Why are alpha particles considered virtually harmless as an external radiation source?

Alpha particles are considered virtually harmless as an external radiation source primarily because they do not have the ability to penetrate the skin. Their relatively large mass and electric charge result in a very short range in air and other materials. This means that when alpha particles are emitted, they can travel only a few centimeters in air and are easily stopped by barriers such as paper or even the outer layer of human skin.

This limited penetration capability is crucial because it implies that if a person is exposed to alpha radiation from an external source, the particles will not reach living tissues underneath the skin. Therefore, the primary danger of alpha particles arises not from external exposure but from internal exposure, such as ingestion or inhalation of alpha-emitting materials, where they can cause significant biological harm.

While it's true that alpha particles can be absorbed by clothing, the essential factor is their limited ability to penetrate living tissue, which greatly mitigates their potential for harm in external exposure situations.