External exposure:

This may concern only a small part of the body (the head, a hand or a few square centimetres of skin in the case of external contamination).

Internal exposure:

Inside the body, some elements spread throughout the organism while others tend to accumulate in certain organs: thus iodine accumulates in the thyroid gland and strontium in the bones, whereas sodium is carried everywhere with the blood and produces the same effects as total exposure.