Dosimetry of Tritium in Water Courses

This is of course a major issue right now in Japan because of the tritiated water from Fukushima. It's also an issue about tritium being released from Indian Point in New York and also into Massachusetts Bay from the Pilgrim station. This is an international issue that we are dealing with. Explain how humans become exposed to tritium. Where do they get it from and what does it do to them?