An important physical cause of toxic effects is a variety of forms of radiation. Ionizing radiation possesses enough energy to strip an electron from an atom. This can result in the formation of damaging free radicals or directly damage bonds in biochemical substances. The most sensitive system in living things is the DNA, since damage to a single molecule can transform a cell to malignancy. It is not necessary for a radioactive emission to damage a DNA molecule directly. The most abundant molecule in living things is water. Water can form free radicals when irradiated, and these in turn can produce toxic effects, including genotoxicity.
Ionizing radiation may be electromagnetic (g-rays, x-rays, ultraviolet rays) or particulate. The major particulate forms of radiation are a and p, but they may include neutrons, other subatomic particles, or larger particles; such as various atomic nuclei found in cosmic rays. Of primary interest here the a, p, and g emissions from the decay of radioactive atoms, or radionuclides. Each particle of radiation possesses energy, commonly measured in millions of electron volts (MeV). A change to SI units is under way that will replace the MeV with the joule: 1MeV = 1.6 x 10~13 J.
The energy of a particle relates to the amount of damage a particle can do to a tissue. As radiation passes through material such as tissue, it is absorbed, depositing its energy in the material. Much of this energy goes into stripping electrons from atoms. This requires an average of 33.85 eV per electron. This ability to eject electrons confers a capacity to disrupt chemical bonds with potentially damaging results.
Gamma (g) radiation is electromagnetic radiation (photons) of very high energy, usually less than 1 MeV. They are produced by radioactive decay of unstable atomic nuclei. X-rays are somewhat less energetic, up to 0.25 MeV for radiation from medical x-ray machines. Ultraviolet (UV) are even less energetic, ranging from about 4.13 to 155 eV. X-ray and UV photons are produced by excitation of an atom's electrons. Alpha (a) particles consist of two protons and two neutrons (helium nuclei). They have energies that range from 3 to 9 MeV. Beta (p) particles are high-energy electrons, with energies ranging up to 3.5 MeV. Alpha, beta, and gamma particles are all emitted by spontaneous reactions in the nuclei of unstable (radioactive) isotopes.
Although alpha particles are high in energy, they have the least penetrating power, due to their mass and charge. They are strongly absorbed and deposit their energy rapidly, producing a large amount of damage. For example, alpha particles with 5 MeV penetrate about 40 mm of soft tissue or about 5 cm in air. Thus, they do not penetrate the skin. However, if they are inhaled or ingested, they do not need to penetrate very far to reach cell nuclei, potentially causing genetic damage. Beta particles are charged, but have little mass. They penetrate about 3 m in air, but they are stopped by thin sheets of many solids. Thus, alpha and beta radiation is a problem mostly if their emitters are ingested. Gamma rays are the most penetrating. Their range in air is about 4 m, and they will penetrate several centimeters of lead. Consequently, g rays can cause their effects on living things either when emitters are ingested or by external radiation sources.
Many of the environmentally important radionuclides are products of the sequence of nuclear decay reactions that start with 238U. Each beta emission increases the atomic number without significantly changing the atomic mass by converting a neutron to a proton. An alpha emission reduces the atomic number by two and the atomic mass by four. Figure 21.4 shows the members of the series, with their half-lives and the energy carried
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