Which events occur within fractions of a second after exposure to radiation?

• A. Absorption of radiation energy
• B. All of the above
• C. Formation of active radicals
• D. Biochemical changes in cells

Answer: (B)

When radiation interacts with tissue, energy is deposited immediately as the particles or photons transfer energy to molecules, causing ionization and excitation. In most body tissue, water is abundant, so radiolysis of water occurs right away, producing reactive radicals such as hydroxyl and hydrogen radicals. These active radicals rapidly react with cellular components, leading to chemical changes inside the cell that can affect DNA, membranes, and other macromolecules. Because all of these steps unfold in a very short timescale—within fractions of a second—the events described (energy absorption, radical formation, and biochemical changes) all occur almost immediately after exposure. That’s why “All of the above” best captures what happens in that tiny time window.