Different people have different sensitivity to epoxy. The most sensitive can show allergic reactions to fumes. It won't hurt you to wear a respirator. Since you can smell it when you work with it, some portion of the epoxy is evaporating into the air, but unlike, say, varnish, the percentage of solvents is much lower. Most of your can of epoxy ends up on your boat (or in lumps on the floor). With varnishes and paints a significant portion evaporates into the air.