Cytotoxicity describes how harmful a substance is to living cells, a measure toxicologists use when vetting food contaminants, packaging migrants and new ingredients. In the laboratory, cultured cells are exposed to the chemical and changes in survival, membrane damage or metabolism are recorded. A strong response flags the substance for deeper safety work, but it does not by itself prove danger to people. These tests, interpreted alongside exposure data, help regulators decide which compounds need strict limits in everyday foods and beverages.