³H, also known as tritium, is a radioactive isotope of hydrogen. It contains one proton and two neutrons in its nucleus, making it heavier than the most common hydrogen isotope, protium, which has no neutrons. Tritium is produced naturally in the atmosphere through cosmic ray interactions and can also be generated in nuclear reactors. Tritium is used in various applications, including nuclear fusion research and self-luminous devices like exit signs and watch dials. Due to its radioactive nature, tritium emits low-energy beta particles, which are not harmful to humans unless ingested in large quantities.