I don't think it is poisonous, we always had enameled pots that had these type of places. Most of the time the enamel has come off at sometime or another and that is bare metal showing thru and we continued to use the pot until the spots would rust thru and then my Mom would mend them with a fix=it mender, (A rivet and washer of some kind or a small short screw and washer) These would not work on today's smooth top stoves, but I don't think there is any health concern with them. In fact I have a couple of enameled pots that I use yet when canning that have those kinds of places in them. They are not a pretty sight to behold, but don't think they are harmful. If they were, my grandparents and their large family would all be dead an so would my immediate family for that matter as well as my parents and the elders lived to a ripe old age and we are still a going okay. My thoughts for the day for what it is worth.