Exposure to radiation and to electric and magnetic fields associated with passenger screening devices are not expected to cause adverse health effects in passengers. Radiation levels used by the technologies under consideration in this report are far below the levels that have been linked to health effects. The level of ionizing radiation1 in passenger screening technologies is compared to the levels of exposure to ionizing radiation from other sources in figure 6-1. Low-energy electromagnetic radiation includes radio waves, microwaves, radar, and power-frequency radiation from electric and magnetic fields associated with electric currents. (See figure 3-1 for the different wavelengths and frequencies of the components of the electromagnetic spectrum.) Passenger screening devices operate using radiation and electric- and magnetic-field levels well below the maximum allowable exposure levels established to protect public health. The health effects of theoretical concern in passenger screening include cancer, reproductive and teratogenic effects, and cardiac effects in passengers with pacemakers.
Cancer is the second leading cause of death in the United States. Approximately 1.25 million new cases (excluding skin cancer) are diagnosed each year, and cancer accounts for 550,000 (or one out of five) deaths annually. The major causes of cancer are tobacco use and dietary factors. It is known that various external factors (e.g., tobacco, viruses, and radiation) and internal or host factors (e.g., hormones, immune status, and genetic factors) can combine or interact sequentially to initiate and promote carcinogenesis and to facilitate tumor growth (American Cancer Society, 1995; Doll and Peto, 1981).
With the possible exception of cigarette smoking, ionizing radiation is probably the most thoroughly studied human carcinogen. We know more about the cancer-causing effects of ionizing radiation than about most other known or suspected human carcinogens. Ionizing radiation is less carcinogenic than other known human carcinogens, and it is responsible for only a small part of the cancer burden in the United States. Less than 5 percent of all cancer deaths may be attributable to exposure to ionizing radiation. The principal sources of radiation exposure to the U.S. population are the natural radiation background and medical and dental radiodiagnostic procedures. The primary factors contributing to cancer are tobacco consumption and dietary habits. Together, these two causes account for about two-thirds of all cancer deaths (Doll and Peto, 1981).
Evidence of ionizing radiation as a human carcinogen is derived from a number of epidemiological studies involving exposures of human populations from the military, medical, and occupational uses of radiation. Such evidence is derived almost exclusively from epidemiological studies of populations exposed to high doses of radiation. Radiation-induced cancer has not been observed in populations exposed to radiation doses of less than 10 to 20 rem (0.1 to 0.2 Sv).2The single most important study involves the long-term evaluation of the Japanese survivors of the atomic bombings of Hiroshima and Nagasaki in 1945. In the Life Span Study (one of several cohort-based epidemiological studies) conducted by the Radiation Effects Research Foundation, more than 75,000 atomic bomb survivors are being studied. Mortality rates and causes of death are continuously updated. Individuals in the study received doses ranging from less than 10 rem (0.1 Sv) to more than 500 rem (5 Sv). The average dose to survivors of the bombings was approximately 20 rem (0.2 Sv). In the mortality survey from 1950 to 1985, a total of 6,000 cancer deaths have been observed; only about 350 excess cancers that have been observed to date could be attributable to radiation exposure (NRC, 1990; UNSCEAR, 1994).
Epidemiological studies have also been conducted to evaluate the health effects of exposure to natural background radiation. Individuals in the United States are exposed to 200
1 In the context of this report, ionizing radiation is defined as high-energy electromagnetic radiation capable of disrupting chemical bonds and causing biological injury through the process of ionization. Ultraviolet light, visible light, and radio waves are examples of lower energy electromagnetic radiation that interacts with matter and causes biological damage by other mechanisms. These types of radiation are referred to in this report as nonionizing radiation.
2 Rem is a measure of the effect of radiation on the human body. It takes into account both the amount of radiation deposited in body tissues and the type of radiation. A millirem is one-thousandth of a rem. A newer unit is the sievert (Sv). I Sv = 100 rem.
to 300 millirem (2 to 3 mSv) annually from natural background radiation, including radioactive materials from the earth's crust and cosmic rays from outer space (NRC, 1990). No evidence has been found of an increase in cancer or other diseases among people living in areas where natural background radiation is several times higher than average (up to 500 to 600 millirem [5 to 6 mSv] per year), such as in Han, China; Kerala, India; or Araxa-Tapira, Brazil (NCRP, 1987; NRC, 1990).
Although no epidemiological studies have shown conclusively that ionizing radiation at low doses (less than 10 rem [0.1 Sv]) causes cancer, it is nevertheless assumed that low doses of radiation are carcinogenic. A popular view within the scientific community is expressed through the linear no-threshold model, which assumes that the risk of developing cancer is directly proportional to dose, even at very low doses, and that any dose, no matter how small, may be damaging. Dose extrapolation under this or other doseresponse models must be viewed with caution. First, there is a paucity of data in the low-dose region. Thus, large uncertainties exist for estimates of health effects at low doses. Second, the extrapolation process, especially for the well known linear no-threshold model, assumes that carcinogenic mechanisms operative at high dose levels are also relevant at low dose levels. This assumption is probably not valid
because significant cell killing, which occurs in the high-dose range (doses above 100 rem [1 Sv]), probably occurs to a much smaller degree, if at all, at low dose levels. Cell killing may play an important role in carcinogenesis by promoting cell division in surviving cells. Furthermore, the body is capable of repairing radiation damage in cells, and damage would be more likely repaired at low doses.
Linear extrapolation to very low doses is often without practical meaning. Although it is theoretically possible that a small exposure might result in a cancer, the probability at very low doses is vanishingly small and on the order of one in a billion. For example, if one airport screening with an x-ray imaging device results in 0.003 mrem (0.00003 mSv) (Smith 1995), and if the lifetime probability of cancer mortality is 1 in 1,000,000 per mrem (per 0.01 mSv) (UNSCEAR 1994), then the theoretical risk associated with cancer mortality from a single screen approaches 3 in 1 billion. Considering that the lifetime cancer mortality rate for Americans from all causes is about 20 percent (or 1 in 5), these small probabilities are practically meaningless.
Compared to ionizing radiation, much less evidence exists that nonionizing fields, such as extremely low-frequency (e.g., 60 Hz [cycles per second]) electric and magnetic fields, cause cancer. Although some epidemiological studies suggest statistical associations of this type of radiation with cancer (primarily childhood leukemia), others do not, and experimental studies have not yielded reproducible evidence of carcinogenic mechanisms (ORAU, 1992; American Cancer Society, 1995). The above statements apply to both transient fields, such as those being considered for future passenger screening technologies, and quasi-static fields, such as those found near power lines.
Although laboratory studies have confirmed that low levels of electromagnetic radiation may cause biochemical and physiological changes in cells, they do not appear to damage DNA directly, and, therefore, would be unlikely to initiate cancer. Cancer was first associated epidemiologically with exposure to low-frequency electromagnetic fields in 1979 when a higher-than-expected occurrence of leukemia was reported in children residing in homes adjacent to high-current power lines (Wertheimer and Leeper, 1979). Since this initial report, a number of other studies have shown statistical associations between cancer and exposure to various sources of electromagnetic fields. However, these epidemiological studies have been difficult to interpret because of various inadequacies in experimental design, including the lack of individual exposure measurements and incomplete consideration of confounding factors. Accordingly, a causal relation between electromagnetic-field exposure and cancer has not been established (NRPB, 1992; ORAU, 1992).
Recent studies of childhood brain cancer and of electrical utility workers provide little or no support for the hypothesis that low-frequency electromagnetic field exposures (60 Hz) represent a health hazard (Kheifets et al., 1995; Preston-Martin et al., 1996; Gurney et al. 1996).
REPRODUCTIVE HEALTH EFFECTS
The U.S. Bureau of the Census estimates that there are 6.3 million pregnancies3each year in the United States (U.S. Bureau of the Census, 1994). Developing human embryos or fetuses are subject to risks from a spectrum of effects, including mental retardation and malformations, from relatively high doses of ionizing radiation that pose no risks to adults. This enhanced sensitivity to radiation is due to the high rate of cellular proliferation and differentiation during fetal life. Damage to a single embryonic cell may be multiplied enormously during growth and development. Enhanced sensitivity is not restricted to radiation; exposures to various chemical and biological agents may also be associated with increased reproductive risks. For instance, a higher incidence of adverse pregnancy outcomes, such as intrauterine growth retardation, occurs in women with a history of smoking and chronic alcohol consumption during pregnancy (Mossman and Hill, 1982).
Screening devices based on exposure to electromagnetic radiation or magnetic fields are common in everyday situations, such as in libraries and stores. Passenger screening devices based on these technologies involve exposure at insignificant levels compared to radiation levels from other commonly accepted radiation sources (such as those shown in figure 6-1). Thus, these technologies are not expected to cause adverse health effects to developing embryos or fetuses. At much higher doses, ionizing radiation is known to cause developmental effects in humans. Furthermore, substantial evidence exists that a threshold dose of 10 to 20 rem (0.1 to 0.2 Sv) must be exceeded in order to produce prenatal effects, such as developmental anomalies and mental retardation. Significant adverse pregnancy outcomes, such as birth defects and spontaneous abortions, have been observed in cases where the fetus has been given doses in excess of several hundred rem (several Sv) (Brent, 1980). Radiation doses to passengers undergoing x-ray screening are many orders of magnitude below these thresholds and much lower than doses from exposure to natural background radiation or even doses from the cosmic ray exposure during a transcontinental flight.
Epidemiological studies have not been conducted to evaluate directly the reproductive risks of exposure to screening
3 This was the total number of pregnancies in 1988 (the most recent data available). Pregnancies include live births (3.9 million) and induced abortions and fetal losses (2.4 million).
devices considered in this report. However, a large number of epidemiological studies have been conducted on reproductive and teratogenic effects (e.g., birth defects and spontaneous abortions) in human population groups exposed to electromagnetic and magnetic fields from video display terminals (VDTs), power lines, and household appliances (ORAU, 1992). The results of these studies strongly suggest that the radiation and field exposures at levels associated with passenger screening devices do not have reproductive or teratogenic effects.
No statistically significant associations have been found in epidemiological studies involving VDTs, which typically expose the operator to maximum magnetic fields of about 2 milligauss (mG)4at 30 cm from the screen. This figure is comparable to magnetic fields found near home television sets. A magnetic field strength of 2 mG produces an electric field (at 16,000 Hz) of about 1 millivolt per meter in the abdomen of the operator. For comparison, flying in an airplane through the magnetic field of the earth produces a uniform static electric field of about 10 millivolts per meter through the entire body. In an analysis of 21 epidemiological studies, the overall results indicate that pregnant women exposed to VDTs do not have an increased risk of reproductive defects (ORAU, 1992). The reproductive parameters evaluated in these studies included birth defects, spontaneous abortions, stillbirths, and intrauterine growth retardation (ORAU, 1992).
Fewer epidemiological studies have been conducted on the reproductive risks of exposure to power lines, electric substations, and home appliances. These studies have been difficult to analyze because radiation or field exposures rarely have been determined, and studies frequently involve small sample sizes. Magnetic fields for some common household appliances, such as coffee makers, stereos, refrigerators, and toasters, vary considerably; at 30 cm from the source, the measured magnetic field may vary from 0.1 to 10 mG. House wiring produces a background measured magnetic field of 1 mG or less. In an analysis of five epidemiological studies of various sources of electromagnetic radiation, no statistically significant increase in spontaneous abortions, birth defects, or fetal growth retardation was observed (ORAU, 1992).
HEART DISEASE AND PACEMAKERS
Individuals with cardiac arrhythmia may be fitted with an artificial pacemaker, which emits a series of rhythmic electrical discharges to control the heart rate. It has been estimated that there are over 500,000 patients with pacemakers in the United
States; over 100,000 pacemaker procedures are performed annually (American Heart Association, 1995). The effects of radiation-producing equipment on pacemakers vary from no effect to the theoretical possibility of rendering the pacemaker nonfunctional, leading to possible fatal cardiac arrhythmias.
As shown in figure 6-1, passenger screening devices involve exposures to insignificant levels of electromagnetic radiation or magnetic fields. Accordingly, these devices will not produce radiation at levels high enough to damage pacemaker circuitry, cause heat damage, or affect normal pacemaker operation through electromagnetic interference. Certain medical procedures expose patients to radiation levels substantially higher than those used in screening, and these procedures can cause such effects on pacemakers (Hardage etal., 1985).
SOME POSSIBLE HEALTH CONCERNS ASSOCIATED WITH SPECIFIC SCREENING TECHNOLOGIES
Imaging technologies generally involve the use of ionizing radiation (e.g., x-rays) to produce images of individuals and objects that may be concealed under layers of clothing. The images are produced using computer analysis of either reflected, absorbed, or scattered radiation (active imaging), or of natural radiation emitted from the human body (passive imaging). For active imaging, small doses of radiation are used in the imaging process. The level of exposure to x-rays in passenger screening is orders of magnitude below the x-ray levels used in medical diagnosis and represents a fraction of 1 percent of the natural background to which the U.S. population is exposed annually (see figure 6-1). For example, in screening devices using backscatter x-ray, exposures are approximately 0.003 millirem (0.00003 mSv) per individual (Smith, 1995). This radiation level is so low that a passenger would have to go through the screening portal approximately 1,000 times to receive the same radiation dose as would be received from cosmic ray exposure at high altitude during one transcontinental flight from New York to Los Angeles.
Passengers who wear pacemakers are not at additional risk from radiation exposure in passenger screening. Pacemaker malfunction may occur when radiation doses exceed 1,000 rad (10 Gy; for x-rays, 1 rad = 1 rem),5as in the case of patients undergoing radiotherapy for cancer, where pacemaker circuitry may fail as a result of radiation damage to the semiconductor circuitry. Function is not affected by x-ray
4 Magnetic flux density is measured in gauss or tesla. A milligauss (mG) is one thousandth of a gauss. The earth's magnetic flux density at the surface due to current flow in the earth's core ranges from 300 to 700 mG (30 to 70 microtesla). The earth's field is a direct magnetic field and not an alternating one, such as the field from electric power transmission lines and appliances. Everyone is exposed to the magnetic flux density of the earth (NRPB, 1992).
5 The unit of absorbed dose is the rad. One rad is equal to the deposition of 100 ergs of ionizing radiation energy per gram of absorbing substance. A newer is the gray (Gy). I Gy=100 rad.
irradiation at doses below about 200 rad (2 Gy) (Hardage et al., 1985).
Trace-detection technologies for passenger screening may involve person-to-person contact or direct contact between the detection equipment (e.g., trace-chemical sensors) and individuals. Personal contact may be a vehicle for transmitting various microbial diseases (bacterial, fungal, and viral in origin) from one individual to another.
Diseases may be transmitted through the inhalation or ingestion of disease-causing microorganisms, direct contact with individuals, or through wounds and cuts. Diseases also may be transmitted through person-to-person contact or when a passenger's hands touch trace-detection equipment. If the transfer of infectious diseases in the passenger screening setting were to occur, it would most likely result from the hand-mediated transfer of disease-producing microorganisms. The hands can infect other areas of the body through direct contact with the nose, mouth, or minor skin wounds. The hands themselves can easily become contaminated through contact with the nose, mouth, areas of skin infection, or the anal region. Intestinal pathogens such as shigella dysenteriae can contaminate hands through toilet tissue. Pathogens on the hands can contaminate inanimate objects, such as doorknobs, chairs, and towels, which can then transmit infection.
The likelihood of disease transmission during passenger screening is dependent upon numerous disease-specific factors, including the integrity of the skin and other host factors, the virulence of the disease-causing microorganism, and the number of microorganisms transmitted. The probability of transmitting disease as a consequence of using trace-detection technologies appear insignificant in comparison to other more common disease-transmission scenarios, such as using public washroom facilities.
Cleanliness, both for personnel at the checkpoint and for the passenger screening equipment, is important to prevent the spread of infection when contact is required between people or between a person and a piece of equipment. Hand washing, a fairly simple procedure, can physically remove microorganisms. Washing hands in plain water removes viruses that cause colds (Nester et al., 1995). Equipment should be designed to allow frequent cleaning to minimize disease transmission from passenger to passenger.
Nonimaging Electromagnetic Technologies
Passenger screening devices, such as portal metal detectors, millimeter-wave devices, hand-wand devices, and dielectrometers, utilize nonionizing radiation and lowfrequency electric and magnetic fields for detection. The interaction between tissues and the radiation of photon energies lower than ultraviolet (UV) and electric and magnetic fields are complex. These interactions also are markedly different from damage mechanisms associated with x-rays and other forms of ionizing radiation. Measuring energy deposition and field intensities within the body can be difficult (ORAU, 1992).
Many of the biological effects of exposure to electromagnetic fields are well understood and consistent with established mechanisms of physical interaction with tissues. These include effects on perception processes associated with the accumulation and redistribution of electric charge on the surface of the body, effects on electrically excitable tissues such as nerve and muscle tissues, and effects caused by heating tissues. Acute effects and their dependence on the frequency and magnitude of the fields can be predicted from human and experimental animal studies. These acute health effects studies, together with the application of appropriate safety margins, form the basis for establishing international exposure guidelines. A relatively small number of people are likely to be exposed to radiation at levels high enough to cause acute effects. These high radiation levels are generally encountered only in certain medical therapy settings, where exposures are strictly controlled, or in specific occupational settings (NRPB, 1992).
Passenger screening devices emit very low levels of radiation and electric and magnetic fields. No epidemiological studies have been conducted to evaluate the health effects of portal metal detectors and other passenger screening devices. However, available data from epidemiological studies involving comparable radiation and field exposures from other sources suggest that these devices do not pose health risks. For the technologies based on microwave irradiation, the levels of microwave energy that a person being screened would be subject to have been measured to be less than 0.001 times the energy level set by the FDA for emission from microwave ovens (Burnett et al., 1992; Microwave Cooking Handbook, n.d.).
Passengers with pacemakers are not at additional risk from nonionizing radiation and electric and magnetic field exposures from passenger screening. Pacemakers are designed to eliminate electromagnetic interference. Medical therapy devices that operate at much higher energy levels, such as hyperthermia and diathermy units, can cause pacemakers to malfunction by causing permanent damage to pulse generators or temporary changes or total inhibition of the pacing rate (Hardage et al., 1985).
Radiation for electromagnetic fields from passenger screening devices does not harm the individuals undergoing screening or operating the equipment. Measured radiation
levels and electromagnetic fields from these devices are very low and are well below the levels known to have harmful effects.
The panel has determined that the health issue is primarily a perception of risk rather than an actual health threat. Concerns about health effects may still affect public acceptance of imaging and nonimaging electromagnetic radiation technologies, especially because people distinguish between radiation received voluntarily (such as radiation during a transcontinental flight) and radiation received involuntarily (such as radiation from living in areas built over piles of uranium mill tailings). People may perceive the radiation they receive to facilitate aviation security as an involuntary dose of radiation that they are unwilling to be exposed to. For example, x-ray screening technologies do not pose a health problem, but people may believe that they do. Therefore, people may object to a technology that exposes them to x-rays, even though the radiation dose is extremely small. This perception may be true especially for aircrews and airport employees exposed to frequent screening. False perceptions may be addressed effectively by disseminating information regarding the insignificant exposure levels used in screening technologies and the benefits of the screening procedures in reducing threats. However, the information must be presented in a way that is understandable to all audiences. Comparing radiation doses received in passenger screening to greater, but still safe, doses used in common or familiar circumstances (e.g., a chest x-ray) is a meaningful and effective strategy, as long as the information is framed in the appropriate context. This information, which should be presented at the screening site, could be part of a public education effort.
No health risks are associated with trace-detection technologies either. However, passengers may perceive the equipment as unhealthy if it appears unclean or unsanitary. The development of passenger screening equipment and the implementation of screening procedures should include measures that minimize the risk of disease transmission.