a Calcium absorption and excretion is dependent on blood calcium level.

b Iron absorption from the gut is influenced by the level of iron present in the blood and liver. Once iron enters into the metabolism of the animal, the excretion rate for that iron is very low.

As: Retention in all tissue is very low.

Do. Bone: M....do... High.. Bone: M. Urine High..| Bone: M

Liver.

(8)...All pro- do... Same.. Same..... Same.. Same.. Same.... Same... 5

High.

tein. Body flu

ids: H.

d Sr: Absorption and deposition (5 to 80 percent in bone) vary with age and existing Ca level. • Doses in excess of 0.1 gm give a low retention.

'H, L, and M indicate high, low, or moderate percentage of deposit out of total amount absorbed.

Indicates chiefly dependent on compound.

"Same" implies that the property under "Inhalation" or "Parenteral" is the same as the corresponding property listed under "Ingestion" for each isotope. The ditto marks "do" or "Do" apply to the vertical columns.

TABLE 2. Hazard from absorption into the body

Selected radioisotopes grouped according to relative radiotoxicity, with the amounts considered as low, intermediate, or high level, in laboratory practice.

Effective radiotoxicity is obtained from a weighting of the following factors:

Half-life.

Energy and character of radiations.

Degree of selective localization in the body.
Rates of elimination.

Quantities involved and modes of handling in typical experi

ments.

The slant boundaries between levels indicate border-line zones, and emphasize that there is no sharp transition between the levels and the associated protection techniques.

The principal gamma-emitters are indicated by asterisk (e. g., *Na24). The above level system does not apply to the hazards of external irradiation.

Table 2 shows the common isotopes subdivided into three groups, according to relative radiotoxicity following accidental intake into the body. The diagram attempts to define the ranges of low level, intermediate level, and high level activity used in the laboratory, with respect to intake hazards.

When the hazard is confined to that from external radiation, low, intermediate, and high levels are normally considered in terms of the emitted gamma radiation. These levels are not defined in this report, because the required

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".