safeguards are more widely understood in this case. It will be clear from the context whether a level is considered from the toxicity or external irradiation standpoint. Those isotopes that are preeminently gamma emitters are indicated by an asterisk in the table. The handling of those few isotopes that are alpha emitters is not specifically included in this report.

3. Hazards in Handling Radioisotopes

The known hazards may be classified in the order of their importance as follows:

1. Deposition of radioisotopes in the body.

2. Exposure of the whole body to gamma radiation. 3. Exposure of the body to beta radiation.

4. Exposure of the hands or other limited parts to beta or gamma radiation.

The hazards may be briefly described as follows:

1. Deposition may result from ingestion, inhalation, or absorption through the intact or injured body surface. Ingestion may occur as an acute problem through the accidental drinking of an active solution.

More generally, it will be a chronic problem caused by accumulation of small amounts of activity on contaminated hands, cigarettes, and other items brought to the mouth, or as a secondary result of inhalation. Following ingestion, the hazard may be due to direct irradiation of the alimentary tract, or more probably due to chronic irradiation of the organs in which the particular active materials are concentrated (e. g., strontium isotopes will accumulate in the bone, and iodine isotopes in the thyroid gland). Inhalation of active gas, vapor, spray, or dust may occur. Exposure to spray or dust is considered particularly hazardous because of the large fraction of such contamination retained by the lungs. Following inhalation, the hazard is threefold:

1. Direct irradiation of the lungs, etc.

2. Absorption of the active material directly from the lung.

3. Elimination from the lung by ciliary action followed by ingestion.

Chronic deposition of unabsorbable particles in the lungs is a major hazard since it is extremely difficult to demonstrate the accumulation of such particles. Once radioactive

material has entered the body and been deposited in the organs governed by its metabolism, it is difficult or impossible to expedite the natural rate of elimination from the organ. It is, therefore, essential to avoid all ingestion or inhalation of radioactive materials and to test potentially exposed personnel for such accumulations whenever a suitable method exists.

Absorption of active materials, through an open cut or even through the intact skin, is a potential hazard when more than tracer doses are handled. Retention of activity in the skin itself is known to be able to produce tumors.

2. The whole body exposure to gamma radiation shall not exceed 300 mr/week, measured in air (i. e., without backscatter). According to present knowledge, this general exposure to gamma radiation is believed to be safe as far as any bodily injury is concerned, when there is no other type of radiation exposure. The importance of possible genetic change effective in later generations has not been established. 3. When the body is exposed to an external source of beta radiation, only the superficial layers up to a few millimeters in thickness are irradiated. Nevertheless, for safety, the limiting general exposure to external beta radiation should be taken as 500 mrep 1/week in the surface layers. The outermost layer of skin is considered to be a dead hornified layer, which acts as a filter, and the dose is computed for the zone immediately below this. In general the filter thickness is taken as 7 mg/cm2. For the palm of the hand the thickness is greater and a value of 40 mg/cm2 is often used.

4. Earlier practice in the handling of radium and related compounds condoned the acceptance of greater exposure of limited parts of the body, specifically the hands or the head, in comparison with whole body exposure. The recommended practice, however, is to limit the exposure of all parts, except the hands, to that which is acceptable for the whole body. In the case of the hands an exposure of 1 r/week measured in air, or 1.5 rep/week in the basal layer of the epidermis, is considered permissible. The calculated exposure is to in

1 In the absence of an internationally accepted unit, the "rep" is a convenient shorthand notation for statements of dose of ionizing radiation not covered by the definition of the roentgen. It represents that dose which produces energy absorption of 93 ergs/gram of tissue. The actual energy absorption in tissue per roentgen is a function of the tissue composition and of the wavelength of the radiation. It ranges between 60 and 100 ergs/gram. For calculations of permissible exposure this variation is ignored, and a beta-ray dose of one rep is said to be physically equivalent to an X-ray dose of one roentgen at a given point in the body. The numerical coefficient of the "rep" has been deliberately changed to 93, instead of the earlier 83, to agree with L. H. Gray's "energy-unit".

clude that due to radioactive contaminants deposited in the skin.

In connection with the four types of hazard enumerated above it is important to bear in mind that the permissible exposure as quoted applies under conditions where only one hazard exists. In the handling of radioisotopes, all four hazards exist together, and this may reduce the permissible exposure to each. Simple summation of the ionization contributions at any point in the body from the four enumerated causes is assumed.2

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In particular, the total irradiation of any part of the body (with the exceptions noted above), should not exceed 300 mrep/week. It is advisable to keep well below the quoted permissible exposures whenever mixed exposures may occur, because of the increased difficulty of registering such exposures accurately.

4. Principle Underlying Protective Measures

The fundamental purposes of protective measures in the handling of radioisotopes are:

1. To prevent ingestion, inhalation, interstitial, or other modes of entry into the body.

2. To reduce the amounts of external irradiation to permissible levels.

The first requirement is fulfilled by good housekeeping and work habits, and by operation in a laboratory properly equipped for the handling of isotopes, including protective covering, manipulative devices, suitable ventilation, and waste disposal facilities. The second requirement, maintenance of satisfactory levels of external radiation, is governed by procedures such as those contained in the National Bureau of Standards Handbook 23, "Radium Protection," which should be available to all persons working with radioisotopes. Special requirements arise when isotopes with beta activity, essentially free from gamma activity, are used.

Laboratories that specialize in the use of a few isotopes should become familiar in detail with the published data on the metabolism and estimated maximum permissible concentration values applicable to these cases. Where many

2 Note that the contributions from internal deposition or skin contamination I will be effective 24 hr/day, and 7 days/week. Contributions from external sources are limited to the normal workweek (48 hours).

3 A more detailed discussion of permissible limits of exposure will be found in the report of the Subcommittee on Permissible Dose from External Sources, now in preparation.

types of isotopes are in use, the following values form a provisional guide to maximum permissible contamination:

(1) For atmospheric contamination: 10-" uc/cm3 (2) For water contamination: 10-μc/cm3

II. Personnel

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1. Selection and Instruction of Personnel

Persons who are neat and careful are preferred workers with radioisotopes. A rigid physical examination should be made of all prospective workers. Careful inspection of the hands, and evaluation of possible previous exposure to radiation, are recommended. All individuals employed in radiation work shall be informed in detail of all known dangers involved. They shall be instructed regarding local rules and regulations for protection, and should be expected to observe them in all details. It is particularly important that all users of radioisotopes should be considered as potential full-time users.

2. Effects of Radiation

Effects of external radiation are adequately described in the National Bureau of Standards Handbooks 23 and 41.

When the active materials are deposited in the body, the effects depend upon the site of deposition, the physical halflife, and the biological half-life, which is determined by the elimination rate. The bone-seekers (for example, strontium) will produce effects similar to those found in radium poisoning. Other materials may produce changes in liver or kidney function, and occasionally in other organs. An essential feature of all the effects is that they may not appear until the dangerous material has resided in the body for many years, and irreparable latent damage may have been produced. There usually are no definite clinical symptoms which can be relied upon to guard against possible impending injury.

3. Blood Count

A complete blood count shall be made by a qualified hematologist before any individual begins work involving the handling of radioactive materials. Counts on two successive days at a stated hour are desirable. No one should be em

ployed who shows pertinent abnormalities in the blood count. Blood counts should be made at regular intervals (normally 3 months) during employment, with more attention given to the trend of successive counts and especially of the differential count than to absolute values. It should not be considered that overexposure of the individual will be detected by changes in blood count. Poor protection techniques may be detected by blood count findings before permanent injury to the individual occurs.5

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4. Physical Examinations

(a) General

A thorough medical examination should be made of each individual potentially exposed to significant amounts of radiation before employment, and annually thereafter. An examination for possible radioactivity, by a person with special knowledge and equipment, should be given each individual, and form a part of the annual physical examination, whenever the exposure potential includes significant internal deposition. More frequent examinations are warranted when the exposure potential is high. The nature of such tests will depend upon the particular isotopes to which the individual may have been exposed. Sufficiently sensitive tests for the deposition of all relevant isotopes may not exist.

(b) Urinalysis and Other Tests

An analysis of radioactivity of the urine is a desired procedure. Normal urine contains radiopotassium in amounts which may mask the added radioisotopes for which tests are made. Either potassium should be separated from the sample and the residual activity measured, or when the possible exposure is restricted to one isotope, this should be chemically separated from the urine. Examination of the feces may be required when the predominant elimination is by feces. Special tests for specific isotopes are in order when they exist (e. g., radioiodine may be estimated in the thyroid gland in terms of the emitted gamma radiation measured by a Geiger counter or ionization chamber). Where exposure

A single exposure of 25 r can apparently escape detection by standard blood counting techniques.

5 A more detailed discussion on blood counts will be found in National Bureau of Standards Handbook 41, Medical X-ray Protection up to Two Million Volts.