Where does all the rubber go???

You know ... when after about 40 thousand miles you go down to the donut store
(tires) and have a brand new set of radials stretched around your rims.

I'm not talking about the used carcasses which we all know about, but, where
does all the rubber go that used to make up the tread on your tires.  Yes of 
course, it wears off, but where does it go?  You don't see Cal Trans shoveling
the rubber dust off of the shoulders do you?  Does it go into the pavement??
I don't think so.  Does it go into the air we breathe?  I'm sure that
Goodyear Tire and Rubber would like to elaberate on this.

More to think on.