Its wise to have some means of alternative heat. We have a woodstove that sits on the fireplace hearth and uses the existing chimney. We only use it if we loose power in the winter or sometimes when we get a patch of frigid temps. I think the certification may entail a UL listing to satisfy some insurance companies. Commercially manufactured stoves would have that, I imagine, as opposed to the homemade stoves. The EPA also got involved with the wood stove industry. Our’s has a catalytic feature as Crowbar mentioned that is designed to help it burn cleaner.
Don’t overlook propane heaters or natural gas heaters. Even if you don’t have a fireplace, ventless heaters can be installed on a wall and supplied by an outside tank.