What distinguishes external exposure from internal contamination?

• A. External exposure comes from sources outside the body.
• B. Internal contamination arises from sources outside the body.
• C. External exposure is measured as dose rate in air only.
• D. External exposure occurs from ingestion of radioactive material.

Answer: (A)

When distinguishing external exposure from internal contamination, the key idea is where the radioactive source is relative to the body and how the dose is delivered. External exposure occurs when the radioactive source is outside the body and radiation reaches the body from outside, giving dose without the material being inside. In contrast, internal contamination happens when radioactive material is taken into the body by ingestion, inhalation, or wound contamination, and the radiation comes from inside as the material decays and distributes to organs.

You might hear dose rate measurements in air or at the body surface for external exposure, but the defining difference isn’t limited to where you measure it—it’s that the source is external to the body. Ingestion or inhalation would lead to internal contamination, not external exposure.