In addition, a lot of enamel will just stain after some time, which is normal; This is why some of the vendors are just going with black enamel these days, instead of the long-used white. I haven't tried baking soda to clear up the stains on mine, I'll have to give it a shot.
Also, if the enamel cracks inside the cooking area of the pan, you may want to throw out the pan; it may expose your food to contact with toxins used in adhering the enamel.
I'd be more worried about eating chips of (sharp, hard) enamel than any toxins - the firing process is not going to leave much other than the ceramic behind.
Also, if the enamel cracks inside the cooking area of the pan, you may want to throw out the pan; it may expose your food to contact with toxins used in adhering the enamel.