Radiation has two slightly different meanings depending upon scientific context. When referring to radioactivity, radiation is the emission of either waves (electromagnetic radiation) or particles (particle radiation). Light can fall into either of these categories, as it is both a wave and particle. In astronomy or when referring to heat transfer, radiation refers only to light in either of its forms.
Radiation is produced by radioactive decay, nuclear fission and nuclear fusion, chemical reactions, hot objects, and gases excited by electric currentss.
Radiation is often separated into two categories, ionizing and non-ionizing, to denote the energy and danger of the radiation. Ionization is the process of removing electrons from atoms, leaving electrically charged particles (ions) behind.
Many forms of radiation such as heat, visible light, microwaves, or radio waves do not have sufficient energy to remove electrons from atoms and hence, are called non-ionizing radiation. In the case of heat, for objects at room temperature, most of the energy is transmitted at infra-red wavelengths.
The negatively charged electrons and positively charged nuclei created by ionizing radiation may cause damage in living tissue. The term radioactivity generally refers to the release of ionizing radiation.
Radioactive materials usually release alpha rays (particles similar to the nuclei of helium), beta rays (quickly moving electrons) or gamma rays. Alpha and beta rays can often be shielded by a piece of paper or a thin sheet of steel. They cause most damage when they are emitted inside the human body. Gamma rays are less ionising than either alpha or beta rays, but protection against them requires thicker shielding. They produce damage similar to that caused by X-rays such as burns, cancer, and genetic mutations. Human biology resists germ-line mutation by aborting most mutated conceptuses.
| Table of contents |
|
2 The Effects of Ionizing Radiation on Animals 3 Minimizing Health Effects of Ionizing Radiation 4 External links |
The earth, and all living things on it, are constantly bombarded by radiation from space, similar to a steady drizzle of rain. Charged particles from the sun and stars interact with the earth's atmosphere and magnetic field to produce a shower of radiation, typically beta and gamma radiation. The dose from cosmic radiation varies in different parts of the world due to many factors including differences in elevation, the effects of the earth's magnetic field and local differences in terrain.
Radioactive material is found throughout nature. It occurs naturally in the soil, water, and vegetation. The major isotopes of concern for terrestrial radiation are uranium and its decay products, such as thorium, radium, and radon. Low levels of uranium, thorium, and their decay products are found everywhere. Some of these materials are ingested with food and water, while others, such as radon, are inhaled. The dose from terrestrial sources varies in different parts of the world. Locations with higher concentrations of uranium and thorium in their soil have higher dose levels.
In addition to the cosmic and terrestrial sources, all people also have radioactive potassium-40, carbon-14, lead-210, and other isotopes inside their bodies from birth. The variation in dose from one person to another is not as great as the variation in dose from cosmic and terrestrial sources.
Natural and artificial radiation sources are identical in their nature and their effect. Above the background level of radiation exposure, the NRC requires that its licensees limit man-made radiation exposure to individual members of the public to 100 mrem (1 mSv) per year, and limit occupational radiation exposure to adults working with radioactive material to 5,000 mrem (50 mSv) per year.
The exposure for an average person is about 360 millirems/year, 81 percent of which comes from natural sources of radiation. The remaining 19 percent results from exposure to man-made radiation sources.
By far, the most significant source of man-made radiation exposure to the general public is from medical procedures, such as diagnostic X-rays, nuclear medicine, and radiation therapy. Some of the major isotopes would be I-131, Tc-99m, Co-60, Ir-192, Cs-137, and others.
In addition, members of the public are exposed to radiation from consumer products, such as tobacco (polonium-210), building materials, combustible fuels (gas, coal, etc.), ophthalmic glass, televisions, luminous watches and dials (tritium), airport X-ray systems, smoke detectors (americium), road construction materials, electron tubes, fluorescent lamp starters, lantern mantles (thorium), etc.
Of lesser magnitude, members of the public are exposed to radiation from the nuclear fuel cycle, which includes the entire sequence from mining and milling of uranium to the disposal of the used (spent) fuel. The effects of such exposure have not been reliably measured. Estimates of exposure are low enough that proponents of nuclear power liken them to the mutagenic power of wearing trousers for two extra minutes per year (because heat causes mutation). Opponents use a cancer per dose model to prove that such activities cause several hundred cases of cancer per year.
In a nuclear war, gamma rays from fallout of nuclear weapons would probably cause the largest number of casualties. Immediately downwind of targets, doses would exceed 30,000 roentgen/hr. 450 R (more than a thousand times the background rate) is fatal to half of a normal population. No survivors have been documented from doses above 600 R.
Occupationally exposed individuals are exposed according to their occupations and to the sources with which they work. The exposure of these individuals to radiation is carefully monitored with the use of pocket-pen-sized instruments called dosimeters.
Some of the isotopes of concern include cobalt-60, cesium-137, americium-241 and Iodine-131. Examples of industries where occupational exposure is a concern include:
We tend to think of biological effects of radiation in terms of their effect on living cells. For low levels of radiation exposure, the biological effects are so small they may not be detected. The body repairs many types of radiation and chemical damage. Biological effects of radiation on living cells may result in four outcomes:
Cancers associated with high dose exposure include leukemia, breast, bladder, colon, liver, lung, esophagus, ovarian, multiple myeloma, and stomach cancers. Department of Health and Human Services literature also suggests a possible association between ionizing radiation exposure and prostate, nasal cavity/sinuses, pharyngeal and laryngeal, and pancreatic cancer.
The period of time between radiation exposure and the detection of cancer is known as the latent period. Those cancers that may develop as a result of radiation exposure are indistinguishable from those that occur naturally or as a result of exposure to other chemical carcinogens. Furthermore, National Cancer Institute literature indicates that other chemical and physical hazards and lifestyle factors (e.g., smoking, alcohol consumption, and diet) significantly contribute to many of these same diseases.
Although radiation may cause cancer at high doses and high dose rates, public health data do not certainly establish the occurrence of cancer following exposure to low doses and dose rates -- below about 10,000 mrem (100 mSv).
Most studies of occupational workers exposed to chronic low-levels of radiation above normal background have shown no adverse biological effects. Even so, the radiation protection community conservatively assumes that any amount of radiation may pose some risk for causing cancer and hereditary effect, and that the risk is higher for higher radiation exposures.
The linear dose-response model suggests that any increase in dose, no matter how small, results in an incremental increase in risk. The LNT hypothesis is accepted by the NRC as a conservative model for estimating radiation risk.
High radiation doses tend to kill cells, while low doses tend to damage or alter the genetic code (DNA) of irradiated cells. High doses can kill so many cells that tissues and organs are damaged immediately. This in turn may cause a rapid whole body response often called Acute Radiation Syndrome. The higher the radiation dose, the sooner the effects of radiation will appear, and the higher the probability of death.
This syndrome was observed in many atomic bomb survivors in 1945 and emergency workers responding to the 1986 Chernobyl nuclear power plant accident.
Approximately 134 plant workers and firefighters battling the fire at the Chernobyl power plant received high radiation doses (70,000 to 1,340,000 mrem or 700 to 13,400 mSv) and suffered from acute radiation sickness. Of these, 28 died from their radiation injuries.
In fact, there are no linear dose-relationships in nature that hold true over all dosage scales.
See also: radiation poisoning.
Geiger counters and scintillometers measure the dose rate of ionizing radiation directly.
In addition, there are four ways in which we can protect ourselves:
Shielding: Barriers of lead, concrete, or water give good protection from penetrating radiation such as gamma rays and neutrons. This is why certain radioactive materials are stored or handled under water or by remote control in rooms constructed of thick concrete or lined with lead. There are special plastic shields which stop beta particles and air will stop alpha particles. Inserting the proper shield between you and the radiation source will greatly reduce or eliminate the extra radiation dose.
Shielding can be designed using halving thicknesses, the thickness of material that reduces the radiation by half. Halving thicknesses for gamma rays are discussed in the article gamma rays.
Containment: Radioactive materials are confined in the smallest possible space and kept out of the environment. Radioactive isotopes for medical use, for example, are dispensed in closed handling facilities, while nuclear reactors operate within closed systems with multiple barriers which keep the radioactive materials contained. Rooms have a reduced air pressure so that any leaks occur into the room and not out of it.
In a nuclear war, an effective fallout shelter reduces human exposure at least 1000 times. Most people can accept doses as high as 100 R, distributed over several months, although with increased risk of cancer later in life. Other civil defense measures can help reduce exposure of populations by reducing ingestion of isotopes and occupational exposure during war time. One of these available measures could be the use of potassium iodide (KI) tablets which effectively block the uptake of dangerous radioactive iodine into the human thyroid gland.
See also: Electromagnetic radiation, Particle radiation, Gamma rays, radioactivity, radiation therapy, adaptive radiation, fallout shelter, nuclear war, nuclear weapon, civil defense.