Which radiation is least dangerous as an external hazard but dangerous if ingested or inhaled?

• A. Alpha
• B. Gamma
• C. Electromagnetic
• D. Ultraviolet

Answer: (B)

Alpha radiation is the type that fits this pattern. Its particles are heavy and carry a strong charge, so they don’t travel far in air and are effectively stopped by just a sheet of paper or the outer layer of skin. That means external exposure to alpha emitters is usually not dangerous. But if the material emitting alpha is inhaled or ingested, those particles end up inside the body and deposit a lot of energy in nearby tissues, causing significant internal damage.

Gamma radiation, by contrast, is highly penetrating—external exposure can be dangerous even with shielding, and internal exposure doesn’t remove that risk. Ultraviolet can harm surfaces like skin or eyes but isn’t a hazard in the same sense when ingested or inhaled, since it’s an electromagnetic wave that doesn’t cause the same internalized dose. Electromagnetic radiation is a broad category and isn’t the specific hazard type described here.

So, the radiation that is least dangerous as an external hazard but dangerous if ingested or inhaled is alpha radiation.