Which type of radiation is least dangerous to external portions of the body but very dangerous if ingested or inhaled?

Alpha particles are indeed the type of radiation that is least dangerous to the external portions of the body but can be extremely hazardous if ingested or inhaled. This is primarily due to their relatively large mass and positive charge, which makes them unable to penetrate the outer layers of skin. When alpha emitters are outside the body, the skin acts as an effective barrier against them, resulting in minimal external exposure risk.

However, if alpha particles are ingested or inhaled, they pose a significant health risk. Inside the body, they can cause intense ionization and damage to surrounding biological tissues due to their high mass and charge, leading to increased likelihood of cellular damage and potential cancer development. This characteristic makes alpha radiation particularly dangerous when it enters the body, despite its low danger in external exposure situations.

In contrast, beta particles, gamma rays, and X-rays have different properties that allow them to penetrate the skin and cause harm externally. Beta particles can penetrate the skin more effectively, while gamma rays and X-rays can penetrate deep into the body, potentially affecting internal organs even without ingestion or inhalation.