Ionizations lead to chemical changes:

• Free radical production
• Broken bonds, importantly double-strand DNA breaks

Since the intracellular environment is essentially aqueous, water is the most likely molecule encountered by radioactive energies. Radiolysis of water may produce H⁺, OH^-.

Damage caused by such free radicals represents the INDIRECT action of ionizing radiation. Most biological effects of low LET radiation can be attributed to free radicals. Less commonly, nucleoproteins or DNA may be ionized directly by charged particles, but not electromagnetic radiations.