Even the most energetic alpha particle from radioactive decay can be stopped by the outermost layer of dead skin that covers the body. Therefore, exposure to most alpha particles originating outside the body is not a serious hazard.

On the other hand, if alpha-emitting radioactive materials are taken inside the body (for example, by ingestion or inhalation), they can be the most damaging source of radiation exposure. The short range of the alpha particle causes the damaging effects of the radiation to be concentrated in a very localized area.