Radiation, Biological Damage
█ ABDEL HAKIM BEN NASR
█ BRIAN HOYLE
The nuclear explosions at Hiroshima and Nagasaki, Japan on August 6 and 9, 1945, demonstrated the immense power of the nuclear bomb. The effects of the explosion were immediate. The radiation that was released by the explosions, however, caused the deaths of many people weeks, months, and even years later. It is this radiationinduced biological damage that can ultimately claim more lives than those lost in the blast of a nuclear weapon.
Radiation released in a nuclear explosion consists of particles that have a high energy. When these particles encounter biological material, in particular deoxyribonucleic acid (DNA), they can break the DNA strands. The breakage can be so severe that a cell's repair machinery cannot compensate. Because DNA is the blueprint for the structure and all the activities that occur in cells, the radiation-induced damage to DNA is lethal to the affected cells.
Radiation exposure that does not kill cells outright can cause sublethal damage that scrambles the sequence of information contained in the DNA. As a result, when the DNA is used to make proteins, proteins that are altered from the intended forms will be made. These represent mutations.
Mutations occur naturally at a very low rate. Using special agents called mutagens can increase the frequency of these mutations. Ionizing radiation was the first mutagen that efficiently and reproducibly induced mutations in a multicellular organism. Radiation is often classified as ionizing or non-ionizing depending on whether ions are emitted in the penetrated tissues or not. Examples of ionizing radiation include x rays, gamma rays, beta particle radiation, and alpha particle radiation (also known as alpha rays). The ultraviolet radiation that is a component of sunlight is an example of a nonionizing radiation.
Different types of radiation have different energies, and so have different effects. With alpha radiation, ionizations produce an intense but more superficial and localized deposition of energy. The energy of x rays and gamma radiation traverses deeper into tissues. This penetration leads to a more even distribution of energy as opposed to the more concentrated or localized alpha rays.
The different behaviors of different types of radiation can be used to some extent to tailor the radiation to selected cellular components. Experiments conducted on animals have shown that repeated exposure to radiation produces a higher frequency of mutations than a single exposure to a higher level of radiation. In other words, exposure to a low level of radiation can be damaging over time.
The relative efficiencies of the different types of radiation in producing mutations can be compared, and is known as the mutagenic effect. Investigation of radiation's mutagenic effects on different tissues, cells, and subcellular compartments is becoming possible by the availability of techniques and tools that allow the precise delivery of small doses of radiation and that provide better monitoring of effects.
Cells that are irradiated release a form of oxygen that is unstable, and which reacts with cellular components in a way that is damaging. DNA can be damaged, as can components called bases, which are assembled to form DNA strands. As well, the reactive oxygen can damage enzymes that function to repair damaged DNA. There is evidence that radiation damage in one cell can be passed on to neighbouring cells. Even the neighbouring cells may be damaged genetically. Thus, radiation damage, especially due to low levels of radiation, may be more extensive than previously assumed.
This increased risk of radiation damage is of concern, as terrorist organizations such as al Qaeda have made efforts to develop and deploy "dirty bombs"—conventional explosives that release a payload of radioactive material. In 2002, an American citizen was arrested for his alleged involvement with al Qaeda to detonate a dirty bomb inside the United States. The spray of radiation in a mid-level dirty bomb could produce a relatively low level of radiation over a fairly localized area. In a denselypopulated city, thousands of people could be exposed to harmful levels of radiation from an explosion from a dirty bomb.
█ FURTHER READING:
Cheung, Kin P. Plasma Charging Damage. Berlin: Springer Verlag, 2000.
Mangano, Joseph, J. Low level radiation and Immune System Damage: An Atomic Era Legacy. Boca Raton: Lewis Publishers, 1998.
Azzam, E. I., S. M. de Toledo, and J. B. Little. "Direct evidence for the participation of gap junction-mediated intercellular communication in the transmission of damage signals from alpha-particle irradiated to nonirradiated cells." Proceedings of the National Academy of Sciences no. 98 (2001): 473–478.