All matter is potentially damaged when exposed to radiation. This article examines radiation exposure to living systems. Radiation exposure occurs when the energy associated with radiation is deposited in a living organism, which is said to "receive a radiation dose." In the United States, radiation doses are measured in "rems" or "millirems" (mrems); "rem" stands for r adiation e quivalent m an. The international unit of radiation is the Grey, and 100 rem = 1 Grey.
Radioactivity is a natural and spontaneous process that occurs when unstable atoms of an element emit or radiate excess energy in the form of particles or waves. Such emissions are called ionizing radiation. Ionization, the process by which molecules lose electrons, is a particular characteristic of the radiation produced when radioactive elements decay. The capacity of radioactive emissions to cause ionization of molecules is the basis for health hazards and provides the means by which radiation can be detected.
Low levels of radioactivity are commonly found in the air, the rocks, and the soil, in the water and oceans, and even in our building materials. Several important radioactive nuclides occur naturally, such as carbon-14 (C-14) and potassium-40 (K-40). These isotopes are generated by cosmic ray interactions and make their way into the food chain. Once ingested, they can decay and deposit an internal dose of radioactivity, however mild. Carbon-14 is a particularly useful isotope. All living organic material has a constant natural ratio of carbon-14 to nonradioactive carbon-12. Once an organism dies, the remaining organic material is no longer involved in the cycle of absorbing carbon and moving the carbon along the living metabolic chain. For example, plants absorb CO 2 and during their life processes produce a variety of carbohydrates (C 6 H 12 O 6 ) x . As long as the plant lives, all of the carbon in the plant is in the naturally occurring ratio and is in equilibrium with the natural ratio of carbon-14 to carbon-12 in the atmosphere. When the plant dies, that ratio in the dead plant starts changing because the carbon-14 decays, whereas the carbon-12 does not. By measuring that ratio of C-14 to C-12 in the dead plant, the approximate date of death can be determined. This process of establishing the date of the death of a living organism is called "carbon dating." Potassium-40 is naturally found in the soil and taken up by plants such as bananas. This fruit with the incorporated K-40 then becomes part of a food chain cycle.
Radiation in small doses, usually below 10 rem, generally does not have demonstrable biological effects. And there are suggestions that small doses may provide a health benefit in the form of radiation hormesis. Hormesis is a modification of the word hormone that describes stimulation induced at low doses of agents that are harmful or even lethal at high doses. Generally, hormesis is any stimulatory or beneficial effect, induced by low doses of an agent that cannot be predicted by the extrapolation of detrimental or lethal effects induced by high doses of the same agent.
Large doses of radiation, at the level of several hundred rems, may cause serious injury if received in a short time period (days or hours). This condition is called "acute radiation syndrome." Much larger doses can cause death. Doses between the large doses and small doses increase the risk of cancer.
Types of Radiation
Common types of ionizing radiation are alpha ( α ), beta ( β ), neutron, x-ray, and gamma ( γ ) radiation. Radioactive elements are often called radioactive isotopes or "radionuclides." An alpha particle consists of 2 protons and 2 neutrons; since there are no electrons in the alpha particle, it carries a charge of +2. Beta particles are electrons and carry a −1 charge. Both are referred to as "charged particles." Neutrons carry no charge. Gamma and x rays have neither mass nor charge. They are electromagnetic waves and are sometimes referred to as "photons." Cosmic radiation is divided into two types: primary and secondary. Primary cosmic radiation consists of high-energy particles, such as protons, that come from the Sun or other sources outside our solar system. The Sun-derived cosmic radiation is produced during solar flares. Only a small fraction of the primary cosmic radiation penetrates to Earth's surface; most interacts with the atmosphere, producing the secondary cosmic radiation, which produces other lower-energy radiation in the form of photons, electrons, neutrons, and muons that eventually reach the surface.
Earth's atmosphere and magnetic field act as shields against cosmic radiation, reducing the amount that reaches Earth's surface. The average radiation dose from cosmic radiation in the United States is 27 mrem per year. This dose roughly doubles with every increase of 6,000 feet of elevation. Traveling at high altitudes in an airplane increases exposure to cosmic radiation. A typical exposure obtained from flying in a commercial jet across the United States (coast to coast) provides a radiation dose of approximately 0.5 rem for each hour traveled.
Cosmic radiation can cause genetic changes. Some scientists believe that this radiation has been important in driving the development of life on our planet. While cosmic radiation can cause some damage to individuals, it also may have played an important role in the development of humans.
Earth's atmosphere is a natural shield against the harmful effects of primary cosmic radiation. If humans were to leave Earth and travel to some distant planet, exposure to high levels of primary radiation is a serious potential risk. Future space travelers will have to be shielded from exposure to cosmic rays.
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