Hydrogen-3, a long-lived radioactive hydrogen isotope — convenient for autoradiography because it is readily built into complex molecules for research. Tritium labelling lets researchers trace metabolic pathways, nutrient absorption, and contaminant fate with exquisite sensitivity: a tritiated compound‘s journey through a plant, animal, or food system can be followed molecule by molecule. It occurs naturally at trace levels from cosmic rays and in higher amounts near nuclear facilities, where environmental monitoring tracks it. Laboratory tritium work follows strict radiological controls — it is a research tool, never a food ingredient.