Ionizing radiation as a factor of environment harmfulness and production. Antiradiation protection in medical establishments, including dentalprofile. Bìoetic aspects of the effects of radiation on human factor
Naturally occurring ionizing radiation originates both from outside the body, in the form of cosmic radiation and radiation from natural radio-isotopes in the environment, and from inside the body from natural radio-isotopes deposited there from food, drink and air.
During the present century, mankind has been subjected to increasing levels of ionizing radiation from man-made sources, such as X-ray equipment, nuclear weapons, the nuclear fuel cycle, and artificial radioisotopes used for medical and other purposes.
radiations may: be divided into two main groups: (1) electrmagnetic
radiations (X-ray, and gamma rays), which belong to the same family of
electromagnetic radiations as visible light and radio waves; and (2) corpuscular
radiations, some of which—alpha particles, beta particles (electrons), and
protons—are electrically charged, whereas others (neutrons) have no electric
charge. This distinction between the
two groups becomes. blurred, however, when their mode
of absorption in materials is considered. The corpuscular types may be regarded
as projectiles whose energy is greater than that binding the atoms in chemical
compounds. They are therefore capable
of breaking chemical bonds and dividing the electrically neutral molecules into
positively and negatively charged ions.
When X-rays and gamma rays are absorbed, high-energy electrons are
released in the irradiated materials, and it is these electrically charged
particles--which are similar to the beta particles emitted by radioisotopes—that
are the effective ionizing agents. The
action of neutrons is more complex. If
they collide with the nuclei of hydrogen atoms, these nuclei (or protons) are
set in motion, thus producing ionization.
Neutrons may also enter atomic nuclei, causing such'instability
that the atoms disintegrate and emit radiation that, in turn, produces
ionization. Thus the common
characteristic of all the types of radiation referred to, whether
electromagnetic or corpuscular, is that particles are responsible for the
ionization they ultimately produce. Whilst the .exact nature of the biological
effects of these radiations is not fully understood, they are related to the
ionization that the radiations are capable of producing in living tissue. Thus, the biological effects of all
ionizing radiations are essentially similar. However, the distribution of
damage throughout the body may be very different according to the type, energy
and penetrating power of the radiation involved. Alpha particles from
radioisotopes have ranges of only about 0.001-
Beta particles have ranges in soft tissues of the order of several millimetres, i.e., much greater than those of alpha particles in such tissues. A beta particle therefore irradiates many more cells than an alpha particle, but the number of ions produced in each cell is much less.
For X-rays and gamma rays, depending on the energy of the radiation, penetration may amount to tens of centimetres, or even to metres, in soft tissue. As in the case of beta particles, the ion density along the tracks of the electrons ejected by X-rays and gamma rays from the medium through which they pass is much lower than for alpha particles.
Natural background radiation
This has three components: (1) cosmic radiation originating in outer space and reaching the earth's surface after reacting with, and being partially absorbed by, the earth's atmosphere; (2) terrestrial radiation coming from natural radioisotopes present in the earth's crust; and (3) radiation from natural radioisotopes that have been accumulated in the body as a result of the consumption of food and water and the inhalation of air containing such radioisotopes.
The average values of the dose rates of these three components of environmental radiation lead to a total of about 90 mrad per year to gonodal tissue and bone marrow.
The evaluation of the radiation exposure of the population presented here applies only to highly developed countries; it refers to the genetic dose' received by a whole population, rather than to the exposure of individuals or groups. In many countries, the frequency with which radiation is used is much less than in the highly developed countries; the methods applied and the radiation protection measures adopted are, however, sometimes such that the radiation exposure per capita per application is greater. It is impossible, therefore, to give an accurate estimate of the mean genetic dose to the whole world population. The figures quoted will, however, provide an idea of the order of magnitude to be expected.
The contributions to the total dose from man-made radiation will be considered under the following headings:
(1) radiation to patients from the medical uses of radiation;
(2) radiation to occupationally exposed persons;
(3) radiation from "fallout" from nuclear tests;
(4) radiation from other forms of radioactive contamination; and
(5) radiation from radioactive consumer goods and from electronic devices.
It is necessary to distinguish, in considering radiation units, between the following three quantities of importance in radiation protection: exposure, absorbed dose and dose equivalent.
Exposure is the sum of the electrical charges of the ions of one sign produced in unit mass of air under certain defined conditions. The unit of exposure is the Rontgen, which is applicable only to electromagnetic radiation of moderate energy.
The absorbed dose is the radiation energy imparted to unit mass of a specified medium. The unit of absorbed dose is the rad.
For radiation protection considerations, it is necessary to introduce a modified quantity that takes into account the biological effectiveness of a given absorbed dose, depending on the type and energy of the radiation. This is done by using a quality factor. Other factors may also be introduced, such as the distribution factor, which expresses the modification in the biological effect due to the nonuniform distribution of internally deposited radionuclides. The product of the absorbed dose and the modifying factors is termed the dose equivalent. The unit of dose equivalent is the rem. Where the value of the quality factor is close to unity, as is true for X-rays (where it is unity), beta particles, and gamma rays, the numerical values of the absorbed dose in rads and the dose equivalent in rems are practically identical.
Biological Effects of Ionizing Radiations
Information concerning these effects has been obtained from studies of: (a) patients who have undergone diagnostic or therapeutic procedures with X-rays and radioisotopes; (b) occupationally-exposed persons (for example, pioneer medical radiologists, early workers with radioactive luminous paints, workers, engaged in mining radioactive ores, persons who have been involved in accidents in or around nuclear reactors, and persons who have been exposed continuously to low radiation doses for long periods); and (c) members of general populations who have been affected by atomic bomb explosions or tests of nuclear weapons. This information has been supplemented by evidence from extensive animal experimentation. Despite these studies, there are still many gaps in our knowledge and further investigations are needed. The effects can be regarded as falling into two main groups, namely, somatic effects and genetic effects.
These effects are observable either relatively soon after individuals have been irradiated ("early" or "short-term" effects), or after periods of a few months to several years ("late" or "long-term" effects). A dose of 1000 rad and above of total body irradiation, delivered over a short period of time, results in death within about a week. Doses of 100-1000 rad of total body irradiation delivered over a short period of time can result in damage and death in a proportion of the individuals exposed.
Acute radiation effects can be observed after irradiation of the greater part of the body. A latent period supervenes after initial symptoms of malaise, loss of appetite and fatigue. The length of this period is roughly inversely proportional to the radiation dose received. The end of the latent period is followed by the onset of the illness: early lethality, destruction of bone marrow, damage to the gastrointestinal tract associated with diarrhoea and haemorrhage, central nervous system symptoms, epi-lation, dermatitis, sterility. Pathological acute effects arise after exposure to doses hundreds of times greater than those likely to be received from environmental contamination, except in major accidents.
Much less is known as to the effects of small doses, e.g., up to 100 rad, received over long periods of time, yet it is these effects that are particularly importarit for the population at large. There are many uncertainties here—e.g., the variation of sensitivity to radiation with age and the possible reduction in effect per unit of radiation dose as compared with single large doses (over 100 rad).
It is not known whether the linear relationships between radiation dosage and the incidence of harmful effects that are sometimes observed at high dose levels also apply at low dose levels; present estimates of risk from low dose levels are based on the assumption that a linear relationship docs apply and that there is no "threshold" of radiation exposure below which no effect is produced.
At low dosage levels, leukaemogenesis and carcinogenesis are at present accepted as the most serious long-term risks for the individual. There is also evidence, however, of other late effects following high doses, e.g., cataract formation, and possibly neurological damage and a general shortening of the life span. These are all examples of what are called somatic effects.
The frequency of different types of tumour has been found to be increased in irradiated populations. This is true of thyroid carcinomas in patients given X-ray therapy to the neck in childhood, carcinomas of the lung in workers engaged in mining uranium ores, haematite and fluorspar, haemangioendotheliomas of the liver in patients injected intravenously with Thorotrast,' and miscellaneous types of neoplasms in atomic bomb survivors and in patients subjected to radiotherapy (United Nations Scientific Committee on the Effects of Atomic Radiation, 1972).
An increased incidence of cancers occurred in workers engaged in painting watch and clock dials with luminous paints containing radium. They ingested large quantities of radium and radium daughter elements. These radionuclides, which are preferentially deposited in bone, lead in time to skeletal injury and to osteosarcoma in some victims (United Nations Scientific Committee on the Effects of Atomic Radiation, 1964).
has only recently been possible to attempt quantitative estimates of the
incidence of harmful effects (leukaemia and other
forms of cancer and certain genetic effects) per unit dose of radiation, and
even now the margins of uncertainty are very wide. In general, most knowledge
has been gained of the effects of relatively large doses received at high
intensity, notably from epidemiological studies of the survivors of
Leukaemia is the malignancy whose rate of induction per rad is best known, and risk estimates are available over a fairly broad range of doses. For lung cancer and all solid cancers—the incidence of which is also clearly increased by radiation—estimates are much more uncertain, particularly as none of the surveys of irradiated people carried out so far has been pursued for a time sufficiently long to exclude the possibility that further cases of malignancies, besides those already recorded, will be observed after longer periods of observation, and because it is not known whether, some twenty years after exposure, peak incidence has yet been reached.
Despite the lack of direct, quantitative information on the sensitivity of the human embryo to irradiation, it is generally assumed that small amounts of radiation may carry some risk of teratogenic effects in man, as in other species. Thus, to minimize the risk of accidentally irradiating an embryo in a particularly sensitive stage of development, the International Commission on Radiological Protection has recommended that radiological examinations of the lower abdomen and pelvis in a woman of reproductive age should be limited, as far as possible, to the ten days following the onset of menstruation; an undetected pregnancy in such a woman is most improbable at this time (International Commission on Radiological Protection, 1966b).
Because of the paucity of human data on the teratogenic effects of graded doses of radiation and the marked variation in susceptibility of animals to malformation with stage in development at the time of irradiation and with known- species differences, it is not possible to estimate precisely the risks of radiation injury to the human embryo and fetus.
Likewise, studies aimed: at detecting teratogenic effects associated with increased levels of environmental background radiation have -given inconclusive results (Brill & Forgotson, 1964).
on human populations on ageing and longevity are incomplete. One of the first
indications of life-shortening effects of radiation in man' was the observation
that radiologists in the
Genetic effects are the results of gene mutations or chromosome anomalies that, arising in the germ cells of the irradiated individuals, may become apparent in their descendants, sometimes generations removed from the irradiated ancestor. Genetic effects are generally detrimental but may have various degrees of severity, from prenatal death to major malformations or mental dysfunctions, to mild impairments of an individual's reproductive performance or of his viability. Because they occur among the descendants of irradiated persons, they are of greater concern to the population than to the individuals actually exposed to radiation. Clear evidence of genetic damage in the offspring of irradiated human subjects is so meager that the genetic harm cannot be quantitatively expressed in terms of the social burden to which a given dose of radiation will eventually give rise. However, the possibility that genetic damage, once induced, may persist for generations must be constantly borne in mind when exposing individuals or populations to new sources of radiation (WHO Expert Committee on Radiation, 1959; United Nations Scientific Committee on the Effects of Atomic Radiation, 1966).
For the development of radiation protection guides, the identification of the particular organs or tissues that are critical because of the damage they may suffer is the essential simplifying step. For example, in the case of radioisotopes of iodine, the critical organ is the thyroid, since the concentration of such isotopes in it, and therefore the dose received, is far greater than for any other organ. Since radioiodine is widely used in medicine and may also be of importance in nuclear energy, the thyroid may often be the critical organ, especially among children (United States, National Council on Radiation Protection and Measurements, 1971).
In general, for irradiation from internally deposited sources, whether alone or combined with external irradiation, the critical organ is determined more by the metabolic pathways of nuclides, their concentration in organs, and their effective residence times, than by inherent sensitivity factors. Depending on the individual radionuclide under consideration, the critical organ may be the gastrointestinal tract, lung, bone, thyroid, kidney, spleen, pancreas, muscle or fatty tissue.
For general irradiation of the whole body, the critical organs and tissues are the gonads (fertility, hereditary effects), the haematopoietic organs, or more specifically the bone marrow (leukaemia), and the eye (cataracts).
The relation between choice of a critical organ and the development of radiation protection guides is not always evident. The position has been summarized as follows: "The dose to the critical organ from any particular mode of radiation exposure does not define the overall risk which will always be greater than this to the extent to which other organs are irradiated. The concept of critical organ is administratively convenient and in some circumstances logically justifiable, but it does not allow summation of risks according to the relative radiosensitivities of the irradiated tissues" (International Commission on Radiological Protection, 1969b).
Once someone decides to include radioactive materials in his/her research, he/she must apply for a radioisotope permit. During the process of obtaining the permit, the radionuclide work procedures will be examined together with other aspects such as the applicant's training, previous work experience with radioactive materials, adequacy of workplace facilities and preparation, dosimeters used, protective equipment, etc.
As explained earlier, it is better to order radioactive materials only when they are needed or as close as possible to the date of the experiment from both an economic and ALARA perspective. This will also reduce the risks associated with long-term storage, source leakage, external irradiation, etc.
There are three essential methods used to minimize external exposure to radiation in radiation safety: time, distance, and shielding
Reduce the time spend working with radioactive materials as much as possible. A good work practice is to perform the experiment without radioactive material first, to get used to the procedures, and perform the first experiment (if possible) with the smallest amount of radioactive material that will give a readable result. After becoming familiar with the procedures and safe handling of these materials, the quantities used can be increased.
The second method involves increasing the distance between the body and radioactive materials. Always store radioactive materials and radioactive waste far from other working areas and/or offices. What if the procedure requires working with radioactive materials close to the body? Whenever possible, especially for strong beta and gamma emitters, use tools. Don't touch the materials with hands unless strictly necessary. However, if hand contact cannot be avoided, manipulation of the materials with gloved hands is required.
Most work with radioactive materials at the University will require that the user be quite close to the material. Therefore, working behind shielding is recommended. As explained earlier, different kinds of shielding must be used for different radionuclides. No shielding is required for pure alpha or pure low energy beta emitters. Plexiglass shielding is required for beta emitters, metal for gamma or X-rays, water, and wax or concrete for neutrons. Large enough layers of air, water, or concrete can protect the human body from all types of radiation.
Always check the effectiveness of the shielding before starting an experiment.
Biological Effects of Radiation
There are two types of biological effects of radiation. One is acute, where the amount of damage is proportional to the value of the dose equivalent received by the person. These effects typically relate to high dose levels. This type of biological damage is called a non-stochastic effect of radiation. Sometimes, when controlled, this type of effect may be beneficial to our health. For instance, some forms of cancer therapy utilise high doses of radiation to kill cancerous cells. In our university, large doses causing acute effects are not commonly encountered.
The second types of effects are delayed and statistical (or stochastic) effects. These effects are related to intermediate and low-level doses received by a person. There is no dose-response relationship. The dose relates to a statistical probability of developing a certain effect. The best example is cancer. Exposure to a certain dose can increase the risk of developing cancer. With respect to the foetus, if the dose was received in the first two months of gestation, mental retardation may occur in the offspring.
Radiation is one of the best known carcinogens. Since the last half of the 20th century, our knowledge of this type of cancer has increased dramatically. A statistical proportionality between the level of dose received by a large number of people and the expected effects was proven at a high-to-intermediate level of dose equivalent. Only at much higher doses than those encountered at the University is there a statistical proportionality between cause and effect. A linear extrapolation of this data has been made to low and very low levels of dose equivalent. However, this linear extrapolation method has not been proven scientifically.
Conversely, some studies show that low levels of irradiation are in fact beneficial to our health. However, in the absence of scientific evidence, the regulators adopted a conservative approach and consider all levels of radiation as being damaging to the human body. Because of this, any procedure that involves radioactive materials must abide by a principle called ALARA, keeping all doses 'As Low As Reasonably Achievable'.
Two types of methods are used to measure external dose. One consists of using instruments such as survey meters, surface contamination meters, and neutron detectors. These instruments comprise the gas filled detectors (Geiger-Müller or proportional detectors), the scintillation detectors, and some special detectors for neutrons.
When performing monitoring the following actions are required:
1. Check first if the right method (direct or indirect monitoring) is required for the type of radionuclide used.
2. Check if the instrument has been calibrated less that a year ago (check the sticker on your instrument) ensure that the battery and HV (when there are available) indicators are in the correct range.
3. Measure the background before reading the values in the work area.
4. Subtract the background after taking readings in the work area.
5. With the remaining number and the instrument's efficiency for that particular radionuclide (see sticker), estimate the level of contamination.
6. Take the necessary actions to reduce the contamination below the set limits.
The second method for measuring external dose is personal dosimetry.
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including:
· Type of radiation involved. All kinds of ionizing radiation can produce health effects. The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have. Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues.
· Size of dose received. The higher the dose of radiation received, the higher the likelihood of health effects.
· Rate the dose is received. Tissue can receive larger dosages over a period of time. If the dosage occurs over a number of days or weeks, the results are often not as serious if a similar dose was received in a matter of minutes.
· Part of the body exposed. Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso. See radiosensitivity page for more information.
· The age of the individual. As a person ages, cell division slows and the body is less sensitive to the effects of ionizing radiation. Once cell division has slowed, the effects of radiation are somewhat less damaging than when cells were rapidly dividing.
· Biological differences. Some individuals are more sensitive to the effects of radiation than others. Studies have not been able to conclusively determine the differences.
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic. These two terms are discussed more in the next few pages.
As discussed in the introduction, concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895. Over the years, numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups. In general, the guidelines established for radiation exposure have had two principle objectives: 1) to prevent acute exposure; and 2) to limit chronic exposure to "acceptable" levels.
Current guidelines are based on the conservative assumption that there is no safe level of exposure. In other words, even the smallest exposure has some probability of causing a stochastic effect, such as cancer. This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure "as low as reasonable achievable" (ALARA). ALARA is a basic requirement of current radiation safety practices. It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible.
Regulatory Limits for Occupational Exposure
Many of the recommendations from the ICRP and other
groups have been incorporated into the regulatory requirements of countries
around the world. In the
1) the more limiting of:
· A total effective dose equivalent of 5 rems (0.05 Sv) or The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (0.5 Sv).
2) The annual limits to the lens of the eye, to the skin, and to the extremities, which are:
· A lens dose equivalent of 15 rems (0.15 Sv)
· A shallow-dose equivalent of 50 rems (0.50 Sv) to the skin or to any extremity.
The shallow-dose equivalent is the external
dose to the skin of the whole-body or extremities from an external source of
ionizing radiation. This value is the dose equivalent at a tissue depth of
The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of
The deep-dose equivalent is the whole-body dose
from an external source of ionizing radiation. This value is the dose
equivalent at a tissue depth of
The total effective dose equivalent is the dose equivalent to the whole-body.
The three basic ways of controlling exposure to harmful radiation are: 1) limiting the time spent near a source of radiation, 2) increasing the distance away from the source, 3) and using shielding to stop or reduce the level of radiation.
The radiation dose is directly proportional to the time spent in the radiation. Therefore, a person should not stay near a source of radiation any longer than necessary. If a survey meter reads 4 mR/h at a particular location, a total dose of 4mr will be received if a person remains at that location for one hour. In a two hour span of time, a dose of 8 mR would be received. The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area.
Dose = Dose Rate x Time
When using a gamma camera, it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source. Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators. This is illustrated in the images at the bottom of this page.
Increasing distance from the source of radiation will reduce the amount of radiation received. As radiation travels from the source, it spreads out becoming less intense. This is analogous to standing near a fire. The closer a person stands to the fire, the more intense the heat feels from the fire. This phenomenon can be expressed by an equation known as the inverse square law, which states that as the radiation travels out from the source, the dosage decreases inversely with the square of the distance.
Inverse Square Law: I1/ I2 = D22/ D12
The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation. In general, the denser the material the more shielding it will provide. The most effective shielding is provided by depleted uranium metal. It is used primarily in gamma ray cameras like the one shown below. The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera. Depleted uranium and other heavy metals, like tungsten, are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms. Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials. Concrete is commonly used in the construction of radiation vaults. Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside.