RADIATION STANDARDS, INCLUDING FALLOUT 113 Iodine 131 exposure can occur as a result of inhalation of the radionuclide and absorption from the lungs. It is useful to provide some estimate of the contribution of this source of intake. The average contribution of iodine 131 to the total gross beta activity in air as reported by the radiation surveillance network for October 1961 was 20 percent. The highest average gross beta activity in air for Octover was 19uye/m* in Phoenix, Ariz. If 20 percent of this was iodine 131, then 20 percent x 19np¢/m°=3.8uucl""/m3. Assuming that a 1-year-old infant breathes 1 cubic meter of air daily, his intake would be 3.8uucI™ per day from inhalation. On the other hand, if an adult breathes 20m* air per day then his average daily intake would have been 20m* air <3.8aucl'"/m* air=76ypcl™. The average level of iodine 131 in milk for Phoenix for October 1961 was 60uzucI™ per liter. If both the child and the adult consumed 1 liter of milk daily, intake by ingestion for both would have been 60uuce. In the case of the child, inhalation would have represented a very small fraction of the total daily intake (4uyc out of a total of G4puc). It can be seen that theoretically inhalation could be a significant contributor to total intake of iodine 131 in the case of adults. It must be remembered, however, that for an adult it takes 10 times the daily intake of iodine 131 to deliver the same radiation dose to the thyroid gland. It would thus appear that inhalation in the situation encountered during the fall of 1961 was not a significant source of radiation exposure to the thyroid gland. There are some direct data to support the conclusion that fresh milk was the only significant source of radiation exposure to the thyroid gland of the population during the period of Soviet nuclear weapons test in the fall of 1961. Kisenbud studied in vivo six adults who reported drinking from 1 pint to more than 1 quart of milk daily. These averaged 57+33uycI™ per thyroid, while three adults who were not milk drinkers averaged only an insignificant amount of iodine 131 4.3+4.9upnceI™ per thyroid. PTBbie wae alga eA 3 BBOEECHL USEMEPte DATA ON RADIOIODINE IN MILK The results of the iodine 131 analyses from the milk-sampling network are published regularly in radiological health data. The early results were presented in previous Joint Committee on Atomic Energy hearings. The data for the fall of 1961 are summarized in table I. The following averages for the daily iodine 131 concentration per liter of milk are given. 1. The average for each station for each month during the period September 1961 through January 1962. 2. The average for each station during the entire period. 3. The average for all of the stations during each month. 4. The average forall stations during the entire period. To estimate the average daily concentration of radioiodine in the milk at each of the reporting stations during the months of October, November, December, and January, a simple average of the measurements reported during the month was derived. For those stations reporting no milk samples during the second half of September, the October average was used a second time for the September value. These monthly averages at each station constitute the basis for estimating the average daily intake of a person consuming a liter of milk per day in the vicinity of the reporting station. ESTIMATE INTAKES OF RADIOIODINE Some evaluation of the significance of these iodine 131 concentrations in milk can be obtained by comparing estimated total intakes of iodine 131 with the guidance of the Federal Radiation Council. For the purposes of estimating intake, it has been assumed that (1) the average consumption of fresh milk in the critical age group is 1 liter per day and (2) that milk is the onlysignificant source of intake of radioiodine in this group. On the basis of these asstmptions the values in table 1 can be considered as representing the average daily intake of iodine 131. SRT +