Which statement best describes alpha particle range, shielding, and hazard?

• A. They travel 1-2 inches in air and are stopped by paper; external hazard is low, internal hazard is high
• B. They travel several feet in air and require thick shielding
• C. They are stopped by thin barriers like paper or skin; external hazard is low but internal hazard is high
• D. They can penetrate skin deeply, posing external hazard

Answer: (C)

Alpha particles are heavy, highly ionizing but have very limited penetration. They can be stopped by a thin barrier such as a sheet of paper or even the outer layer of skin, which means external exposure risk is low. The real hazard comes when alpha-emitting material is inside the body—through ingestion or inhalation—where they cause intense local ionization and tissue damage, making the internal hazard high. That combination—stopped by thin barriers, with low external hazard but high internal hazard—best describes their range, shielding, and hazard. The other statements either exaggerate how far alpha particles travel in air or incorrectly imply deep penetrating capability, which isn’t the case.