Not polyurethane. Something less gassy, like linseed oil. There may be some water-based finishes out there that our organic friends can speak to...

Dan

Many of the water based sealers, lacquers and stains are significantly more toxic to use than their oil based counterparts. I'd say just use linseed oil. It's natural, non toxic, and works fairly well. If you really get a wild hair up you know where you can boil lumber in linseed oil and wax and it'll last darn near forever. It's just hazardous and expensive to do yourself. If the oil/wax gets a little too hot......poof! One heck of a fire.