Ionizing radiation

Ionizing radiation is radiation in which an individual particle (for example, a photon, electron, or helium nucleus) carries enough energy to ionize an atom or molecule (that is, to completely remove an electron from its orbit). If the individual particles do not carry this amount of energy, it is essentially impossible for even a large flood of particles to cause ionization. These ionizations, if enough occur, can be very destructive to living tissue.

The composition of ionizing radiation can vary. Electromagnetic radiation can cause ionization if the energy per photon is high enough (that is, the wavelength is short enough). Ultraviolet light, X-rays, and gamma rays are all ionizing radiation, while visible light, microwaves, and radio waves are not. Ionizing radiation may also consist of fast-moving particles such as electrons, positrons, or small atomic nuclei.

Types of radiation

Ionizing radiation is produced by radioactive decay, nuclear fission and nuclear fusion, extremely hot objects (thermal or blackbody radiation), and accelerated charges (bremsstrahlung, particle radiation, synchrotron radiation).

In order for radiation to be ionizing, the particles must both have a high enough energy and interact with electrons. Photons interact strongly with charged particles, so photons of sufficiently high energy are ionizing (the energy at which this begins to happen is in the ultraviolet region; sunburn is the result of this ionization). Charged particles such as electrons, positrons, and alpha particles also interact strongly with electrons. Neutrons, on the other hand, do not interact strongly with electrons, and so they cannot directly ionize atoms. They can interact with atomic nuclei (depending on the nucleus and their velocity; see fast neutron and slow neutron), often producing radioactive nuclei, which produce ionizing radiation when they decay.

The negatively charged electrons and positively charged nuclei created by ionizing radiation may cause damage in living tissue. If the dose is sufficient, the effect may be seen almost immediately, in the form of radiation poisoning. Lower doses may cause cancer or other long-term problems. The effect of very low doses is a subject of current debate.

Radioactive materials usually release alpha rays (particles similar to the nuclei of helium), beta rays (quickly moving electrons or positrons) 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 ionizing 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.

Non-ionizing radiation is thought to be essentially harmless below the levels that cause heating. Ionizing radiation is dangerous in direct exposure, although the degree of danger is a subject of debate. Humans and animals can also be exposed to ionizing radiation internally: if radioactive isotopes are present in the environment, they may be taken into the body. For example, radioactive iodine is treated as normal iodine by the body and used by the thyroid; its accumulation there often leads to thyroid cancer. Some radioactive elements also bioaccumulate.

Sources of ionizing radiation

Natural background radiation

Natural background radiation comes from three primary sources: cosmic radiation, external terrestrial sources, and radon.

Cosmic radiation

The earth, and all living things on it, are constantly bombarded by radiation from outside our solar system of positively charged ions from protons to iron nuclei. This radiation interacts in the atmosphere to create secondary radiation that rains down, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons. The dose from cosmic radiation is largely from muons, neutrons, and electrons.

The dose rate from cosmic radiation varies in different parts of the world based largely on the geomagnetic field and altitude.

External terrestrial sources

Radioactive material is found throughout nature. It occurs naturally in the soil, rocks, water, air, and vegetation. The major radio nuclides of concern for terrestrial radiation are potassium, uranium and thorium. Each of these sources has been decreasing in activity since the birth of the Earth so that our present dose from potassium-40 is about 1/2 what it would have been at the dawn of life on Earth.

Radon

radon gas seeps out of uranium containing soils found across most of the world and may concentrate in well sealed homes. It is often the single largest contributor to an individual's background radiation dose and is certaintly the most variable in the United States.

Man-made radiation 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.

Some man-made radiation sources affect man through direct radiation, while others take the form of radioactive contamination and irradiate man from the inside.

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 used would be I-131, Tc-99m, Co-60, Ir-192, Cs-137, and others. These are rarely released into the environment.

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/h. 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, caesium-137, americium-241 and iodine-131. Examples of industries where occupational exposure is a concern include:

Fuel cycle
Industrial Radiography
Radiology Departments (Medical)
Radiation Oncology Departments
Nuclear power plant
Nuclear medicine Departments
National (government) and university Research Laboratories

The effects of ionizing radiation on animals

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:

injured or damaged cells repair themselves, resulting in no residual damage
cells die, much like millions of body cells do every day, being replaced through normal biological processes
cells incorrectly repair themselves resulting in a biophysical change.
Low levels of ionizing radiation may be beneficial to many types of cells; this phenomenon is termed radiation hormesis, see below.

Chronic radiation exposure

Exposure to ionizing radiation over an extended period of time is called chronic exposure. The natural background radiation is chronic exposure, but a normal level is difficult to determine due to variations. Location and occupation often affect chronic exposure.

Acute radiation exposure

Acute radiation exposure is an exposure to ionizing radiation which occurs during a short period of time. There are routine brief exposures, and the boundary at which it becomes significant is difficult to identify. Extreme examples include

Instantaneous flashes from nuclear explosions.
Exposures of minutes to hours during handling of radioactive material.
Laboratory and manufacturing accidents.
Intentional and accidental high medical doses.

The effects of acute events are more easily studied than those of chronic exposure.

Radiation levels

The associations between ionizing radiation exposure and the development of cancer are mostly based on populations exposed to relatively high levels of ionizing radiation (e.g., Japanese atomic bomb survivors, and recipients of selected diagnostic or therapeutic medical procedures).

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 not shown conclusive 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.

All ionizing radiation attacks living tissue by causing ionization, which disrupts molecules directly and also produces highly reactive free radicals, which attack nearby cells. The net effect is that biologicl molecules suffer local disruption. Very high doses of radiation disrupt cells by wrecking large amounts of cellular machinery. Lower doses also wreck cellular machinery, but most cellular machinery can be effectively repaired or replaced. An important exception is the DNA itself, which may be damaged and not repaired. Thus radiation doses that are not sufficient to desrtroy cells outright affect cells in the process of replication more severely. In humans, this includes cells in the stomach lining, hair follicles and blood. Thus 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.

Ionizing radiation level examples

Recognized effects of acute radiation exposure are described in the article on radiation poisoning. The exact units of measurement vary, but light radiation sickness begins at about 50 - 100 rad (0.5 - 1 Sv, 50 - 100 rem, 50,000 - 100,000 mrem).

Chronic radiation levels and standards are often measured in millirems, 1/1000th of a rem.

The following table includes some short-term dosages for comparison purposes.

Level (mrem)	Ionizing radiation standards	Example
1 / yr		USA dose from nuclear fuel and nuclear power plants. [1]
1 / day		Daily natural background radiation, including radon. [2]
2.5 / 6 h		Cosmic dose on flight from New York to Los Angeles. [3]
2 / hour	USA NRC public area exposure limit.
10 / yr		USA average dose from consumer products. [4]
15 / yr	USA EPA cleanup standard.
25 / yr	USA NRC cleanup standard for individual sites/sources.
27 / yr		USA dose from natural cosmic radiation. 16 coastal plain - 63 eastern Rocky Mountains. [5]
28 / yr		USA dose from natural terrestrial sources. [6]
39 / yr		Global level of human internal radiation due to radioactive potassium.
46		Estimate of largest off-site dose possible from March 28 1979 Three Mile Island accident.
66 / yr		Average USA dose from human-made sources. [7]
100 / yr	USA NCR total public exposure.
110 / yr		1980 average USA radiation worker occupational dose. [8]
200 / yr		USA average medical and natural background. [9] Human internal radiation due to radon, varies with radon levels. [10]
220		Average dose from upper gastrointestinal diagnostic X-ray series.
300 / yr		USA average from all natural sources. [11]
366 / yr		USA average from all sources.
few hundred / yr		Estimate of cobalt-60 contamination within about 0.5 mile of "dirty bomb".
500 / yr	USA NCR occupational limit for minors (10% of adult limit). USA NCR limit for visitors.	Orvieto town, Italy, natural. [12]
500 / 9 months	USA NCR occupational limit for pregnancy.
640 / yr		HBRA of Yangjiang, China. [13]
760 / yr		Fountainhead Rock Place, Santa Fe, NM natural.
1,000 - 5,000	USA EPA nuclear accident emergency action level. [14]
1,000 - 19,000 acute		Nagasaki bomb survivors have lower incidence of cancer.
1,500 / yr		Taiwan cobalt-60 10 year exposure, 97% lower cancer than population.[15]
5,000 / yr	USA NCR occupational limit (10 CFR 20).
10,000 acute	USA EPA acute dose level estimated to increase cancer risk 0.8%. [16]
12,000 / yr		30 year exposure, Ural mountains, lower cancer mortality rate.[17]
15,000 / yr	USA NCR occupational eye lens exposure.
17,500 / yr		Guarapari, Brazil natural.[18]
25,000 acute	USA EPA voluntary maximum dose for emergency non-lifesaving work. [19]
50,000 / yr	USA NCR occupational whole skin, limb skin, or single organ exposure.	30 year exposure, Ural mountains, somewhat lower cancer mortality rate.[20]
75,000 acute	USA EPA voluntary maximum dose for emergency lifesaving work. [21]
79,000 / yr		Ramsar, Iran natural.[22] Guarapari, Brazil, natural, maximum on beach.
50,000 - 100,000 acute	Low-level radiation sickness due to short-term exposure.	World War II bomb victims.

Radiation hormesis

The United States based National Council on Radiation Protection and Measurements (NCRP), a body commissioned by Congress, recently released report written by the national experts in the field which states that, for the sake of caution, radiation's effects should be considered to be proportional to the dose an individual receives, regardless of how small that dose is. However, it is not possible to prove or disprove this conclusion. The number of people required for such a study would have to include the entire population of the Earth.

In the absence of conclusive evidence in humans, some scientists have turned to studying other living things. From these studies, some have concluded that small amounts of radiation may actually be good for a person and decrease their chance of cancer. This theory is called "Hormesis."

In the absence of conclusive data, and with significant amounts of money at stake as well as a number of people's livelihoods, a controversy has arisen. There may not be a person in the field of radiation physics or health physics who is both knowledgeable and unbiased about this topic.

An argument for hormesis

It has never been proven that very low doses of ionizing radiation are harmful. A small but growing number of studies offer evidence that they may even have some beneficial effects.

A linear, no-threshold (LNT) dose response relationship is widely assumed to be valid by most policy makers and many scientists. However, a small but growing number of scientists hold that this relationship is grossly misleading, and may be totally wrong. One problem is that this relationship ignores known cellular repair mechanisms; cells in all organisms have efficient methods to detect and repair damage.

Some scientists hold that these linear-response curves were created with an anti-nuclear political and social agenda in mind, and have little or no scientific validity.

It is easy to show the fallacy of the linear no-threshold relationship: Background radiation in our everyday environment does not kill people, yet radiation blasts from nuclear fission events (e.g. worst-case meltdowns or nearby atomic bombs) can almost immediately kill a person. These deadly radiation events are nearly a million times more powerful than background radiation. Compare this to taking one aspirin a day (which we may call background level); this has been proven to be harmless for most people, and actually has substantial medical benefits for many people. If one were to take one million aspirin a day, that person would die immediately. The same is true of most essential vitamins and minerals; small amounts are harmless, or even necessary for life. Doses a million times larger are not healthy, and potentially fatal. No scientist would make a linear graph for these phenomena, and then work backwards to prove that aspirin is deadly, or that vitamins and minerals are deadly. Yet this same flaw in logic is often applied to radiation.

In fact, there are no linear dose-relationships in nature that hold true over all dosage scales.

Some scientists point out that when life first arose over 2 billion years ago, it evolved in an environment that had thousands of times more background radiation than we are exposed to today. They suggest that, although that life was very different from contemporary life and the human organism in particular, this means that there must be much more room for life to live safely -- long enough to reproduce -- in a low-radiation environment. None of this, of course, is meant to minimize the risks of higher levels of ionizing radiation.

Minimizing health effects of ionizing radiation

Although exposure to ionizing radiation carries a risk, it is impossible to completely avoid exposure. Radiation has always been present in the environment and in our bodies. We can, however, avoid undue exposure.

Although people cannot sense ionizing radiation, there is a range of simple, sensitive instruments capable of detecting minute amounts of radiation from natural and man-made sources.

Dosimeters resemble pens, and can be clipped to one's clothing. They measure an absolute dose received over a period of time. They must be periodically recharged, and the result logged.

Geiger counters and scintillometers measure the dose rate of ionizing radiation directly.

In addition, there are four ways in which we can protect ourselves:

Time: For people who are exposed to radiation in addition to natural background radiation, limiting or minimizing the exposure time will reduce the dose from the radiation source.

Distance: In the same way that the heat from a fire is less intense the further away you are, so the intensity of the radiation decreases the further you are form the source of the radiation. The dose decreases dramatically as you increase your distance from the source.

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.

External links

The Nuclear Regulatory Commission regulates most commercial radiation sources and non-medical exposures in the US:
Biological Effects of Low Level Exposures: Radiation Hormesis
UNSCEAR 2000 Report, Volume 1:Sources
UNSCEAR 2000 Report, Volume 2:Effects