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Common Questions About Radiation

Physical Facts

After being x-rayed for a diagnostic examination, how much radiation stays in my body?

No x-rays remain in your body. The x-rays are gone as soon as the x-ray machine shuts off in the same way that the light from a light bulb vanishes when the switch is turned off. X-rays must not be confused with radioactivity, where the radiation slowly decreased with time.

What are the main sources of radiation to which the public is exposed?

We assume in our answer that the question refers to ionizing radiation and thus excludes radio waves and light. The largest source for the average person is background radiation which accounts for approximately 43% of the exposure of the average person in the U.S.A. Background radiation consists of cosmic rays that reach us from the sun and outer space, radiation from radioactive materials which are always present naturally in the earth and buildings around us and radiation from radioactive materials naturally present in the body, particularly from a radioactive form of potassium, which is present in all potassium. Close behind is the radiation from diagnostic x-ray examinations, which accounts for about 40% of the radiation to which the average person is exposed. Another 7% is contributed by medical procedures involving the use of radioactive materials to diagnose illness such as heart disease and cancer. All other sources of radiation, such as radioactivity in consumer products, radioactive fall-out from past nuclear explosions, nuclear power, and occupational exposure, contribute only 10% to the exposure of an average person.

Is there only one kind of radiation which exposes the public?

So far as ionizing radiation is concerned, exposure is mainly from three types of radiation which have essentially the same effect for each unit of dose; these are x-rays and gamma rays, which are the same radiation (differing only in their origin) and beta rays which are electrons emitted by radioactive materials. There is also some exposure, particularly to the lungs, from a normally occurring radioactive gas called radon which emanates from the ground and is breathed by all of us. The radon gas emits alpha particles, relatively massive and slow-moving compared to electrons, which have little penetrating power but will affect the surface of the lungs when inhaled. Radon levels vary with the type of soil and building material, and are higher near granite rock areas. The public is also exposed to neutrons which are produced in the atmosphere by cosmic rays, but the neutron dose is only a small fraction of the background dose.

I read that besides natural background, most of the radiation exposure to people comes from medical x-ray examination. Do all medical x-ray examinations give about the same dose?

The dose from diagnostic x-ray examination varies greatly and depends mainly on the thickness of the body section traversed by the x-ray beam and the number of films in the study. The very common chest x-ray examination requires a relatively small skin dose of about 30 millirems (0.03 rem) due to the ease with which x-rays penetrate the lung. Extensive studies in the pelvis can lead to doses of several rems. If there is a medical need for an x-ray examination, it should be carried out since the modest risk from the radiation dose is usually far outweighed by the benefit of the information gained.

How can I tell how much radiation I have received even if I didn't wear a radiation badge?

A rough estimate of exposure can be made if the physical factors controlling the exposure are known: time, distance, primary beam intensity, field size, source activity and type. It is possible to deduce the dose received, if relatively high, by assaying the frequency of chromosome aberrations in circulating lymphocytes. This technique is not useful in estimating an unknown dose in the average radiation worker because of the residual effects of previous exposures. It is also expensive.

How accurate are the exposure reported by the use of film badges and how much exposure can one receive before it is readable on the badge?

The use of film badge provides a simple method of measuring personnel exposure for radiation workers. The accuracy of the reading is typically 20%, the error increasing as the dose level falls. Because of its limited area the film badge provides only a sample of exposure of an individual. The minimum dose readable varies with the radiation energy but is generally of the order of 10 mrem.

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Radiation Hazards: General

I have gotten the impression that little is known about the effects of radiation, other than that it is very dangerous. Is this true?

This is untrue. We probably know more about radiation than any other cancer-producing agent (carcinogen), physical or chemical. Experience goes back about 80 years and the information is probably better documented than that for any other carcinogen. Radiation is, in fact, a relatively weak carcinogen and mutagen.

It is sometimes suggested that if a person is exposed to radiation he/she will develop cancer or at the least is likely to. Is this true?

This is untrue. Radiation is not very effective in causing cancer and even in large populations exposed to high doses of radiation the increase over the normal cancer rate is small. For instance, follow-up studies (through 1978) of 82,000 exposed Japanese A-bomb survivors have produced an estimated 250 radiation-induced cancers.

Can the radiation exposure associated with a diagnostic radiologic procedure be more dangerous than the associated illness?

This is almost always untrue. For individual patients undergoing radiologic procedures related to illness, the benefits of the procedure far outweigh the risks.

I have had several x-ray exams over the past couple of years. Will this sterilize me?

No. The radiation dose required to sterilize (500-600 rems) is at least a factor of 100 larger than the gonadal radiation exposure from "several" x-ray exams, even if the exams are directly of the gonadal area. If the x-ray exams are directed at other regions, the radiation exposure to the gonads would be even less.

What happens to you when you work with x-rays?

With the very small amounts of radiation by persons working with x-rays and with the small amounts delivered to patients in typical diagnostic x-ray or nuclear medicine examinations, we expect nothing to happen to patients or personnel. Studies of persons occupationally exposed to x-rays or gamma rays for the past 30 years have not shown an increase in cancer.

What is considered to be the minimum lethal dose of radiation for humans?

The mean lethal dose (L.D. 50/30), or the dose that wound be lethal to 50% of the human population within 30 days after irradiation, is approximately 350 rems of x-rays or gamma rays given in a single exposure to the whole body. The minimum lethal dose which may cause death in a few percent of exposed persons is probably about 250 rems. The mean lethal dose will be considerably higher if only part of the body is irradiated or if the irradiation is spread out over a longer time period, such as 1 week. The administration of special medical care, including infection control and bone marrow transplants, will also increase the LD 50/30.

Just how much radiation will cause injury to my body?

Obvious injury such as skin burn, loss of hair and cataracts may be produced after doses of several hundred rems have been received in a short time such as one day. At low doses such as a few rems the only possible injury is long delayed cancer, but the possibility is very small. Depending on the assumption made, the risk has been estimated by expert committees as about 2 in 10,000 persons over a lifetime after receiving 1 rem of radiation dose to the entire body.

How much danger, if any, are x-ray technicians in, if they work with radiation for their entire lifetime?

The "danger" to which an x-ray technician is exposed, resulting from contact with radiation during a working lifetime, is very small. If the typical radiation exposure of about 0.5 rem per year is received, no observable effects are expected.

Is it true that there is no dose below which there is no damage to cells?

This is not clearly known but may be true. Even very small amounts of ionizing radiation can produce damage at the sub-microscopic level in some of the exposed cells. Whether this damage affects the properties of the cell, such as its ability to divide or its transformation to a malignant form, depends or many factors including where the damage occurs, the extent of the damage, and healing ability of the cell. It is clear, however, than the malignant transformation of a cell is extremely unlikely at low doses.

What are the delayed effects on people of low and moderate doses of x-rays?

In the great majority of persons there is no evident effect, immediate or delayed. However, there is a very small chance that cancer may develop after a delay of several or many years, and that genetic effects may appear in future generations.

How much radiation is considered an acceptable level exposure? How were these levels arrived at?

The acceptable level depends on who you are, where you are, your age, and a judgement on whether that level can readily be reduced. There are maximum permissible exposure levels which have been promulgated by the International Commission on Radiological Protection and in the USA by the National Council on Radiation Protection and Measurements. These recommended levels have been endorsed by various federal agencies such as the Nuclear Regulatory Commission, by State governments and by numerous employers. There are dose limits for the whole body and for specific organs of the body. The limits are different for persons who are occupationally exposed and persons who are not occupationally exposed but are in the vicinity of radiation sources; for the whole body these limits are 5 rem per year and 0.1 rem per year, respectively. The limit to persons younger than 18 and the fetus in occupationally exposed pregnant women is 0.5 rem per year.

Very few occupationally exposed persons receive the 5 rem limit per year. The national average for occupationally exposed persons is less than 0.5 rem per year. All such persons carry a radiation monitor. It is a requirement of the Nuclear Regulatory Commission that radiation doses received occupationally must be reviewed periodically by radiation safety personnel and that efforts be made to maintain such doses at levels which are "as low as reasonably achievable."

These maximum levels were arrived at by considering combined cancer and genetic risk from radiation and choosing a dose level such that radiation work would carry a risk comparable with the less hazardous occupations in our society. The general public limit has been set somewhat arbitrarily at 50 times less than the occupational limit with an average to the whole population not to exceed 170 millirem per year which is somewhat less than an additional background dose.

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