Cs is the chemical symbol for caesium, an alkali metal that occurs naturally in trace amounts and can enter the food chain from soil and water. Its radioactive isotope caesium-137, released by nuclear accidents and weapons testing, is the form of food safety concern because it behaves like potassium and accumulates in muscle tissue and crops. After contamination events, authorities monitor milk, meat and produce against strict maximum limits, and caesium measurement remains a standard part of radiological food surveillance programmes worldwide.