Effects of Radiation

Radiation affects atoms and molecules, and living tissues as well. It has two main effects on atoms and moleculesexcitation and ionization. Excitation is a process in which radiation energizes an atom or molecule so that its electrons move to higher-energy shells. Usually, the excited atom retains this energy for only a fraction of a second, releases it in the form of a photon, and reverts to its initial level of energy. Ionization is a process in which radiation energizes an atom or molecule so that its electrons leave the atom and move through space. Atoms that lose electrons become positive ionsparticles that have a positive charge. The electrons may then join other atoms.

Radiation may be ionizing or non-ionizing. Ionizing radiation is radiation of two typesthe type that is powerful enough to take away electrons directly from atoms on and around their path, and the type that has to first transfer energy to an atom. The first type includes alpha and beta particles and protons, and is more dangerous than the second type, which includes x-rays, gamma radiation, and neutron radiation. Non-ionizing radiation has photons too weak to ionize particles. It includes radio waves, microwaves, infrared radiation, and visible light. Each of these causes excitation only.

Excitation and ionization also affect living tissues. Electrons bind many of the molecules in the body's cells. When these molecules are excited or ionized by radiation, chemical bonds may break and molecules may change shape, upsetting intracellular chemical processes and consequently destroying or distorting cells. Mutation, which is a permanent change in physical characteristics, may occur if radiation affects molecules of deoxyribonucleic acid (DNA), the transmissible material in living cells. If radiation causes mutation, disagreeable characteristics may be passed on to progeny in rare cases. Excitation caused by low-energy photons, especially ultraviolet light from the sun, may cause damage. If the damage is critical, the cell develops cancer or dies while trying to divide. The degree of damage depends on the ionizing ability of the radiation, the dose, and the type of tissue. Birth defects, cancer, and death are the chief effects of radiation.

The quantity of radiation taken in by a substance is referred to as the dose of radiation. There are two systems used to measure dosage. The older system uses a unit called rad (radiation absorbed dose). One rad is produced when 1 gram of material absorbs 100 ergs. (An erg is an extremely small unit of energy.) The newer system, introduced in 1975, uses a unit called gray. It is named after Louis H. Gray, a British radiation biologist. One gray is equal to 100 rads, or 1 joule per kilogram of material. A joule is a unit of energy equal to 10 million ergs.

The effect of a dose of radiation depends on its type. This is measured by the quality factor, which indicates how much the radiation damages living tissue compared with an equal dose of x-rays. The quality factor of alpha particles is 10; that means a dose of alpha particles damages living tissue around 10 times as much as x-rays. The quality factor of x-rays, gamma radiation, and beta particles is 1, and that of neutrons varies between 2 and 11.

The damage caused is calculated by multiplying the dose of radiation by its quality factor; the measure is called dose equivalent. If the unit of the dose is a rad, the unit of the dose equivalent is rem (roentgen equivalent in man). It is the amount of radiation necessary to cause the same effect on a human being as 1 rad of x-rays. If the dose is reported in grays, the unit of the dose equivalent is sievert. Grays and sieverts are metric measurements.

Radiation sickness may occur from large doses of radiation. Doses above 100 rems damage red and white blood cells (the hematopoietic effect). Doses above 300 rems may cause death in several weeks. When subjected to doses above 1,000 rems, the cells lining the digestive tract die, bacteria from the intestines invade the bloodstream (the gastrointestinal effect), and infection causes death within a week. At doses of several thousand rems, the brain is injured, and death may occur within hours. Deaths from radiation sickness happen only very rarely. People have suffered large doses in reactor accidents, in a few cases where radioactive material was mishandled, and in the 1945 bombings of Hiroshima and Nagasaki, during World War II. The worst reactor accident in history was a 1986 explosion and fire at the Chernobyl nuclear power plant in Ukraine, which killed 31 workers.

The dosage of radiation received in daily life is much smaller, and is sometimes called background dose. Sources are radon, a gas released by radioactive rocks and soil; medical and dental x-rays; nuclear power plants; waste disposal sites; and radioactive isotopes in tobacco smoke. Repeated exposure to small doses of radiation increases the risk for cancer and congenital defects.

Experts from many countries formed the International Commission on Radiological Protection to set exposure protocols and to protect people from the effects of radiation. The commission has set the annual maximum permissible dose (MPD) for nuclear workers at 5 rems per year, and that for the public at 0.5 rem per year. Other agencies, including the National Council on Radiation Protection and Measurements in the United States and the Atomic Energy Control Board in Canada, set similar guidelines.