An Evaluation of Radiation Exposure Guidance for Military Operations: Interim Report (1997)

To understand how to protect soldiers from ionizing radiation,⁵ it is necessary to understand its characteristics, how it interacts with tissues in the body, and the effects these interactions may have on immediate and long-term health.

All matter is made up of atoms, each consisting of a nucleus containing neutrons and positively charged protons. Negatively charged electrons surround the nucleus. The nucleus of a radioactive atom has excess energy causing it to be unstable. To become more stable, the radioactive nucleus will eventually release energy in the form of either particulate (e.g., alpha and beta particles) or electromagnetic (e.g., gamma rays) radiation.

When these forms of radiation strike atoms of any material, they may have enough energy to eject electrons. This process, called ionization, can result in the breaking of electron bonds that hold atoms together. Ionization and other radiation-induced effects, such as excitation and free radical formation, cause chemical changes in components of the living cell, including deoxyribonucleic acid (DNA), the genetic material in the cell that is located in the chromosomes within its nucleus.

Alpha particles colliding with atoms give up their energy in a very short distance, such as the thickness of a sheet of paper, less than the thickness of skin, or a few centimeters of air. Consequently, alpha particles are not likely to be

harmful when striking the outside of a body that is protected by clothing and the outermost dead layer of the skin. However, when these same alpha-emitting radionuclides are taken into the body, they can directly irradiate nearby cells of tissues in which they are deposited and may cause cellular changes. Such changes may result in adverse health effects in the short-or long-term, depending on the nature of the changes. Alpha particles may be encountered in contamination created by intentional or accidental dispersion of nuclear weapon-source materials (e.g., plutonium) or as a result of fallout from a nuclear detonation. Alpha particles from naturally occurring sources of radiation, such as radium and radon, contribute to normal background levels.

Beta particles penetrate to a greater depth than alpha particles before the transfer of all their energy to tissues is complete. However, even high-energy beta particles will give up most of their energy within about one centimeter of plastic, one to two centimeters of tissue, or 4 to 5 meters of air. Therefore, beta particles striking the outside of the body will penetrate only a short distance, but they may travel far enough to damage the actively dividing cells of the skin. Beta particles are of greater concern after they have entered the body and can transfer their energy to nearby cells of internal organs. Beta radiation may be found in contamination consisting of fission products from a nuclear detonation or resulting from dispersion of nuclear reactor waste or radiotherapy sources (e.g., radiocesium and radiocobalt).

Gamma rays and x rays are the most penetrating forms of ionizing radiation and consist of electromagnetic energy. While randomly colliding with electrons in the body, gamma rays may give up all their energy in tissue, or they may pass all the way through the body without interacting. Therefore, exposure to gamma or x rays from sources outside the body may cause ionizations in tissues at any location in their path. Gamma rays are characteristic of a wide variety of radioactive contaminants associated with nuclear weapons and nuclear waste and also with radioactive sources used in medicine and industry, while x rays are most commonly encountered in medical applications (including those in combat medical facilities).

The energy of ionizing radiation is measured and described in a number of ways. One can use a meter or other device to measure exposure—ionization in air caused by radiation. Exposure is measured in coulombs per kilogram (C/kg)

of air, formerly⁶ the roentgen (R). This measurement of exposure applies only to ionizing electromagnetic radiation, such as gamma and x rays, not to particulate radiation (e.g., alpha or beta particles). In the field (outside the laboratory), exposure is the quantity that is measured, although for convenience, it is commonly assumed that exposure and absorbed dose (see below) are the same when expressed in traditional units (i.e., 1 R = 1 rad).

While beta and alpha radiations can be detected in the field, determination of their contribution to tissue dose is a complex process not reasonably implemented except under laboratory conditions. Exposure to alpha-and beta-emitting radionuclides is expressed as the concentration of these radionuclides in air, food, and water. The primary dose to persons exposed to these concentrations results from ingestion and inhalation of the radionuclides.

A useful quantity in radiation physics is the energy actually deposited in a certain amount (mass) of tissue. This unit is referred to as absorbed dose. The unit of absorbed dose is the gray (Gy), formerly the rad. One Gy equals 100 rad; 1 milligray (mGy) equals 100 millirad (mrad). However, the amount of energy deposited in tissue does not account for differences in biologic effects of different radiation types.

The dosimetric quantity that accounts for the differences in biologic effectiveness of various types of radiation and allows doses from different radiations to be combined is called the equivalent dose. It is calculated by multiplying the absorbed dose by the appropriate radiation weighting factor, ''WR'' (ICRP, 1991a). For example, the factor for alpha particles is 20 and that for gamma and beta radiation is 1, indicating that it takes about 20 times more gamma or beta radiation than alpha radiation to cause a given effect. The unit of equivalent dose is the sievert, formerly the rem. One sievert (Sv) equals 100 rem; 1 millisievert (mSv) equals 100 millirem (mrem).

Just as different radiation types have greater or lesser effectiveness in damaging tissue, different tissues types have varying sensitivity to that damage. For a

given dose of radiation, highly sensitive tissues show a greater increase in cancer rate than do less sensitive tissues. For radiation protection purposes ICRP has developed weighting factors for tissues (called "W_T") that describe the sensitivity of different tissues. Tissue weighting factors facilitate combining doses to allow quantitative comparison of long-term risk from partial body exposure to that from total body exposure. From that combination of doses one can estimate the risk of radiation effects for the entire body. Tissues that are very sensitive to long-term effects from radiation have high weighting factors (e.g., bone marrow W_T = 0.12), while less sensitive tissues have lower weighting factors (e.g., skin W_T = 0.01).

The effective dose to the whole body is found by multiplying the equivalent dose in each tissue type by its corresponding tissue weighting factor and adding the results for each tissue type. This composite dose is proportional to the increased risk from cancer and genetic effects. Like the equivalent dose, the effective dose is expressed in units of sieverts (Sv) or millisieverts (mSv). Dose limits set for occupational exposures are expressed as effective dose and include both internal and external doses. Table 2-1 compares the characteristics of the three ways in which dose in biological tissue may be expressed.

It is critical that radiation measurement equipment be suited to its measurement task. Important considerations are the accuracy and sensitivity of the instrument chosen. Alpha, beta, and gamma (or x-ray) radiation measurements each require different instruments because of the way in which each radiation type interacts with matter. An instrument designed for alpha detection, for ex-

ample, will not give accurate information for the other types of radiation. A radiation safety program specifies the appropriate equipment to be used in estimating exposure to an individual from external radiation sources.

For directly measuring individual doses of gamma radiation (and, under some conditions, beta radiation), a thermoluminescent dosimeter (TLD) is often used. The TLD will give a reasonable measure of the dose to the whole body from gamma rays from a broadly distributed source. Because of the short range of beta particles, however, a TLD will only indicate the dose received from this type of radiation in its immediate location.

Doses to individuals may also be calculated indirectly if exposure rates are known from radiation surveys (radiation measurements made in the field). Two types of radiation survey instruments are helpful for assessing the potential for exposure to military personnel in the field. The first type measures the radiation exposure or dose to which personnel may be subjected. This category of instrument includes devices such as microroentgen meters and ion chambers. The second type of meter is represented by Geiger-Mueller or sodium-iodide detectors. These meters are used for finding contamination, although the Geiger-Mueller detector may be calibrated to provide exposure readings.

An ion chamber is designed to measure exposure, that is, ionization in air due to gamma rays (in C/kg or R). This instrument measures the quantity of radiation energy at a point in the air. Ion chambers normally come equipped with a moveable cover over the detection chamber. When the cover is open, the instrument will respond to alpha and beta particles, as well as gamma rays. However, these instruments are not usually calibrated for alpha and beta particles, so the instrument reading may not be accurate for them.

A Geiger-Mueller detector is primarily designed to measure the number of alpha, beta, or gamma rays emanating from a source and striking the detector in a given time. This meter does not normally provide information about the energy of incident radiation or about exposure. However, it can be calibrated to relate the number of gamma rays to a known ionization in air to give readings in units of C/kg (or roentgen).

The devices discussed briefly above are useful for detecting or measuring contamination on surfaces (e.g., on the ground or on a vehicle such as a tank), but they cannot directly detect low levels of airborne radioactivity that might be hazardous. To determine whether airborne contamination is a health problem, an additional device—the air sampler—is required. This device removes radioactive contamination from the air and concentrates it sufficiently to be measurable by a detector similar to those discussed above.

Determining dose for internal exposures from inhaled or ingested radionuclides is much more difficult and time-consuming than determining external dose. It requires measurements of air (or water or food) contamination, identification of significant radionuclides, measurement of amounts excreted, and the application of sophisticated biomathematical models to determine doses to specific organs. Gamma-emitting radionuclides deposited in the body can be detected and measured with instruments external to the body. Under battlefield

conditions, rough measurements of environmental contamination can be made as a basis for estimating dose. If calibration factors are available for open window ion chambers and GM counters, those instruments may be used to get a very crude estimate of airborne contamination. Under less adverse conditions, more sophisticated instrumentation and techniques should be applied.

In this section we provide a perspective for the radiation doses soldiers could receive in the course of military operations. Under normal conditions, everyone is exposed to background ionizing radiation from two major sources: continuous, naturally occurring radiation from space and from radioactive elements and technology-enhanced (often referred to as manmade) radiation sources. Natural sources of radiation constitute the major source of radiation exposure to the populations of most, if not all, countries, with the next largest source being applications of medical technology.

In the United States the average annual effective dose of naturally occurring background radiation is about 3 mSv (0.30 rem) per year (NCRP, 1987). Of this, about 2 mSv (0.20 rem) come from exposure to radon, 0.28 mSv (0.028 rem) from cosmic rays, 0.39 mSv (0.039 rem) from naturally occurring radionuclides in the human body, and finally, 0.28 mSv (0.028 rem) comes from naturally occurring radioactive materials within the ground (terrestrial). The effective dose from all natural sources during a 70-year lifetime is approximately 200 mSv (20 rem). Levels of background radiation vary significantly across geographic areas. In the United States, for example, the natural background radiation from cosmic rays and terrestrial sources in Denver, Colorado, is 50 percent higher (NCRP, 1987) than the national average. Thus a resident of Denver receives about 0.3 mSv [0.03 rem] per year more than the average resident of the United States.

In addition to the doses of background radiation received annually, some soldiers are engaged in duties in which they are at risk of exposure to higher levels of ionizing radiation. Examples of such duties include repairing and maintaining radioactive commodities (such as depleted uranium ammunition and luminescent sights containing tritium), flying at high altitudes, and administering medical diagnostic and therapy procedures. Table 2-2 shows the distribution of occupational doses for Army radiation workers.

Apart from routine occupational exposures, the only exposure of large numbers of U.S. military personnel to radiation has been to those who were in the occupation forces near Nagasaki and Hiroshima, Japan at the close of World War II and to those who participated in the above-ground nuclear test program conducted between 1945 and 1962. Of these 210,000 military personnel, about 1,200 received doses that were estimated to exceed 50 mSv (5 rem) (DSWA, 1995a)—the present annual dose limit set by the U.S. Nuclear Regulatory Commission (CFR, 1991) for individuals occupationally at risk of exposure to radiation. About 20,000 participants (DSWA, 1995b) have been assigned esti-

mated doses that exceed the more conservative annual occupational limit—20 mSv (2 rem)—recommended by the ICRP (1991a). A total of 0.07 percent of the doses exceeded 100 mSv (10 rem), and the average estimated dose for an Atomic Veteran is 6 mSv (0.6 rem).

The most critical target of ionizing radiations passing through living tissues is generally accepted to be the DNA that constitutes the genes in the nucleus of every cell. Ionizing radiation can damage DNA directly or indirectly. For direct damage to occur, the radiation must hit this genetic material. Since the volume of the DNA is very small compared with the total volume of the cell, the probability of this occurring is low. Indirect damage occurs when radiation interacts in close proximity to the genetic material—the interaction can create a free radical in water that can subsequently damage DNA. Two-thirds of the tissue damage created by radiation is caused by these indirect processes.

Complete and accurate biological repair of such DNA damage is a normal process that occurs millions of times daily. However, under certain conditions, radiation-induced DNA damage can be irreparable, or the repair can be incomplete or inaccurate. This can result in the appearance of acute adverse health effects (within about 2 months) or delayed effects (over many years, even decades after the exposure).

Most radiation at environmental levels (background) does not result in detectable health effects. The reason is that most radiation interactions occur in the water in the cells of the body, producing free radicals that rapidly dissipate without doing biological damage.

Generally, when radioactive contaminants enter the body, the radionuclides are not uniformly distributed. As a result the dose may be highly localized. Uniform irradiation of the entire (whole) body, by radionuclides deposited inside it,

is very rare and occurs only with very soluble, usually beta-or gamma-emitting, radionuclides. Animal studies have demonstrated that nonuniform distribution of energy through tissues, such as from radioactive particles, is less hazardous than uniform distribution because of the lower number of cells at risk (EPA, 1976; Nenot and Stather, 1979). Cancers resulting from intake of radionuclides are more likely to arise in those tissues that contain the highest concentrations of radionuclides. Some tissues, such as lymph nodes, are much less susceptible to radiation-caused cancers than others, such as red bone marrow.

For a tissue to be affected by radiation it must be directly irradiated. For example, radiation to the hand from an x-ray machine cannot cause primary health effects in other parts of the body. Most acute effects of radiation are due to cell killing and are deterministic in nature. Long-term effects are usually due to mutations and are termed stochastic effects.

Irreparable radiation-induced DNA damage results in premature cell death or inability of the cell to divide. If cells are damaged faster than they can be replaced or repaired, health may be adversely affected. If this damage-vs.-repair differential is present, clinical signs will be detectable and symptoms may develop early in the postexposure period (within about 2 months).

The type and severity of deterministic effects depend upon the type of ionizing radiation involved, the magnitude of the dose, and the rate at which the dose is accumulated (dose rate). As described above, gamma and x-ray radiations emitted by sources outside the body can penetrate several tens of centimeters of tissue to interact with DNA in cells deep within the body. High-energy beta emitters on or close to the skin can penetrate the skin's outer layer of dead and aging cells to reach the actively dividing cells beneath. These exposures have the potential to cause local skin injuries and effects within the range of the radiation. Such manifestations of acute radiation-induced health effects can occur alone, in combination with each other, and with nonradiation-induced trauma, including thermal burns, or other serious medical conditions. Combined injuries of these types tend to have a greater effect on the health of the exposed person than the sum of the effects of the individual injuries.

Accidents involving humans, medical experience, and animal studies indicate that doses of radiation must exceed a threshold in order to cause the various types of acute health effects (injuries) that have been described. Thresholds for several radiation effects of interest are shown in Table 2-3.

If the dose is accumulated instantaneously or within a short time, the threshold doses for early radiation effects may be quickly reached or exceeded, resulting in acute effects. This can occur in the event of a high dose from a source outside the body (e.g., nuclear weapon detonation) at a high dose rate. If, however, the same total dose is accumulated over a longer period of time (i.e., is fractionated or protracted), the types of deterministic health effects due to the exposure are likely to be fewer in number and less severe. The effects of pro-

tracted or fractionated doses are less than acute doses because the numbers of cells being killed by the radiation over time will be less than the number of new cells being produced in the body's tissue systems during the same period and because repair of radiation injury occurs within most cells. Distribution of doses over long time periods can occur with external exposures and when long-lived radionuclides are deposited inside the body. The likelihood of alpha and beta emitters deposited inside the body causing generalized symptoms of radiation exposure early in the postexposure period is minimized by their limited penetrating power, which restricts their biological effectiveness to nearby cells.

Another key factor in the body's response to ionizing radiation is the relative sensitivity to radiation of the various cell types that comprise body tissues. Bergonié and Tribondeau's Law (1906) implies that rapidly dividing cells (e.g., cells of the blood forming tissues and certain groups of immature sperm cells) are among the most sensitive to acute effects of radiation. The more highly differentiated cells (e.g., muscle and nerve cells) are less vulnerable to acute injury by radiation. Other factors that influence the expression of the deterministic effects of radiation include the region of the body irradiated and variation between individuals in their physiologic response to radiation.

A small group of deterministic effects tends to appear beyond the characteristic early (2 month) postexposure period. This group reflects irreparable DNA damage incurred at the time of exposure and subsequent cell death. It includes cataracts, infertility in males and females, suppression of thyroid gland function, and fibroatrophy as a consequence of radiation-induced damage to connective tissue and blood vessels. These effects are associated with practical threshold doses that are typically higher than those of concern in this report.

Of special concern in the modern military would be the radiation-induced damage that could occur in the embryo or fetus as the result of the inadvertent exposure to radiation of a pregnant soldier. A dose of greater than 50 mSv (5 rem) to the embryo or fetus is associated with an increase in risk (relative to the nonexposed) of nonspecific deterministic effects in the forms of embryonic

death, congenital malformations, or mental and growth retardation, depending on the period of gestation during which the exposure occurred (Brent, 1989).

Incomplete repair, or misrepair, of radiation-induced DNA damage increases the risk of tumors and hereditary effects that may appear many years later, unless the damage is inconsistent with cell survival and division. Such damaging effects occur randomly among individuals in exposed populations or their offspring. The frequency and probability of their occurrence, but not their severity, increase with increasing radiation dose. The type of late effects that can occur depends on the type of cells affected.

Radiation-induced gene mutations in some types of cells (somatic) can result in abnormal cell growth that may be benign (noncancerous) or malignant (cancerous). In theory, these abnormal growths can be initiated in a single irradiated (and transformed) cell, but a variety of biological factors protect against every transformed cell progressing to become a malignant focus for cancer or leukemia development. Such factors include the age of the individual at exposure, gender, genetic heritage, and the immune system's ability to resist cancer. Theoretically, and for radiation protection purposes, it is assumed that there is no dose below which the probability of such effects occurring is zero, that is, there are no threshold doses for radiation-induced tumors.

It takes time for damage to DNA to result in a radiation-induced tumor. The interval between the exposure and the detection or diagnosis of a tumor attributable to the exposure is termed the latent period. The latent period is generally accepted to be a minimum of 2–5 years for radiation-induced leukemias and 10 years for most solid cancers.

While all cell types are assumed to be susceptible to malignant transformation by ionizing radiation, cells in certain tissues appear to be more susceptible. Increased risks of benign (noncancerous) nodules in thyroid gland and female breast tissue, several types of cancer (e.g., lung, thyroid gland, and female breast), and all forms of leukemia except chronic lymphocytic leukemia have been strongly associated with external exposure to ionizing radiations, primarily at high dose rates. Examples of populations in which these associations have been found include the Japanese atomic bomb survivors, some groups who had medical diagnostic or treatment exposure, and some occupationally exposed individuals.

In the atomic bomb survivor population, statistically significant associations between increases in death rates for certain cancers and leukemia and radiation dose have been reported among groups who received doses of 200 mSv (20 rem) or more (Shimizu et al., 1990). A recent update of cancer mortality among the same population suggests an increased risk for cancer mortality at a lower level—above doses of 50 mSv (5 rem) (Pierce et al., 1996).

Increased risks of death due to cancers of certain organs and other malignant tumors related to exposure to radiation from radionuclides deposited in suscep-

tible tissues have been reported among females occupationally exposed to radium and patients treated with radium. Thyroid cancers have occurred in persons exposed to radioactive iodines in radioactive fallout and lung cancers have been observed in uranium and other miners exposed (by inhalation) to the radioactive decay products of naturally occurring radon underground (NRC, 1988). Several of these populations were exposed to high levels of radiation for long periods of time.

The lifetime risk of fatal cancers associated with exposure to low doses of radiation (100 mSv [10 rem]) at low dose rates is estimated to be a factor of 2 to 4 less than the risk associated with exposure to higher doses and dose rates (NRC, 1990).

Studies of cancer mortality among radiation workers whose exposures were controlled by stringent radiation protection standards (i.e., at low doses and low dose rates) yield risk estimates consistent with those derived for low doses and low dose rates from studies of cancer mortality among atomic bomb survivors (Cardis et al., 1995). Several studies of the mortality experience of American and British Atomic Veterans (e.g., Johnson et al., 1996; Darby et al., 1988, 1993a, 1993b; Watanabe et al., 1995) have been completed, but they have not provided convincing evidence of detrimental radiation effects on long-term survival.

Gene mutations in reproductive cells (sperm or ova) can increase the risk of stochastic effects in the form of nonspecific heritable genetic diseases among the offspring of irradiated organisms. Experimental animal and plant studies show that the probability of such effects occurring is related to radiation dose. However, no increased risk (compared to nonexposed populations) of such diseases has been documented among the children of atomic bomb survivors who were exposed before conceiving their children (NRC, 1991).

Recently, there was considerable interest in determining whether the Atomic Veterans and their families may have experienced adverse reproductive outcomes (e.g., stillbirths, infertility, birth defects, etc.) as a result of their exposure to radiation. The Institute of Medicine considered the feasibility of such a study and reported as follows (IOM, 1995):

There are three primary means of reducing radiation dose from sources external to the body: time, distance, and shielding.

For a given source of radiation, the amount of radiation energy deposited in the body is related to how long one is exposed. Therefore, reducing the duration of an individual's exposure to radiation will decrease dose.

Increasing the distance between an individual and a radiation source is an important means of reducing radiation exposure, because the intensity of the radiation is inversely proportional to the square of the distance from the radiation source. For example, when the distance from a localized source is doubled, the intensity of the radiation is reduced by a factor of 4 (22).

Shielding is useful for absorbing radiation energy. If enough interactions occur in the shielding material, then much of the radiation is prevented from reaching the body's tissues. Alpha particles can be stopped by a piece of paper. Beta particles are blocked by about a centimeter of plastic. Clothing and the outer layers of skin cells provide some protection from beta particles outside the body. Gamma rays, however, may require many centimeters of lead or meters of concrete for shielding.

Once a radioactive material is taken into the body, the protective measures of distance and shielding cannot be applied. However, the duration of internal exposure may be reduced by increasing the rate of excretion of the radioactive material through elimination of body fluids or solids. The primary means of protection from internal radiation exposure is to prevent radioactive materials from entering the body in the first place. Appropriate respiratory protection can prevent the inhalation of airborne radioactive materials. Ingestion is prevented by not eating, drinking, or smoking where radioactive materials are present.