We live in a sea of radiation. In recent years, people have learned to fear the effects of radiation. They don't want to live near nuclear reactors. They are frightened by reports of links between excess exposure to sunlight and skin cancer. They are afraid of the leakage from microwave ovens, or the radiation produced by their television sets.
Several factors combine to heighten the public's anxiety about both the short-range and long-range effects of radiation. Perhaps the most important source of fear is the fact that radiation can't be detected by the average person. Furthermore, the effects of exposure to radiation might not appear for months or years or even decades.
To understand the biological effects of radiation we must first understand the difference between ionizing radiation and non-ionizing radiation. In general, two things can happen when radiation is absorbed by matter: excitation or ionization.
- Excitation occurs when the radiation excites the motion of the atoms or molecules, or excites an electron from an occupied orbital into an empty, higher-energy orbital.
- Ionization occurs when the radiation carries enough energy to remove an electron from an atom or molecule.
Because living tissue is 70-90% water by weight, the dividing line between radiation that excites electrons and radiation that forms ions is often assumed to be equal to the ionization of water: 1216 kJ/mol. Radiation that carries less energy can only excite the water molecule. It is therefore called non-ionizing radiation. Radiation that carries more energy than 1216 kJ/mol can remove an electron from a water molecule, and is therefore called ionizing radiation.
The table below contains estimates of the energies of various kinds of radiation. Radio waves, microwaves, infrared radiation, and visible light are all forms of non-ionizing radiation. X-rays, -rays, and - and �/i>-particles are forms of ionizing radiation. The dividing line between ionizing and non-ionizing radiation in the electromagnetic spectrum falls in the ultraviolet portion of the spectrum. It is therefore useful to divide the UV spectrum into two categories: UVA and UVB. Radiation at the high-energy end of the UV spectrum can be as dangerous as x-rays or -rays.
Energies of Ionizing and Non-Ionizing Forms of Radiation
Radiation Typical Frequency (s -1) Typical Energy (kJ/mol) Particles -particles 4.1 x 108 �/i>-particles 1.5 x 107 Electromagnetic radiation ionizing cosmic rays 6 x 1021s-1 2.4 x 109 radiation -rays 3 x 1020s-1 1.2 x 108 x-rays 3 x 1017s-1 1.2 x 105 ultraviolet 3 x 1015 s-1 1200 visible 5 x 1014 s-1 200 infrared 3 x 1013 s-1 12 non-ionizing microwaves 3 x 109 s-1 1.2 x 10-3 radiation radio waves 3 x 107 s-1 1.2 x 10-5
When ionizing radiation passes through living tissue, electrons are removed from neutral water molecules to produce H2O+ ions. Between three and four water molecules are ionized for every 1.6 x 10-17 joules of energy absorbed in the form of ionizing radiation.
The H2O+ ion should not be confused with the H3O+ ion produced when acids dissolve in water. The H2O+ ion is an example of a free radical, which contains an unpaired valence-shell electron. Free radicals are extremely reactive. The radicals formed when ionizing radiation passes through water are among the strongest oxidizing agents that can exist in aqueous solution. At the molecular level, these oxidizing agents destroy biologically active molecules by either removing electrons or removing hydrogen atoms. This often leads to damage to the membrane, nucleus, chromosomes, or mitochondria of the cell that either inhibits cell division, results in cell death, or produces a malignant cell.
From the time that radioactivity was discovered, it was obvious that it caused damage. As early as 1901, Pierre Curie discovered that a sample of radium placed on his skin produced wounds that were very slow to heal. What some find surprising is the magnitude of the difference between the biological effects of non-ionizing radiation, such as light and microwaves, and ionizing radiation, such as high-energy ultraviolet radiation, x-rays, -rays, and - or �/i>-particles.
Radiation at the low-energy end of the electromagnetic spectrum, such as radio waves and microwaves, excites the movement of atoms and molecules, which is equivalent to heating the sample. Radiation in or near the visible portion of the spectrum excites electrons into higher-energy orbitals. When the electron eventually falls back to a lower-energy state, the excess energy is given off to neighboring molecules in the form of heat. The principal effect of non-ionizing radiation is therefore an increase in the temperature of the system.
We experience the fact that biological systems are sensitive to heat each time we cook with a microwave oven, or spend too long in the sun. But it takes a great deal of non-ionizing radiation to reach dangerous levels. We can assume, for example, that absorption of enough radiation to produce an increase of about 6C in body temperature would be fatal. Since the average 70-kilogram human is 80% water by weight, we can use the heat capacity of water to calculate that it would take about 1.5 million joules of non-ionizing radiation to kill the average human. If this energy was carried by visible light with a frequency of 5 x 1014 s-1, it would correspond to absorption of about seven moles of photons.
Ionizing radiation is much more dangerous. A dose of only 300 joules of x-ray or -ray radiation is fatal for the average human, even though this radiation raises the temperature of the body by only 0.001C. -particle radiation is even more dangerous; a dose equivalent to only 15 joules is fatal for the average human. Whereas it takes seven moles of photons of visible light to produce a fatal dose of non-ionizing radiation, absorption of only 7 x 10-10 moles of the -particles emitted by 238U is fatal.
There are three ways of measuring ionizing radiation.
- Measure the activity of the source in units of disintegrations per second or curies, which is the easiest measurement to make.
- Measure the radiation to which an object is exposed in units of roentgens by measuring the amount of ionization produced when this radiation passes through a sample of air.
- Measure the radiation absorbed by the object in units of radiation absorbed doses or "rads." This is the most useful quantity, but it is the hardest to obtain.
One radiation absorbed dose, or rad, corresponds to the absorption of 10-5 joules of energy per gram of body weight. Because this is equivalent to 0.01 J/kg, one rad produces an increase in body temperature of about 2 x 10-6C. At first glance, the rad may seem to be a negligibly small unit of measurement. The destructive power of the radicals produced when water is ionized is so large, however, that cells are inactivated at a dose of 100 rads, and a dose of 400 to 450 rads is fatal for the average human.
Not all forms of radiation have the same efficiency for damaging biological organisms. The faster energy is lost as the radiation passes through the tissue, the more damage it does. To correct for the differences in radiation biological effectiveness (RBE) among various forms of radiation, a second unit of absorbed dose has been defined. The roentgen equivalent man, or rem, is the absorbed dose in rads times the biological effectiveness of the radiation.
rems = rads x RBE
Values for the RBE of different forms of radiation are given in the table below.
The Radiation Biological Effectiveness of Various Forms of Radiation
|x-rays and -rays||1|
|�sup>- particles with energies larger than 0.03 MeV||1|
|�sup>- particles with energies less than 0.3 MeV||1.7|
|thermal (slow-moving) neutrons||3|
|fast-moving neutrons or protons||10|
|-particles or heavy ions||20|
Estimates of the per capita exposure to radiation in the United States are summarized in the table below. These estimates include both external and internal sources of natural background radiation.
Average Whole-Body Exposure Levels for Sources of Ionizing Radiation
|Source||Per Capita Dose (rems / y)|
|nuclear test fallout||0.005|
|consumer and industrial products||0.005|
|nuclear power industry||0.001|
External sources include cosmic rays from the sun and -particles or -rays emitted from rocks and soil. Internal sources include nuclides that enter the body when we breathe (14C, 85Kr, 220Rn, and 222Rn) and through the food chain (3H, 14C, 40K, 90Sr, 131I, and 137Cs). The actual dose from natural radiation depends on where one lives. People who live in the Rocky Mountains, for example, receive twice as much background radiation as the national average because there is less atmosphere to filter out the cosmic rays from the sun. The average dose from medical x-rays has decreased in recent years because of advances in the sensitivity of the photographic film used for x-rays. Radiation from nuclear test fallout has also decreased as a result of the atmospheric nuclear test ban. The threat of fallout from the testing of nuclear weapons can be appreciated by noting that a Chinese atmospheric test in 1976 led to the contamination of milk in the Harrisburg, PA, vicinity at a level of 300 pCi (3.00 x 10-10 Ci) per lite. This was about eight times the level of contamination (41 pCi per liter) that resulted from the accident at Three Mile Island.
The contribution to the radiation absorbed dose from consumer and industrial products includes radiation from construction materials, x-rays emitted by television sets, and inhaled tobacco smoke. The most recent estimate of the total radiation emitted from the mining and milling of uranium, the fabrication of reactor fuels, the storage of radioactive wastes, and the operation of nuclear reactors is less than 0.001 rem per year.
The total dose from ionizing radiation for the average American is about 0.170 rem per year. The Committee on the Biological Effects of Ionizing Radiation of the National Academy of Sciences recently estimated that an increase in this dose to a level of 1 rem per year would result in 169 additional deaths from cancer per million people exposed. This can be compared with the 170,000 cancer deaths that would normally occur in a population this size that was not exposed to this level of radiation.
|Practice Problem 10:
Calculate the rems of radiation absorbed by the average person from the 14C in his or her body. Assume the activity of the 14C in the average body is 0.08 mCi, the energy of the b-particles emitted when 14C decays is 0.156 MeV, and about one-third of this energy is captured by the body.
The principal effect of low doses of ionizing radiation is to induce cancers, which may take up to 20 years to develop. What is the effect of high doses of ionizing radiation? Cells that are actively dividing are more sensitive to radiation than cells that aren't. Thus, cells in the liver, kidney, muscle, brain, and bone are more resistant to radiation than the cells of bone marrow, the reproductive organs, the epithelium of the intestine, and the skin, which suffer the most damage from radiation. Damage to the bone marrow is the main cause of death at moderately high levels of exposure (200 to 1000 rads). Damage to the gastrointestinal tract is the major cause of death for exposures on the order of 100 to 10,000 rads. Massive damage to the central nervous system is the cause of death from extremely high exposures (over 10,000 rads).