Tritium is an isotope of hydrogen which has one proton and two neutrons. It emits beta particles only and has a half-life of 12.3 years. The radiation from tritium will not penetrate human skin so tritium is not an external radiation hazard, but when taken into the body it becomes an internal hazard. It usually occurs as heavy water where one H in the water molecule is tritium and the other Hydrogen. You can find more info at the Tri-University Meson Facility (TRIUMF) in Vancouver, Canada.