Cell damage can be caused by direct or indirect ionization (Figure 1). Direct ionization occurs when charged particles, e.g., electrons, with sufficient kinetic energy interact with cellular atoms or molecules to create free radicals.4,6,10,11 The process is known as direct because the interaction occurs directly between a particle and a cellular component without an intermediary step. The charged particle can continue to interact with other molecules until all its kinetic energy is lost.
Indirect ionization occurs when non-charged particles, e.g., photons, interact with cellular water. The energy absorbed by an H2O molecule results in the formation of ion pairs and reactive oxygen metabolites such as hydroxyl radicals.4,6,11 In turn, these free radicals interact with cellular atoms and molecules damaging cellular proteins and may form additional free radicals.2,4,6,10,11 The process is known as indirect because of the intermediate step of H2O-based free radical formation.
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