Which of the following substances is most potent in direct health risks through inhalation?

Inhalation of alpha particles presents the most significant direct health risks compared to the other options listed. Alpha particles are heavy, positively charged particles emitted from the nucleus of certain radioactive materials. When alpha-emitting substances are ingested or inhaled, they can cause considerable damage to sensitive internal tissues and organs. This is primarily due to their high mass and charge, which allows them to impart a large amount of energy over a very short distance, causing ionization of nearby atoms and extensive biological effects in a localized area.

On the other hand, beta particles, gamma rays, and X-rays are forms of radiation that do not pose the same level of risk via inhalation. Beta particles, while also harmful, cannot penetrate as deeply into tissue as alpha particles do and can be more effectively shielded against. Gamma rays and X-rays are high-energy electromagnetic radiation that, although they can pose risks through external exposure, are less damaging when inhaled because they generally pass through biological tissues with less interaction compared to alpha particles.

Therefore, the answer accurately reflects the risk associated with inhalation, as alpha particles pose a unique threat due to their physical properties, leading to heightened potential for internal damage.