Scare stories do the rounds. Acrylamide is a substance produced when sugars and amino acids are heated together. It is present in burnt toast - as well as in unburnt toast, roasted vegetables, chips and cooked meat. Eating huge quantities of acrylamide has been linked to cancer - as has eating huge quantities of just about anything else.
No-one has tested the theory, either by comparing the incidence of cancer among different groups of people or by feeding burnt toast to rats. It has been established that worry can give you cancer.