Radiation interacts with water in the human body primarily through a process called radiolysis, where the energy from radiation breaks down water molecules into highly reactive fragments. Since the human body is about 60-70% water, this interaction is fundamental to how radiation affects biological tissues.
When ionizing radiation—such as X-rays, gamma rays, or particles like alpha and beta particles—passes through the body, it deposits energy into water molecules. This energy is sufficient to break the chemical bonds holding the water molecules (H₂O) together, producing free radicals. The most common free radicals generated are hydroxyl radicals (·OH), hydrogen atoms (·H), and hydrated electrons (e⁻_aq). These free radicals are extremely reactive and can cause damage to nearby biological molecules, including DNA, proteins, and lipids.
The sequence begins with the absorption of radiation energy by water, leading to ionization and excitation:
1. **Ionization:** Water molecules lose an electron, forming a water ion (H₂O⁺) and a free electron.
2. **Excitation:** Water molecules are excited to a higher energy state without losing electrons but become unstable.
3. **Radical Formation:** The water ion quickly reacts with another water molecule to form hydroxyl radicals and hydronium ions. The free electron becomes solvated, forming a hydrated electron.
These reactive species then diffuse through the cellular environment and interact with critical biomolecules. The hydroxyl radical, in particular, is highly damaging because it can abstract hydrogen atoms from DNA, causing strand breaks or base modifications. This damage can lead to mutations, cell malfunction, or cell death if not properly repaired.
Besides direct damage to DNA and other molecules, the interaction of radiation with water also leads to indirect effects. Because water is abundant, most radiation damage in cells is actually caused by these free radicals rather than direct hits to DNA. This indirect action accounts for a significant portion of radiation-induced biological effects.
The extent of damage depends on several factors:
– **Type and energy of radiation:** High linear energy transfer (LET) radiation like alpha particles causes dense ionization tracks, leading to clustered DNA damage that is harder to repair. Low LET radiation like X-rays causes more sparse ionizations.
– **Oxygen presence:** Oxygen can react with free radicals to form peroxides, which “fix” the damage, making it permanent and more harmful. This is known as the oxygen effect.
– **Cellular environment:** The presence of antioxidants and repair enzymes can mitigate damage.
Radiation-induced water radiolysis also produces molecular products like hydrogen peroxide (H₂O₂) and molecular hydrogen (H₂), which can further influence cellular oxidative stress.
In the context of the whole body, radiation interacting with water leads to a cascade of molecular events that can manifest as tissue damage, inflammation, and increased risk of cancer. For example, in radiation therapy, the goal is to maximize damage to cancer cells by exploiting these interactions while minimizing harm to normal tissues.
On a larger scale, water in the body also plays a role in distributing radioactive contaminants if ingested or absorbed. Radioactive isotopes dissolved in bodily fluids can irradiate tissues internally, causing localized damage depending on the isotope’s decay properties.
In summary, radiation interacts with water in the body by breaking water molecules into reactive free radicals through radiolysis. These radicals then cause indirect damage to vital biomolecules, especially DNA, leading to cellular injury or death. This mechanism underlies much of the biological impact of radiation exposure.
