I'd heard that toner was (could be?) carcinogenic.
It's a well-known fact that research causes cancer in rats :-)
More seriously, toner probably is carcinogenic simply because it's a fine
powder. And like flour or fine sawdust, a cloud of toner could be
explosive.
Which to me means you avoid making clouds of the stuff. But when I've
taken toner cartridges apart, you seem to get less loose toner around
than when you clean up an old laser printer.
-tony