Tritium is a radioactive form of hydrogen with a physical decay half-life of 12.3 years. It 
emits very low-energy beta radiation, which is completely absorbed by common materials 
such as sheets of plastic, glass or metal, and cannot penetrate the top dead layer of skin in 
humans. Exposure can nevertheless pose a risk if the element is ingested in drinking water 
or food, or inhaled or absorbed through the skin. 
In Canada, the control of tritium releases to the environment is particularly important, since 
CANDU reactors produce significantly more tritium than most other types of reactors due to 
the use of heavy water (deuterium) in the moderator and heat transport system. Tritium is also 
used by a few industries to produce gaseous tritium light sources. Much smaller quantities 
are used in research applications, and as a tracer in oil and gas exploration.