|MadSci Network: Medicine|
Brittney, The quick answer to your questions is "Yes, alpha emitters can be dangerous when inhaled into the lungs or consumed in water or food." The explaination is a bit more involved. The first stop is the chapter on Health Physics in Radiochemistry and Nuclear Methods of Analysis by W.D. Ehmann and D.E. Vance, John Wiley & Sons, Inc., New York, 1991. This is a bit technical but can be summarized as follows: An alpha particle is a helium nucleus (2 protons and 2 neutrons totalling an atomic mass of 4) that can be emitted by many naturally occurring radioactive nuclides such as isotopes of Uranium, Thorium, Radium, Radon, and all the rest of the natural decay chains until they reach lead. For example, Uranium-238 ends up as Pb-206. The 32 mass unit difference means that 8 alpha particles are emitted in the complete decay of one U-238 nucleus to Pb-206. If it is radon gas that is inhaled, then Rn-222 decaying to Pb-206 emits 4 alpha particles to make the 16 mass unit difference. Polonium-210 only emits 1 alpha particle to arrive at Pb-206. Sometimes the chemistry of the radioactive nuclides in the decay chain causes them to be separated. Polonium-210 is naturally concentrated by tobacco leaves, and therefore is inhaled by smokers to a larger degree than by non-smokers. There are two additional natural decay chains; one begins with Th-232 and ends at Pb-208, and the other begins with U-235 and ends with Pb-207. Those three chains, U-238, U-235, and Th-232, provide most of the alpha-particle emitting radionuclides we are exposed to in our lives. Alpha particles outside the body are not dangerous to us. The outer layer of the skin is enough to stop alpha particles before they can contact any of our living cells. This is because alpha particles are large, heavy, charged nuclei compared to electrons. When an alpha particle is emitted, it travels in a straight line because a collision with a heavy nucleus is very rare compared to hitting an electron (remember, almost all of an atom is empty space occupied by electrons with the heavy nucleus a very small particle at the center). But because the alpha particle is so heavy compared to the electron, and because it is positively charged compared to the electron's negative charge, the alpha particle collides with and knocks electrons out of their orbits. This is called ionization, and is why alpha particles (along with beta particles and gamma rays) are called ionizing radiation. Alpha particles knock out a large number of electrons in a very short distance; that is how they are slowed down, and eventually pick up two electrons to become ordinary helium atoms. If you put an alpha emitting nuclide on a piece of photographic film (it has to be put directly on the emulsion because the paper cover would stop the alpha particles) you would see a small dark circle a few millimeters in diameter around the alpha emitting nuclide. This shows that a large amount of energy has been deposited in a small area, and many atoms were ionized by the alpha particle. "Alpha particles will produce much more biological damage than the other two types of radiation [beta and gamma]. Therefore, alphas are the most biologically hazardous if they are ingested and incorporated into internal tissue." (Ehman & Vance page 181) Alphas are very easy to shield against externally, and the usual way of entry into the body is inhalation or ingestion. The next step is to look at the effects that may be caused by ingested alpha emitters. There is a lot of controversy over theories of causation of cancers or other abnormalities from alpha emitters. One theory is called the hot-particle theory. It says that cancer can be caused by the intense, local injury to a cell or group of cells caused by a particle containing an alpha emitter next to the cells. This is countered by the proponents of cell repair, or extermination by other body cells as part of the normal repair/replace function. I recommend going to: http://www.hps.o rg/publicinformation/ate/q806.html This reference presents the answer to the question of ingesting radioactive material. The site is the Health Physics Society, and the information is considered to be reliable. A way to obtain further information is to go to http://www.google.com and enter biological effect of alpha radiation in the search window. There is a lot of information out there on this question. But not all of it is correct, so be sure to look at the source of the information. One site talks about "microscopic nuclear explosions" that yield the alpha particles. This is not how alpha particles are emitted (actually by a complex quantum mechanical phenomenon called tunneling, where the alpha particle all of a sudden is outside the nucleus it was formerly inside by "tunneling through the wall".). One note on your question: all of the information you find is statistical, and would be called epidemiological statistics if applied to human populations. Epidemiological statistics can be applied to whole populations; THEY CANNOT BE APPLIED TO INDIVIDUALS. There are individuals who have never smoked who got lung cancer, and individuals who smoked all their lives who didn't get lung cancer. An individual either has lung cancer or doesn't have lung cancer (probability 1 or 0). Epidemiologic statistics say that out of a population of so many individuals with a given characteristic (such as smoker, or exposed to a radon concentration of so much in their house), there is a probability that so many of that group will develop lung cancer, but you cannot say which ones because that particular instance of the disease may have come from some other source.
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