21.6 Biological Effects of Radiation

Biological Effects and Measurement of Radiation

Effects of Ionizing Radiation

Ionizing radiation damages living organisms through two main mechanisms: direct and indirect.

In direct damage, radiation interacts with biological molecules like DNA, proteins, and lipids, breaking their structure and disrupting their function. In indirect damage, radiation hits water molecules in your cells and produces free radicals and reactive oxygen species (ROS). These highly reactive fragments then go on to damage nearby biological molecules.

DNA damage is the most dangerous outcome. Radiation can cause single-strand breaks, double-strand breaks, mutations, and chromosomal rearrangements. Depending on the severity, this can lead to:

• Cell death
• Impaired cell function
• Uncontrolled cell growth (cancer)

The biological impact depends on several factors: the type of radiation (alpha, beta, gamma, X-rays), the energy it carries, the dose received, the dose rate, and how sensitive the exposed tissue is. At high doses, acute radiation syndrome can occur, with symptoms like nausea, vomiting, and potential organ failure.

Units of Radiation Measurement

There are several units used to quantify radiation, and each one captures something slightly different. Here's how they build on each other:

• Exposure measures how much ionization radiation produces in air. The unit is the roentgen (R), defined as $1 \text{ R} = 2.58 \times 10^{-4} \text{ C/kg of air}$. This only applies to X-rays and gamma rays in air.
• Absorbed dose measures the energy actually deposited per unit mass of any material (including tissue). The SI unit is the gray (Gy), where $1 \text{ Gy} = 1 \text{ J/kg}$. The older unit is the rad, where $1 \text{ rad} = 0.01 \text{ Gy}$.
• Equivalent dose adjusts the absorbed dose to account for how damaging a particular type of radiation is biologically. It's calculated as:

$\text{Equivalent dose} = \text{Absorbed dose} \times W_R$

where $W_R$ is the radiation weighting factor (for example, $W_R = 1$ for gamma rays but $W_R = 20$ for alpha particles, reflecting how much more damage alpha particles do per unit of energy). The SI unit is the sievert (Sv); the older unit is the rem, where $1 \text{ rem} = 0.01 \text{ Sv}$.

• Effective dose goes one step further by accounting for how sensitive different organs are to radiation. It sums up the equivalent doses to each organ, each multiplied by a tissue weighting factor. This is also reported in sieverts.

Methods for Detecting Radioactivity

Different detectors are suited for different situations:

• Geiger-Müller counters detect alpha, beta, and gamma radiation by producing electrical pulses when radiation ionizes gas inside the tube. They're commonly used for radiation surveys and contamination checks. You've probably seen these in movies or labs: they're the ones that "click."
• Scintillation detectors use materials that emit flashes of light when struck by gamma rays or X-rays. Those light flashes are converted into electrical signals. These detectors are used in medical imaging (like PET scans) and radiation spectroscopy.
• Solid-state detectors (often made of germanium) work by creating electron-hole pairs in a semiconductor when radiation passes through. The resulting electrical signal is proportional to the radiation's energy, making these detectors excellent for high-resolution gamma spectroscopy and environmental monitoring.
• Film badges and thermoluminescent dosimeters (TLDs) measure accumulated radiation dose over time, making them ideal for personal dosimetry. Film badges contain radiation-sensitive film that darkens with exposure. TLDs store energy from radiation and release it as light when heated, allowing the total dose to be read. Both are standard tools for monitoring workers in hospitals, labs, and nuclear facilities.

Sources of Everyday Radiation Exposure

You're exposed to small amounts of radiation every day from a variety of sources:

• Natural background radiation is the largest contributor for most people. It includes cosmic rays from space, terrestrial radiation from radioactive elements in the Earth's crust (uranium, thorium, and especially radon gas), and internal radiation from isotopes naturally present in your body like $^{40}\text{K}$ (potassium-40) and $^{14}\text{C}$ (carbon-14).
• Medical sources include diagnostic X-rays (dental, chest, CT scans), nuclear medicine procedures (PET scans, thyroid scans using radioactive iodine), and radiation therapy for cancer treatment.
• Consumer products can contain trace radioactive materials. Tobacco contains $^{210}\text{Po}$ (polonium-210), some building materials like granite emit low levels of radiation, and certain older luminous watches used tritium or $^{147}\text{Pm}$ (promethium-147).
• Occupational sources affect nuclear power plant workers, radiologists, nuclear medicine technicians, and researchers who handle radioactive materials regularly.
• Nuclear fallout from weapons testing and accidents like Chernobyl (1986) and Fukushima (2011) can also contribute to environmental radiation exposure.

Radiation Dose-Response Models

Scientists use different models to estimate the health risk of radiation exposure, particularly for cancer:

• The linear no-threshold (LNT) model assumes that any radiation dose, no matter how small, increases cancer risk proportionally. This is the most widely used model for setting safety standards.
• The threshold model proposes that there's a safe level of exposure below which no harmful effects occur.
• The radiation hormesis hypothesis suggests that very low doses of radiation might actually stimulate protective biological responses and be slightly beneficial.

No model is definitively proven at very low doses, since the effects are too small to measure directly in populations. However, the LNT model is the basis for most current radiation protection guidelines and regulations for both public and occupational exposure.