Americium is a human-made, radioactive element. There are no naturally occurring or stable isotopes of americium. Pure americium is a silvery metal. There are two important isotopes of americium, americium 241 and americium 243, also written as 241Am and 243Am, and read as americium two-forty-one and americium two-forty-three. Both isotopes have the same chemical behavior in the environment.
Quantities of americium, as well as other radioactive elements, can be measured in units of mass (grams), but are usually measured in terms of their radioactivity (curies or becquerels). Both the curie (Ci) and the becquerel (Bq) tell us how much a radioactive material decays every second. The becquerel is a new international unit and the curie is a traditional unit; both units are currently used in the United States. A becquerel is the amount of radioactive material in which 1 atom transforms every second, and a curie is the amount of radioactive material in which 37 billion atoms transform every second.
Each radioactive isotope of an element, including americium, constantly gives off radiation, which changes it into an isotope of a different element or a different isotope of the same element. This newly formed nuclide may be stable or radioactive. This process is called radioactive decay. 241Am and 243Am give off alpha particles (sometimes referred to as alpha radiation) and change into neptunium 237 (237Np) and neptunium 239 (239Np), respectively. Neptunium is also radioactive, so isotopes of this element also give off radiation and change into isotopes of other elements. This process continues and eventually ends when stable isotopes of bismuth and lead are formed.
Half-life is the term that is used to describe the rate of the decay process. Specifically, the half-life is the time it takes for half of the atoms of a radionuclide to undergo radioactive decay and change it into a different isotope. The half-life of 241Am is 432 years. The half-life of 243Am is 7,370 years.
Specific activity is a term that describes the relative rates of decay of the same mass of different radioactive materials. If you have 1 gram of each isotope of americium side by side, the specific activities would be 3.4 curies per gram for 241Am and 0.2 curies per gram for 243Am. Thus, for the same mass, 241Am is about 17 times more radioactive (i.e., its specific activity would be 17 times higher) than that of 243Am. The higher the specific activity of a radioisotope, the faster it is decaying.
The predominant commercial use of 241Am is in ionization-type smoke detectors. This application relies on the alpha particles that are produced when the isotope decays as an ionization source. A typical household smoke detector contains 0.9 microcuries (µCi; a µCi is one millionth of a curie) or 33,000 Bq of 241Am, and 1 g of americium dioxide is sufficient to make 5,000 smoke detectors. 241Am is also used for industrial gauging applications and in medical diagnostic devices. There are research applications, but no commercial applications for 243Am. This will change if the United States and Russia turn nuclear warheads into power reactor fuel by making a mixed oxide fuel of uranium and plutonium. This fuel, called MOX, will contain both 241Am and 243Am, and the plan is to use it to make electricity. Once used, the spent nuclear fuel is to be sent for disposal.
Pathways of americium in the environment
241Am can be released to the environment from nuclear reactors, nuclear explosions, and accidents, as well as from manufacturing products containing americium (such as smoke detectors). Americium is a byproduct of plutonium production.241Am is formed from the radioactive decay of plutonium 241 (241 Pu), which itself is produced from uranium 238 (238U) while it is inside an operating nuclear reactor. In the absence of clean-up procedures, radioactive decay is the only way for decreasing the amount of americium in the environment. Because 241Am has a long half-life, it will be present in the environment for a long time. Since 241Pu can be released along with 241Am in the decay process, the amount of 241Am will slowly increase for decades, even in the absence of additional 241Am releases, and then slowly decrease.
241Am released to the atmosphere will be associated with particles and will settle to earth or be washed from the air in precipitation (e.g., rain, snow). 241Am from atmospheric nuclear weapons tests may remain in the atmosphere for decades before settling to earth. For example, 241Am released in nuclear accidents, like the accident at Chernobyl, will stay in the lower atmosphere and begin to settle to earth near the site from which it was released. Larger particles will settle out more quickly and over a smaller area; smaller particles may remain in the atmosphere for several months and travel far from where they were released. Precipitation removes particles from the air more rapidly.
241Am released into water from nuclear facilities will tend to stick to particles in the water or the sediment. 241Am deposited on soil is so strongly attached to soil particles that it does not travel very far into the ground. Ultimately, most americium ends up in soil or sediment.
While plants may take up 241Am from the soil, the amount taken up is small, especially in the parts of the plant that are most often eaten (i.e., the fruit, grain, and seeds). While fish may take up 241Am, the amount that builds up in the flesh is very small. Most of the 241Am found in shellfish like shrimp or mussels is contained in the shell, rather than in the edible parts of the animal.
Exposure to americium
You may be exposed to 241Am by breathing air, drinking water, or eating food containing 241Am; however, the levels of 241Am in air, water, soil, and food are generally very low, and of little health consequence.
People working at sites where nuclear waste is stored, in nuclear power plants, or in other nuclear facilities that handle 241Am may be exposed to higher levels of americium than the general population. People living near these sites also may be exposed to elevated levels of americium. People who produce or handle 241Am in smoke detectors or other devices may be exposed to higher levels. In the case of a nuclear accident, exposure could occur by breathing or eating dust containing 241Am. Even at sites that contain measurable levels of radioactive contamination, the amount of 241Am that is taken up into plant and animal tissue is so small that it is of little concern.
Pathways of americium in the body
Americium can enter your body from the air you breathe, the food you eat, or the water you drink, or from contact with your skin. If you breathe in air that contains americium, some forms of americium that dissolve, easily in lung fluid stay in your lungs for hours or days. Other forms that do not dissolve easily in lung fluid might stay in your lungs for months or years. Some americium that enters your lungs may get into your blood. If you swallow americium, a very small amount of what enters your digestive tract may also enter your blood. Most of the americium entering your blood leaves your body in your urine and feces. Of the small amount that stays in your body, most goes to your bones, where it can remain for many decades; a smaller amount goes into your liver and other organs, where it may remain for a few years as the body clears it.
Health effects of americium
To protect the public from the harmful effects of toxic chemicals and to find ways to treat people who have been harmed, scientists use many tests.
One way to see if a chemical will hurt people is to learn how the chemical is absorbed, used, and released by the body. In the case of a radioactive chemical, it is also important to gather information concerning the radiation dose and dose rate to the body. For some chemicals, [animal testing alternatives|animal testing]] may be necessary. Animal testing may also be used to identify such health effects as cancer or birth defects. Without laboratory animals, scientists would lose a basic method to get information needed to make wise decisions to protect public health. Scientists have the responsibility to treat research animals with care and compassion. Laws today protect the welfare of research animals, and scientists must comply with strict animal care guidelines.
Additionally, there are vigorous national and international efforts to develop alternatives to animal testing. The efforts focus on both in vitro and in silico approaches and methods. For example, the National Toxicology Program (NTP) at the National Institute of Environmental Health Sciences (NIEHS) created the NTP Interagency Center for the Evaluation of Alternative Toxicological Methods (NICEATM) in 1998. The role of NICEATM is to serve the needs of high quality, credible science by facilitating development and validation—and regulatory and public acceptance—of innovative, revised test methods that reduce, refine, and replace the use of animals in testing while strengthening protection of human health, animal health and welfare, and the environment. In Europe, similar efforts at developing alternatives to animal based testing are taking place under the aegis of the European Centre for the Validation of Alternative Methods (ECVAM).
In the process of radioactive decay, americium releases alpha particles and gamma rays. Alpha particles are relatively high energy particles, but travel only extremely short distances and do not penetrate the skin. However, if americium is taken into the body and enters body tissues, alpha particles may produce damage to nearby cells. Gamma rays can travel much greater distances and can penetrate the entire body. Since alpha particles do not penetrate the skin and the gamma rays released from americium sources are relatively low in energy, external exposure to americium is not usually considered to be a danger to your health.
The radiation from americium is the primary cause of adverse health effects from absorbed americium. Upon entering the body by any route of exposure, americium moves relatively rapidly through the body and is deposited on the surfaces of the bones where it remains for a long time. As americium undergoes radioactive decay in the bone, alpha particles collide with nearby cell matter and give all of their energy to this cell matter. The gamma rays released by decaying americium can travel much farther before hitting cellular material, and many of these gamma rays leave the body without hitting or damaging any cell matter. The dose from this alpha and gamma radiation can cause changes in the genetic material of these cells that could result in health effects such as bone cancers. Exposure to extremely high levels of americium, as has been reported in some animal studies, has resulted in damage to organs such as the lungs, liver, kidneys, and thyroid. It is rare, however, that a person would be exposed to amounts of americium large enough to cause harmful effects in these organs.
Health effects on children
This section discusses potential health effects from exposures during the period from conception to maturity at 18 years of age in humans.
Americium accumulates in human bones and remains there for a long time. Americium emits radioactive alpha particles in the bone that can cause damage to the surrounding tissue. The body normally repairs all of the damage, but if it fails, bone cancer could result after many years. If americium exposure to children occurs, the exposure to the radiation from americium may be for a longer period of time, causing a larger radiation dose over their lifetime and increasing the likelihood of producing cancer. However, there are no published data showing that children are more susceptible than adults to radiation-induced americium toxicity. A developing baby is expected to have some americium in its body. It could be affected by this americium while in the womb if the pregnant mother has high enough levels of americium in her blood.
Reducing risk of exposure to americium
If your doctor finds that you have been exposed to significant amounts of americium, ask whether your children might also be exposed. Your doctor might need to ask your state health department to investigate.
While discharges from nuclear waste sites, nuclear reactors, or plants that manufacture ionization smoke detectors or gauges containing americium are regulated, it is possible that higher-than-normal levels of americium may be in soil near a nuclear waste site, nuclear reactor, or plant that manufactures ionization smoke detectors or gauges containing americium. If you live near any of these types of nuclear or manufacturing sites, discourage your children from putting their hands in their mouths and engaging in other hand-to-mouth activities.
Discharge water and air emissions from facilities that make americium smoke detectors or gauges or produce plutonium for nuclear weapons may contain some americium. These operations are strictly regulated, but you can check local health advisories before consuming fish or other food from these waters. Nuclear reactors are not expected to discharge measurable amounts of americium.
Ionization smoke detectors that contain americium are safe when installed; however, you should not allow your children to play with these detectors. If the detectors are damaged or are no longer being used, they should be promptly returned to the manufacturer for disposal.
Medical tests for exposure to americium
Physicians do not test for americium in their offices, but they can collect samples (that is, biomonitoring) and send them to special laboratories or request that you be sent to such a laboratory. If you are exposed to a large dose of radiation from americium, your blood can be tested for signs of damage to the cell chromosomes. If americium were to enter your body from contaminated air, food, or water, the amount of americium (in Ci or Bq) can be estimated by measuring the radiation given off by the americium present in samples of blood, urine, teeth, or tissue. If in the lungs, the amount of americium can be estimated by measuring the americium gamma radiation that exits the body. Radiation detector systems used for these purposes are at a limited number of locations. The amount in your lungs and excretions falls sharply after exposure, so tests should be done as soon as possible.
- The Agency for Toxic Substances and Disease Registry
- Interagency Coordinating Committee on the Validation of Alternative Methods
- European Centre for the Validation of Alternative Methods
- Institute for Laboratory Animal Research
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