air, but this dose will usually be small compared to the exposure that follows after the debris is deposited on the ground. However, this ratio of doses may not hold for events where most of the radioactivity that escapes beyond the test site is in the form of gases or finely suspended particles that are confined to a relatively shallow layer of air near the surface. Measurements of total fallout activity in air (called gross beta counts) provide only a crude alerting system. It is not a reliable procedure for predicting the amount of fallout to be deposited nor the amount of iodine 131 in milk.22, Because of the transitory nature of the fallout debris remaining in the air (and sometimes because of the particular choice of units used in expressing its concentration) what may sound like an alarmingly large amount may, in fact, result in only minor radiation doses, Altath Meietesipstinrits The Data Water The highest measuredfallout activity in water was at Upper Pahranagat Lake, Nev., in 1955 amounting to 0.14 millionth of a curie per liter. Since this was a total gross beta countit is difficult to give a precise estimate of the potential radiation dose. A crude analysis suggests that if this water had been stored and used as a sole supply for 70 years the total dose might be about one roentgen to the bones and one-quarter roentgen each to the thyroid and lower large intestine. No radioactive fission products nor induced activities includ- ing tritium from underground tests have been found in under- ground water supplies at places of human consumption. Air The highest concentration of radioactive debris in the air in a populated area off-site (except for the Marshallese experience where measurements were made only after the passage of the cloud) was about 1.3 millionth of a curie per cubic meter aver- aged over the 24 hours the activity was present.24 This happened at St. George, Utah, on May 19, 1953. The estimated radiation dose to the lungs from inhaled fallout debris was less than 0.2 roentgen.**> The external whole body exposure from the fallout while it wasstill in the air was roughly estimated to be 0.025 roentgen —only about oo of the whole body exposure that occurred after deposition of the fallout. Evaluation The concentrationsof fission products or tritium in the water supplies have not constituted major sources of radiation exposure to man. Thereis a large dilution factor when surface 745-413 O-64—4 19