There isn't yet enough research on health hazards of toner to really give a good answer here... Black toner for example is mostly polyester plastic and carbon dust. Neither of those are particularly toxic, although they are in a much finer powder than you'd normally encounter them.
Basically I'd still steer clear of them, but exposing yourself to them isn't a certain death sentence like say organic mercury.
I would say akin to smoking a cigarette. One time won't hurt you in a measurable way but if your job is refilling toners or dealing with broken laser printers you need some protection.
Basically I'd still steer clear of them, but exposing yourself to them isn't a certain death sentence like say organic mercury.