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All radioactive nuclides emit high-energy particles or electromagnetic waves. When this radiation encounters living cells, it can cause heating, break chemical bonds, or ionize molecules. The most serious biological damage results when these radioactive emissions fragment or ionize molecules. For example, α and β particles emitted from nuclear decay reactions possess much higher energies than ordinary chemical bond energies. When these particles strike and penetrate matter, they produce ions and molecular fragments that are extremely reactive. The damage this does to biomolecules in living organisms can cause serious malfunctions in normal cell processes, taxing the organism’s repair mechanisms and possibly causing illness or even death.

There is a large difference in the magnitude of the biological effects of nonionizing radiation (for example, light and microwaves) and ionizing radiation, emissions energetic enough to knock electrons out of molecules (for example, α and β particles, γ rays, X-rays, and high-energy ultraviolet radiation).

Energy absorbed from nonionizing radiation speeds up the movement of atoms and molecules, which is equivalent to heating the sample. Although biological systems are sensitive to heat, a large amount of nonionizing radiation is necessary before dangerous levels are reached. Ionizing radiation, however, may cause much more severe damage by breaking bonds or removing electrons in biological molecules, disrupting their structure and function. The damage can also be done indirectly, by first ionizing H₂O, which forms a H₂O⁺ ion that reacts with water, forming a hydronium ion and a hydroxyl radical.

Because the hydroxyl radical has an unpaired electron, it is highly reactive. This hydroxyl radical can react with all kinds of biological molecules (DNA, proteins, enzymes, and so on), causing damage to the molecules and disrupting physiological processes.

The energy delivered by each type of radiation to the tissues is different and is measured in terms of absorbed dose, the SI unit of which is the gray. The deposition of one joule of energy in one kilogram of material corresponds to one gray. The CGS unit, which is the rad, is also still widely used (1 rad = 0.01 Gy).

The biological response to the absorbed dose of each type of radiation is described by a radiation weighting factor, which depends on the ionizing power and penetration ability. Absorbed dose multiplied by radiation weighting factor is known as equivalent dose, which is measured in sievert in SI units. The CGS unit, which is the rem, is also still widely used (1 rem = 0.01 Sv).

Table 1. Radiation weighting factors.

Different body tissues have different sensitivities to ionizing radiation. If the exposure is concentrated in one area of the body or the equivalent dose is otherwise not even throughout the body, tissue weighting factors are used to determine the overall damage to the body given the uneven dose. The effective dose for the body is calculated by summing up the weighted equivalent doses for all the organs.

Several different devices are used to detect and measure radiation, including Geiger–Müller (GM) counters, scintillation counters, and radiation dosimeters. A Geiger–Müller counter has two parts: a cylindrical tube filled with an inert gas like argon or helium and a counter. Within the tube is a pair of electrodes with a high voltage across them. Any ionizing radiation starts a cascade of ionizations of gas molecules, creating a current between anode and cathode owing to the stream of electrons, which is collected, amplified, displayed by the counter as counts per minute or disintegrations per second. GM counters cannot differentiate between radiation types, but the energy-compensated variants can measure dose and thus can be used as personal dosimeters. A scintillation counter contains a scintillator—a material that emits light when excited by ionizing radiation—and a sensor that converts the light into an electric signal. Radiation dosimeters also measure ionizing radiation and are often used to determine personal radiation exposure. Commonly used types are electronic personal dosimeters, film badge, thermoluminescent, and quartz fiber dosimeters.

The effects of radiation depend on the type, energy, and location of the radiation source, and the length of exposure. The average person is exposed to background radiation, including cosmic rays from the sun and radon from uranium in the ground, radiation from medical exposure, including CAT scans, radioisotope tests, X-rays, and so on; and small amounts of radiation from other human activities, such as airplane flights (which are bombarded by increased numbers of cosmic rays in the upper atmosphere), radioactivity from consumer products, and a variety of radionuclides that enter our bodies when we breathe (for example, carbon-14) or through the food chain (for example, potassium-40, strontium-90, and iodine-131).

A short-term, sudden dose of a large amount of radiation can cause a wide range of health effects, from changes in blood chemistry to death. Short-term exposure to tens of rems of radiation will likely cause very noticeable symptoms or illness; an acute dose of 500 rems or 5 Sv is estimated to have a 50% probability of causing the death of the victim within 30 days of exposure. Exposure to radioactive emissions has a cumulative effect on the body during a person’s lifetime, which is another reason why it is important to avoid any unnecessary exposure to radiation.

This text is adapted from Openstax, Chemistry 2e, Section 21.6: Biological Effects of Radiation.