Which type of radiation is generally a health hazard only when absorbed internally?

Alpha particles are indeed a type of radiation that poses a health hazard primarily when they are absorbed internally. This is because alpha particles have a relatively low penetration ability; they can be stopped by a sheet of paper or even the outer layer of human skin. However, if alpha-emitting materials are ingested or inhaled, they can deliver a significant dose of radiation to sensitive tissues, such as those in the lungs or gastrointestinal tract, leading to potential health issues, including cancer.

In contrast, beta particles, gamma rays, and X-rays are forms of radiation that can be hazardous through external exposure since they have greater penetrating power. Beta particles can penetrate the skin and potentially cause harm, and gamma rays and X-rays are highly penetrating forms of radiation that can pass through the body and affect internal organs even without direct absorption. Thus, the unique risk factor associated with alpha particles is that their danger is mostly linked to internal exposure, which makes them particularly hazardous in contexts where radioactive materials are ingested or inhaled.