We've used Pine fresh for more than 7 years with our ferrets.  We've never
had any health problems as a result.  It is the absolute best odor control
of any litter we tried.  I've had vets caution against many clay kitty
litters because ferrets may eat it and get an intestinal blockage.  Pine
Fresh breaks down when wet but the ferrets only like to play with it when
its first put into the litter pan.  Its hard to find sometimes; it's only
carried by the Safeway chain in northern Virginia where I live.  When it's
out of stock, we've had to substitute a clay litter and have not been
satisfied with the results.  We've had four ferrets over the past years and
have seen no respiratory problems with any of them.  I would recommend it
unless someone has strong evidence that it might be harmful because I think
a lot of the other options also have some risks.
[Posted in FML issue 1890]