32.2 Biological Effects of Ionizing Radiation

Ionizing radiation can wreak havoc on our cells, damaging DNA and causing a range of effects from mutations to cell death. Understanding how radiation impacts biology is crucial for assessing health risks and developing protective measures.

Measuring radiation exposure involves different units like rad, gray, rem, and sievert. These units help quantify the energy absorbed by tissue and its biological impact. Radiation can cause acute syndromes and long-term health effects, with risks varying based on factors like dose and individual sensitivity.

Effects of Ionizing Radiation on Biology

DNA and cellular effects of radiation

• Ionizing radiation transfers energy to atoms or molecules in cells causing ionization
  • Directly damages DNA by breaking chemical bonds
  • Indirectly damages DNA by producing free radicals that react with and damage DNA
• DNA damage from ionizing radiation includes
  • Single-strand breaks where one strand of the DNA double helix is broken
  • Double-strand breaks where both strands of the DNA double helix are broken
    • More difficult for cells to repair than single-strand breaks
  • Base damage involving alteration or loss of DNA bases (A, T, C, G)
• Cellular effects of ionizing radiation involve
  • Cell death as high doses can cause immediate cell death
  • Mutations when DNA damage leads to genetic mutations if not repaired correctly
    • Can result in cancer (leukemia) or other diseases (Huntington's)
  • Chromosomal aberrations that change chromosome structure or number
  • Cell cycle arrest which delays cell division to allow time for DNA repair
• DNA repair mechanisms work to fix radiation-induced damage
  • These mechanisms can sometimes fail, leading to mutations or cell death

Units of radiation dose measurement

• Absorbed dose quantifies the amount of energy deposited per unit mass of tissue
  • Measured in rad (radiation absorbed dose)
    • 1 rad = 0.01 J/kg
  • SI unit is gray (Gy)
    • 1 Gy = 100 rad
• Equivalent dose is the absorbed dose weighted by the biological effectiveness of the radiation type
  • Measured in rem (roentgen equivalent man)
    • Equivalent dose (rem) = absorbed dose (rad) × quality factor (QF)
    • QF depends on radiation type (x-rays, gamma rays, alpha particles)
  • SI unit is sievert (Sv)
    • 1 Sv = 100 rem
• Effective dose is the equivalent dose weighted by the sensitivity of each organ or tissue
  • Measured in rem or sievert
  • Accounts for the varying susceptibility of different organs and tissues to radiation damage

Health impacts of radiation exposure

• Acute radiation syndrome (ARS) occurs after high-dose, short-term exposure (> 0.5 Gy)
  1. Hematopoietic syndrome (0.5-2 Gy) damages bone marrow and blood cells
  2. Gastrointestinal syndrome (2-6 Gy) damages intestinal lining
  3. Neurovascular syndrome (> 6 Gy) damages nervous system and cardiovascular system
• Long-term health effects include
  • Cancer with increased risk of various types (leukemia, solid tumors)
    • Latency period is the time between exposure and cancer development (years or decades)
  • Cardiovascular disease with increased risk of heart disease and stroke
  • Cataracts causing clouding of the eye lens
  • Fertility issues involving temporary or permanent sterility
• Dose-response relationship models
  • Linear no-threshold (LNT) model assumes any dose, no matter how small, increases cancer risk
  • Threshold model assumes there is a dose below which no adverse effects occur
  • Radiation hormesis hypothesis suggests low doses of radiation may have beneficial effects
• Factors affecting health risks
  • Age at exposure as children and fetuses are more sensitive to radiation
  • Sex with women generally more sensitive to radiation than men
  • Genetic susceptibility as some individuals may have genetic factors that increase their radiation sensitivity

Cellular responses to radiation

• Bystander effect occurs when non-irradiated cells exhibit radiation-induced effects due to signals from nearby irradiated cells
• Adaptive response is a phenomenon where low doses of radiation can make cells more resistant to subsequent higher doses
• Free radicals produced by ionizing radiation can cause oxidative stress and further cellular damage beyond direct DNA effects