Chilled water contains deuterium, a stable isotope of hydrogen, but it is present in very small amounts—about one atom in 6420 of hydrogen. This means that all water, regardless of temperature, has trace amounts of deuterium. Concerns about deuterium being harmful to health are largely unfounded, as it is not radioactive and behaves similarly to ordinary hydrogen in most chemical reactions. However, it is noted that deuterium can affect reaction rates due to its greater mass, a phenomenon known as the kinetic isotope effect. While high concentrations of deuterium (heavy water) can be toxic, the natural levels found in water are not a health risk. The discussion emphasizes that the dose is critical when considering potential toxicity, and the small amounts of deuterium in water should not cause concern.