studies marine pollution samples.
The majority of studies found polyethylene, polypropylene and polystyrene.
Just one (out of 42) found polyurethane.

The properties (also listed in the above link) reveal why.
PE and PP are lighter than water, so they float in the surface layer and remain there when they break down to microscopic size. Polystyrene is just about as heavy as salt water, but is most used in foam. It floats until it breaks down.
PU is heavier than even salt water. It would sink to the bottom rapidly. It will deposit on the bottom of the wastewater channels and wherever the roads drain to. It won't float out to sea.

Those deposits do get polluted by PU particles, but that would be a relatively confined and concentrated pollution - by an inert material. Those deposits are most likely already heavily polluted anyway.
So not much harm done there, I guess?

Just the cost to worry about.
PU motor and shock mounts last longer than their rubber counterparts, but are not widely used because they are more expensive.
Would a set of PU tires be affordable enough to make them worthwhile?