Evaluation The concentrations of fission products or tritium in the water supplies have not constituted major sources of radiation exposure to man. There is a large dilution factor when surface water supplies are contaminated, and the fission products from underground nuclear detonations become fixed at and near the site of the explosion, Whereas, theoretical calculations suggest that concentrations of tritium in the water may be above acceptable limits for some underground nuclear detonation, this refers only to the water around ground zero. Some dilution is to be expected if it moves off-site and, more importantly, the criterion of acceptability is based on the assumption that all of the water drunk throughout a lifetime will contain the same concentration of tritium as set by the limits. The quantity of water initially contaminated to these limits by an underground nuclear explosion is relatively small and do not constitute the sole supply for a lifetime. Much less radioactive fallout debris enters the body by inhalation than by ingestion, and while it is in the air outside the body the radiation exposure is much less than after the material has been deposited on the ground and would not constitute the sole supply for a lifetime. 26.