What harm can chemicals do to human body?

A, as a friction agent of toothpaste, calcium carbonate can make teeth brush more cleanly and will not cause harm to human health, so A does not meet the meaning of the question;

B, quicklime can react with water to generate calcium hydroxide, which can remove water from food and will not cause harm to human health, so B does not meet the question;

C, sodium nitrite is a carcinogen and cannot be eaten, so industrial salt sodium nitrite can be used instead of salt for cooking, which will cause harm to human health, so C meets the meaning of the question;

D. Eggs are placed in limewater, and calcium hydroxide reacts with carbon dioxide to generate calcium carbonate, which is wrapped around the eggs, so that the eggs cannot contact with oxygen and are easier to store. It will not cause harm to human health, so D does not meet the meaning of the question.

So choose C.