Gamma rays are the most harmful when from an external source, and alpha particles are the most damaging when the source is internal. Ionizing radiation poses risks by damaging molecules and causing cellular malfunctions.
Gamma rays, due to their great penetrating ability, are the most harmful type of radiation when coming from an external source. On the other hand, alpha particles are the most damaging when the radiation source is internal since they are absorbed by internal tissues. Ionizing radiation is the most harmful because it can damage molecules, disrupt cell processes, and lead to somatic or genetic damage.
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