Iodine Basics | Exposure Route | Radiation Dose | Health Risks | Protection Standards

The activity of I-131 (quantity of radioactive material present), the exposure route, and the individual’s age are factors that determine the exposure dose from radiation.

• Infants’ and children’s increased rate of growth and development make them more vulnerable to radiation exposures.
• Ingestion of contaminated milk has been the major I-131 exposure pathway for humans.
• The concentration of I-131 in milk from goats and sheep is 10 times higher than the concentration in cow’s milk.

The amount of I-131 available to expose a person after a release depends on the:

• amount released;
• distance between the populated area and the place of the release;
• height of the release; and
• meteorologic conditions at and after the time of the release.

The exposure pathway of greatest public health significance is the deposition of I-131 on pasture grasses, followed by the ingestion by cows or goats and the subsequent consumption of contaminated milk and fresh dairy products by humans.

Exposure begins immediately for persons in the immediate vicinity of a nuclear release who are in the plume (the visible or invisible cloud of contamination). Internal exposure by inhalation occurs for persons inside the plume. External exposure occurs while the person is in the plume or on land left contaminated by fallout from the plume. Internal exposure by ingestion occurs when persons eat food that is contaminated with the fallout. The oral pathway is the main route of internal I-131 exposure for people. Milk is the major source of internal exposure.

Dietary intake of iodine before exposure is important because a relative iodine deficiency increases the thyroid uptake of I-131. After exposure, the most critical dietary information needed is the amount and type of milk and milk products consumed, their I-131 concentrations, and the time they were consumed relative to the time of the release.

Goat’s milk and sheep’s milk contain approximately 10 times the concentration of radioiodine found in cow’s milk. Inhalation, especially near releases of I-131 in the absence of rain, is another route of internal exposure. However, doses to humans from inhalation and from ingestion of plants, animals, or water are usually small in comparison. The figure below shows the exposure pathways of I-131 from the environment to humans.

Figure. Exposure Pathways of I-131 From Environment to Humans