Which type of radiation is emitted from a strong radiologic source and is the least harmful?

Alpha radiation is considered the least harmful of the types listed because of its physical properties. Alpha particles are relatively large and carry a positive charge, which means they have low penetration power. They can be stopped by a sheet of paper or even the outer layer of human skin, making them less of an external hazard.

However, if alpha-emitting substances are ingested or inhaled, they can cause significant damage due to their high mass and energy, but in terms of external exposure, they pose minimal risk compared to other forms of radiation.

On the other hand, beta particles can penetrate the skin and cause radiation burns, gamma radiation and X-rays are highly penetrating and can damage internal tissues, making them more dangerous in terms of external exposure. Therefore, the primary reason alpha radiation is viewed as less harmful lies in its limited ability to penetrate and affect living tissues from outside the body.