What is the annual radiation exposure limit for radiation workers averaged over a five year period?

5.1. Policy

Work with sources of ionizing radiation will be conducted so that doses received by individuals do not exceed the applicable limit, and so that doses are maintained as low as reasonably achievable (ALARA).

5.2. Definitions

5.2.1. Annual limit on intake (ALI) - the derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year. ALI is the smaller value of intake of a given radionuclide in a year by the "reference man" that would result in a committed effective dose equivalent of 0.05 Sv (5 rem) or a committed dose equivalent of 0.5 Sv (50 rem) to any individual organ or tissue.

5.2.2. Dose equivalent - the product of the absorbed dose in tissue and the quality factor (a value that reflects the biological impact of a particular type of ionizing radiation). Measured in rem or Sievert (Sv).

5.2.3. Occupational dose - the dose received by an individual in a restricted area or while performing assigned duties that involve exposure to sources of radiation.

5.2.4. Member of the public - an individual who is not in a restricted area and who is not performing assigned duties that involve exposure to sources of radiation.

5.2.5. Committed dose equivalent (CDE) - the dose equivalent to organs or tissues of reference that will be received from an intake of radioactive material by an individual during the 50-year period following intake.

5.2.6. Committed effective dose equivalent (CEDE) - the sum of the products of the weighting factors applicable to each of the body organs or tissues that are irradiated and the committed dose equivalent (CDE) to each of these organs or tissues. This is a measure of the overall risk associated with internal deposition of radioactive material.

5.2.7. Eye dose equivalent (LDE) - the dose equivalent to the lens of the eye at a tissue depth of 0.3 cm (300 mg/cm2).

5.2.8. Shallow dose equivalent (SDE) - the dose equivalent at a tissue depth of .007 cm (7 mg/cm2) averaged over 1 cm2; applies to external whole body or extremity exposure.

5.2.9. Deep dose equivalent (DDE) - the dose equivalent at a tissue depth of 1 cm; applies to external exposure.

5.2.10. Total Effective Dose Equivalent (TEDE) - the sum of the deep dose equivalent (DDE) for external exposures and the committed effective dose equivalent (CEDE) for internal exposures.

5.2.11. Total Organ Dose Equivalent, Maximum Organ (TODE) - the sum of the deep dose equivalent (DDE) and the committed dose equivalent (CDE) to the organ receiving the highest dose.

5.3. Occupational Dose Limits for Adults

5.3.1. An annual limit of 5 rem (0.05 Sv) total effective dose equivalent (TEDE).

5.3.2. An annual limit of 50 rem (0.50 Sv) to an individual organ or tissue other than the lens of the eye, as determined by the deep-dose equivalent and the committed dose equivalent.

5.3.3. An annual limit of 15 rem (0.15 Sv) to the lens of the eye.

5.3.4. An annual limit of 50 rem (0.50) Sv) to the skin.

5.3.5. An annual limit of 50 rem (0.50 Sv) to each of the extremities.

5.3.6. The above limits must be reduced by the amount of occupational dose received while employed by someone other than Oregon State University during the current year.

5.4. Occupational Dose Limits for Minors (under age 18)

5.4.1. The occupational dose limits for minors are 10 percent of the above occupational dose limits for adults.

5.5. Dose to an Embryo or Fetus

5.5.1. A limit of 0.5 rem (5 mSv) during the entire pregnancy due to occupational exposure of a declared pregnant woman. The dose equivalent to an embryo/fetus must be taken as the sum of:

5.5.1.1. The deep-dose equivalent to the declared pregnant woman; and

5.5.1.2. The dose equivalent to the embryo/fetus from the radionuclides in the embryo/fetus and radionuclides in the declared pregnant woman.

5.5.2. If the dose equivalent to the embryo/fetus is found to have exceeded 0.5 rem (5 mSv), or is within 0.05 rem (0.5 mSv) of this dose, by the time the woman declares the pregnancy to the licensee, the licensee shall be deemed to be in compliance with subsection (3)(a) of this rule if the additional dose equivalent to the embryo/fetus does not exceed 0.05 rem (0.5 mSv) during the remainder of the pregnancy.

5.5.3. An effort will be made to avoid substantial variation above a uniform monthly dose rate to a declared pregnant woman. The Radiation Safety Officer will investigate any report of a dose in excess of 0.050 rem (0.5 mSv) to a declared pregnant worker within 7 working days of receiving knowledge of the dose.

5.5.4. A woman is not a declared pregnant woman unless she says so in writing without being coerced. Unless a woman declares her pregnancy, she is to be treated as any other radiation worker.

5.6. Dose Limits for Individual Members of the Public

5.6.1. An annual limit of 0.1 rem (1 mSv) total effective dose equivalent (TEDE)

5.6.2. An hourly limit from external sources of 0.002 rem (0.02 mSv) in unrestricted areas.

5.7. As Low As Reasonably Achievable (ALARA)

In addition to maintaining radiation doses below the limits set forth above, work with sources of ionizing radiation shall be planned and conducted to keep doses as low as reasonably achievable.

Editor's Note: The information below compares 1. the radiation exposures to the whole body which are the established federal standard for various activities (Note: The first federal standard for fetuses of pregnant radiation workers went into effect Jan. 1.); 2. amounts of natural background radiation; 3. common sources of additional radiation; 4. amounts from medical treatment (very high radiation to a limited part of the body), and 5. amounts from diagnostic research (low levels from radioactive tracer elements). The source of this information is Francis Masse, director of the MIT Radiation Protection Office. Dr. Masse is a past president of the Health Physics Society and served in 1987-89 as chairman of the National Academy of Sciences panel which reviewed the exposure of soldiers to radiation from atmospheric testing in the 1940s and 1950s.

Astronauts: 25,000 Millirems

The highest recommended limit for radiation exposures is for astronauts-25,000 millirems per Space Shuttle mission, principally from cosmic rays. This amount is beyond the average 300+ millirems of natural sources of radiation and any medical radiation a person has received.

25,000 millirems per year level was the federal occupational limit during World War II and until about 1950 for radiation workers and soldiers exposed to radiation. The occupational limit became 15,000 millirems per year around 1950. In 1957, the occupational limit was lowered to a maximum of 5,000 millirems per year.

Average Natural Background: 300 Millirems

The average exposure in the United States, from natural sources of radiation (mostly cosmic radiation and radon), is 300 millirems per year at sea level. Radiation exposure is slightly higher at higher elevations-thus the exposure in Denver averages 400 millirems per year.

(A milliRem is 1/1000th of a Rem. According to McGraw-Hill's Dictionary of Scientific and Technical Terms, a Rem is a unit of ionizing radiation equal to the amount that produces the same damage to humans as one roentgen of high-voltage x-rays. The name is derived from "Roentgen equivalent man." Wilhelm Roentgen discovered ionizing radiation in 1895 at about the same time that Pierre and Marie Curie discovered radium.)

All of these limits are for the amount of radiation exposure in addition to background radiation and medical radiation.

Adult: 5,000 Millirems

The current federal occupational limit of exposure per year for an adult (the limit for a worker using radiation) is "as low as reasonably achievable; however, not to exceed 5,000 millirems" above the 300+ millirems of natural sources of radiation and any medical radiation. Radiation workers wear badges made of photographic film which indicate the exposure to radiation. Readings typically are taken monthly. A federal advisory committee recommends that the lifetime exposure be limited to a person's age multiplied by 1,000 millirems (example: for a 65-year-old person, 65,000 millirems).

Minor: 500 Millirems

The maximum permissible exposure for a person under 18 working with radiation is one-tenth the adult limit or not to exceed 500 millirems per year above the 300+ millirems of natural sources, plus medical radiation. This was established in 1957 and reviewed as recently as 1990.

Fetus: 500 Millirems Or 50 Per Month (New Rule Jan. 1, 1994)

New federal regulations went into effect New Year's Day, establishing for the first time an exposure limit for the embryo or fetus of a pregnant woman exposed to radiation at work. The limit for the gestation period is 500 millirems, with a recommendation that the exposure of a fetus be no more than 50 millirems per month.

Weight Variables

Like alcohol intoxication levels, levels of exposure to radioactivity (due to radioactivity deposited in the body) depend on a person's weight. A diagnostic tracer of one microcurie of radioactive calcium 45, given orally, would result in an exposure of 3.7 millirems for a 100-pound person, and half of that, 1.85 millirems, for a 200-pound person.

Therapeutic Radiation

Therapeutic radiation treatment that is delivered by administering radioactive material via the mouth or by injection usually results in high, very localized doses to a small part of the body, which absorbs most of the radioactivity. The radioactivity concentrates and remains in the target organ (for example, the thyroid) for a longer period of time than does the radioactivity that is distributed to the rest of the body. The radiation exposure for other parts of the body is a function of the amount of radioactivity per pound and the time the radioactivity is present in the tissue.

George Bush's Hyperthyroid Problem

For example, a hyperthyroid problem such as that experienced by former President George Bush is typically treated with a radioactive iodine drink designed to deliver about 10,000,000 millirems of radioactive iodine to the thyroid. It would coincidentally deliver a dose to the rest of the body of about 20,000 millirems. A slightly lower dose of radioactivity is used for cancerous tumors. Radiation to kill a cancerous tumor often involves a beam delivering 6,000,000 millirems to the cancerous tissue, but the whole-body equivalent dose is much less, as it was in the thyroid example cited above.

What is a lethal dose from a single instance of radiation? According to studies made after the atomic bomb explosions in 1945 at Hiroshima and Nagasaki, half of the people died whose entire bodies were exposed to 450,000 millirems of radiation from the atomic bomb. All persons died whose bodies were exposed to 600,000 millirems of radiation.

Federal Standards, Permissible Levels Of Radiation Exposure to Whole Body (1994 unless noted otherwise)

Millirems above natural background levels (average 300) and medical radiation:

25,000-Astronauts, per Space Shuttle mission. This also was the annual occupational limit for adults from World War II through 1950.

15,000-1950 to 1957 occupational limit per year for adults, including radiation workers and soldiers. Limit changed in 1957 to 5,000 millirems.

5,000-(Since 1957) Occupational limit per year for adult radiation workers, including soldiers exposed to radiation. It is "as low as reasonably achievable; however, not to exceed 5,000 millirems." It is recommended that lifetime cumulative exposure is not to exceed the age multiplied by 1,000 millirems.

500-Occupational limit per year for a minor under 18 exposed to radiation. An embryo or a fetus of a pregnant worker exposed to radiation (a new regulation as of Jan. 1, 1994) is not to exceed more than 500 cumulated total millirems before birth, and it is recommended that the exposure of a fetus be limited to no more than about 50 millirems above background levels per month.

Sources of Naturally Occurring Radiation (Whole Body Equivalents)

25 to 35-Human body's own radiation dose per year from radioactive elements and minerals in the body.

300-Average annual natural background radiation, sea level (includes your own body radiation, cosmic radiation and radon).

400-The city of Denver's average annual natural background radiation (altitude 5,000 feet).

Common Additional Sources of Radiation (Whole Body Equivalents per year in millirems above background levels)

12-Coast-to-coast US round trip flight in airplane at 35,000 feet of altitude.

10-Annual increase due to daily use of salt substitute (potassium chloride) or eating a diet heavy in such potassium-rich foods as bananas and Brazil nuts. Potassium is an essential dietary element that is present mostly in the muscles.

2-Annual exposure due to watching four hours of television every day.

Therapeutic Doses of Radiation to A Part of the Body (Whole Body Equivalents in millirems above background levels)

20,000-Therapeutic radioactive iodine treatment of thyroid gland. A localized dose delivers 10,000,000 millirems to the thyroid and about 20,000 millirems to the rest of the body. A radiation dose to kill a cancerous tumor often sends a beam delivering 6,000,000 millirems to the cancerous tissue, but the whole body equivalent dose is much less, as in the thyroid case.

Doses of Radiation for Medical Diagnosis or Research Purposes (Whole Body Equivalents in millirems above background levels)

500 to 200-Cardiac stress test.

245-Exposure of one 70-pound youth in federal research at the Fernald School by MIT in the 1940s, using trace elements to track iron absorption through eating cereal. The research showed iron supplements are more effective if not be taken with meals.

172-Average exposure of 17 youths, ages 12 to 17, avgerage weight 100 pounds, in the above research.

127-Exposure of heaviest youth, 135 pounds, in the above research.

4 to 11-Exposure received by 45 youths, ages 10 to 16, in federal research in the 1950s by the Fernald School, with the assistance of MIT. The study used radioactive calcium 45 to track calcium absorption. One adult (age 21) was also in the study and received a higher dose, resulting in an exposure equal to 11 millirems for the whole body.

2-One chest X-ray (the whole body equivalent). A typical X-ray exposes the chest to a dose equal to 20 millirems at the entrance and 1 milliRem at the exit. Averaging this exposure over the whole body yields a whole body equivalent of about 2 millirems.
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The US Food and Drug Administration's current regulations state, "The amount of radioactive material to be administered shall be such that the subject receives the smallest radiation dose with which it is practical to perform the study without jeopardizing the benefits to be obtained by the study. Under no circumstances may the radiation dose to any adult research subject from a single study, or cumulatively from a number of studies conducted within one year, be generally recognized as safe if such doses exceed the following:

Single dose for an adult-3,000 millirems;

Annual total dose-5,000 millirems.

For a research subject under 18 years of age at the last birthday, the radiation dose shall not exceed 10 percent of that set forth above."

Therefore, the single exposure limit for a child is 300 millirems (whole body equivalent) and the annual total exposure cannot exceed 500 millirems.

Since 1968, medical researchers at institutions doing the research have been required to follow informed-consent procedures. These procedures require the assent (if feasible) of a child 7 years of age or older, and the consent of both parents if there is any perceived risk involved in the research. For research involving any perceived risk, there also has to be a relationship between the study and a child's disorder or disease.

If there is direct benefit that is likely to accrue from participation in the study, then a researcher needs the assent of the child (age 7 or older) and the consent of at least one of the parents. In such direct benefit research situations, the permitted levels of radiation can be exceeded.

A version of this article appeared in the January 5, 1994 issue of MIT Tech Talk (Volume 38, Number 18).