Radiation dose and health risk must be calculated from continued exposures. The radiation dose from internalized I-131 is estimated on the basis of the following:
activity deposited in the lungs and ingested (quantity of radioactive material measured in units of Becquerels or Curies);
I-131's chemical form and physical properties;
the types, energies, and intensities of the emitted radiation;
physical and biologic half-lives;
the thyroid mass (which is age dependent); and
the thyroid uptake fraction (based on diet and metabolism).
With chronic exposure, the half-life of the radionuclides released becomes less relevant because new releases occur continuously. Many of the earlier off-site radiation exposures from nuclear weapon production facilities were chronic. In this case study, the term “dose” is used to refer to the radiation dose (the amount of energy deposited in tissue). The exposure dose depends on an individual's risk factors (for example, age at time of exposure and the consumption of milk and milk products).
The critical target organ for I-131 is the thyroid gland.
Illustration of the thyroid gland. Source: National Cancer Institute.
The thyroid gland uses iodine to produce thyroid hormones which help regulate growth and metabolism. Iodine has a strong affinity for the thyroid gland, which is the critical target organ for exposure. Iodine is readily absorbed from the gastrointestinal tract and lungs into the bloodstream. Most of the iodine that enters the body quickly becomes systemic (EPA 1988), with approximately 30% depositing in the thyroid. Exposure to I 131, especially in childhood, increases the risk for hypothyroidism, thyroid nodules, and cancer.
Hyperthyroidism or iodine deficiency results in increased uptake of I-131.
The metabolism of iodine is linked closely with the functional activity of the thyroid. The portion of systemic iodine that redistributes to the thyroid ranges from 20% (for hypothyroidism or iodine-rich diets) to 75% (for hyperthyroidism or iodine-deficient diets), with an average of 30%-50% for normal diets. The rest is excreted via urine. The moderate to severe iodine deficiency in the area near Chernobyl was a predisposing factor that caused thyroid doses to be higher than doses in regions where iodine uptake was normal.
Thyroid dose from ingestion of I-131 can be 10 times higher for newborns than for adults.
Thyroid dose from inhalation of I-131 can be two times higher for infants than for adults.
Thyroid dose can be 15-20 times higher than the overall dose to the rest of the body.
Estimating the radiation dose delivered by I-131 radiation to either the thyroid or the whole body involves multiplying the activity inhaled or ingested by an age-specific dose factor. Activity inhaled is the product of the mean air concentration of I-131, respiratory rate, and exposure time. Activity ingested is the product of the mean concentrations of I-131 in both food and water and the amounts of each consumed. These concentrations are functions of time delays between production and consumption as well as the geographic pattern of air concentration and fallout distribution. If the I-131 exposure was chronic, daily totals must be calculated and added using appropriate formulas and methods.
A child's thyroid dose from ingestion can be up to 20 times that of an adult because the same amount of energy is deposited in a smaller tissue mass. A child's thyroid dose from inhalation can be twice that of an adult, and is 15-20 times higher than the overall dose to the rest of the body. Children living in the maximum exposure area of the Hanford Nuclear Reservation were estimated to have received doses that were 10 times the estimated dose of adults over the same period.
Four factors can affect the internal contamination dose of persons who ingested milk containing the same I-131 concentrations.
Time between production and consumption. Because of the short half-life of I-131, even a short delay caused by the processing and transport of milk can decrease the radioactivity of ingested milk. This factor played an important role in past releases, depending on whether a population was urban or rural; in general, urban populations consumed processed milk transported from farms, whereas rural populations consumed unprocessed fresh milk. Most populations today consume processed milk.
Rate of consumption of fresh milk or of dairy products such as cheese. It is important to take into account that milk from cows, goats, and sheep contain different levels of I-131, and that goat's and sheep's milk have the highest concentrations.
Age and sex of exposed groups. Children and older people consume more milk than other groups do. Age at time of exposure is an important factor that influences individual thyroid dose. Because the infant's thyroid is small, the dose conversion factor for milk consumption is strongly dependent on age. After age 50, the thyroid mass and the capacity for uptake of iodine is gradually reduced. During pregnancy, the uptake of iodine is slightly increased because of the relative iodine deficiency of the body.
Geographic distribution of population related to the factors affecting thyroid dose (residence in relation to release or wind pattern).
