A few comments on points raised by earlier posts:
I've used aspen chips for many years, first with Scone and now with Natasha. Except for one bag I got which was large chunks, it's generally small soft shavings and the buns like it. That one bad bag was enough to throw Scone's litter habits out, but he came right back when I found softer stuff by another manufacturer.
There isn't a problem of odor with aspen chips if you line the bottom of the litterbox with a thick layer of newspaper. Aspen itself isn't particularly absorbent, but the urine runs through it and soaks into the paper. The Aspen keeps the smell down, and the paper takes care of the liquid. No problems.
The reason aspen (a hardwood, a/k/a deciduous tree) is good for litter and ordinary pine and cedar (both softwood, a/k/a conifers) aren't is not anything added to them, but rather the natural oils and aromatic chemicals which are present in the wood (phenols). These chemicals are volatile (that's why cedar smells good to us), and the buns inhale them. That can cause systemic problems. Hardwoods lack the volatile oils, so they're safe to use untreated. Heat treating softwoods drives out the phenols, and the resulting wood becomes safe - that's why kiln-dried pine and pine pellets are OK to use.
