Substances are radioactive because their atomic structures are unstable or as a result of nuclear reactions. Radioactive substances can give off or emit different types of energy or radiation. These include alpha particles, beta particles, and gamma rays, which are produced in the atom's nucleus. Some radioactive substances emit one or more types of radiation. The effects of high-level exposures can include burns, acute radiation sickness, and death. High-level exposures usually occur in occupational or medical settings or during accidents or war. The long-term health risks include cancer, birth defects, infertility, and genetic abnormalities. Because each type of radiation has different physical properties, the risks and potential effects on health of each are different. Some forms of radiation can penetrate the skin; others affect the body only if inhaled, ingested, absorbed through the skin, or enter through a wound. The effect of radioactive substances once inside the body depends on a number of factors, including how fast the substances decay (their half-lives), the energy level of the decay, and how long they stay and where they are located in the body.

Alpha Particles

Alpha particles are positively charged and made up of two protons and two neutrons. They lose their energy quickly and do not penetrate the surface of the skin. They can enter the body through a cut in the skin, ingestion, or inhalation. They can cause intense ionizations when they interact with matter and result in significant local damage if taken into the body. Uranium-238 and plutonium-239 are examples of alpha sources.

Beta Particles

Beta particles are positively or negatively charged electrons produced inside the nucleus. They can easily penetrate human tissue before losing all of their energy. Like alpha particles, they cause damage to the body, but do not have as much energy or potential to damage the body as alpha particles. Iodine-131, phosphorus-32, strontium-90, and tritium are examples of beta particle sources. The effects of beta particle sources on the body depend on whether or not they are stored in tissue or excreted.

Gamma Rays

Gamma rays are photons or electromagnetic waves spontaneously given off by certain radioactive substances. Gamma rays are not charged and pass through the human body at the speed of light. As gamma rays pass through the body, they interact with and may damage cells. Uranium-238, iodine-131, and cesium-137 are examples of gamma ray sources.

Some Radioactive Substances in the Environment

Cesium-137

Cesium-137 is the most common of cesium's radioactive isotopes to be encountered. It is produced during fission of either uranium or plutonium fuels and is found in the environment as a result of worldwide fallout associated with atmospheric weapons tests. It is also used in industry as a sealed gamma ray source for measuring the thickness of materials and in medicine as a sealed source for therapy and as a tracer substance. Because its chemical forms can be water soluble, cesium-137 can be distributed almost uniformly in body fluids and is rapidly eliminated by the kidneys. The biologic half-life of cesium-137 ranges from 68 to 165 days. Exposure to cesium-137 can increase the risk of cancers such as leukemia.

Iodine-131

Radioactive iodine's (especially iodine-131, -132, and -129) are important fission products from nuclear weapons tests and nuclear reactors. They are volatile substances and once released into the atmosphere can return to earth via precipitation, contaminating land, vegetation, and ultimately the food and water supply. Iodine-131 has a half-life of 8.05 days and emits several medium-energy beta particles and mostly low-energy gamma rays. Exposure to radioactive iodine usually results from inhalation; however, ingestion of contaminated milk has been the primary route of exposure following fallout events. An increased frequency of thyroid nodules and cancers have been reported in persons exposed to radioactive iodine from nuclear fallout. The risk of thyroid cancer appears to be life-long and dependent on age at time of exposure (e.g., infants and young children are at highest risk) and dose of 1-131 received.

Plutonium-239

Plutonium is an artificially produced element used as a fuel in nuclear power reactors and in nuclear weapons. It emits two high-energy alpha particles and has a radiation half-life of 24,390 years. Inhalation is the most common route of entry into the body; workers may also be exposed through puncture wounds. Inhaled plutonium can destroy small local masses of lung tissue and may result in lung cancer. Some of the inhaled plutonium can reach thoracic lymph nodes and the blood and be distributed elsewhere in the body. Exposure to plutonium may increase the risk of bone, liver, or lung cancer; leukemia; or chromosome aberrations.

Radon-222

Radon results from the radioactive decay of radium, a common element in rock and soil derived from the decay of uranium. As a gas, it can build up in buildings and is a source of alpha radiation. The alpha radiation may contribute to changes in cells in the respiratory tract that result in lung cancer.

Strontium-90

Strontium-90 emits a relatively high-energy beta particle, giving rise to yttrium-90, which then emits a beta particle of even higher energy. It is used in medicine and industry. Once in the body, it becomes deposited in bone where the high-energy beta particles irradiate both the bone and adjacent bone marrow. Exposure to strontium may increase the risk of leukemia, bone cancer, or a weakened immune system.

Uranium

Naturally occurring uranium consists of uranium-238 (99.27%), uranium-235 (0.72%), and uranium-234 (0.0054%). Uranium-235 is extracted or concentrated from natural uranium for use in nuclear weapons or nuclear power reactors. The uranium remaining after uranium-235 has been removed is referred to as depleted uranium; however, this uranium continues to be a radiation as well as a chemical hazard. Radioactive uranium is responsible for significant alpha radioactivity contamination in the environment. The amount of natural uranium that is absorbed by the body depends on the uranium's physical state, chemical form, and the route of exposure. Most uranium that is retained in the body is stored in bone and kidneys. Exposure to uranium may increase the risk of kidney disease, non-malignant respiratory disease, or lung cancer.