It can be man-made, but it is also a naturally occurring substance that is produced by cosmic ray action and by the decay of natural radionuclides in rocks and soil. It is a colorless, odorless gas, lighter than air. It is present in air and water all over the earth and is regularly ingested and breathed by everyone.
All humans contain trace amounts of tritium and several other naturally occurring radioisotopes.
Many isotopes are stable and retain their structure indefinitely. However, some isotopes, including tritium, are not stable and are said to be radioactive. As the tritium nucleus decays it emits an electron, causing energy to be released in the form of beta radiation. A new nucleus is then formed with two protons and one neutron thereby becoming a form of non-radioactive helium. Tritium beta emissions are very weak.