If you're already in the fire, using a respirator type mask will not hurt anything. At least it may make breathing easier even if the oxygen level is marginal. Depending on type, it may cut down on the breathing of toxic gases. Do not use if oxygen is insufficient is a legal warning that the mask will not protect you from everything. If you are trapped by smoke & toxic fumes, anything is worth a try, even the old wet cloth over the nose idea.