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Epidemiology of Health Effects of Radiofrequency Exposure

    Anders Ahlbom, Adele Green, Leeka Kheifets, David Savitz, Anthony Swerdlow. Environmental Health

    Perspectives. Research Triangle Park: Dec 2004.Vol.112, Iss. 17; pg. 1741, 14 pgs

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Author(s): Anders Ahlbom, Adele Green, Leeka Kheifets, David Savitz, Anthony Swerdlow

    Document types: General Information

    Section: Environmental Medicine: Review

    Publication title: Environmental Health Perspectives. Research Triangle Park: Dec

    2004. Vol. 112, Iss. 17; pg. 1741, 14 pgs

    Source type: Periodical

    ISSN/ISBN: 00916765

    ProQuest document ID: 768209171

    Text Word Count 11864

    Document URL: http://proquest.umi.com/pqdweb?did=768209171&sid=3&Fmt=4&clientId=51305&RQT=309&VName=PQD

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    Copyright National Institute of Environmental Health Sciences Dec 2004

    [Headnote]

    We have undertaken a comprehensive review of epidemiologic studies about the effects of radiofrequency fields (RFs) on human health in order to summarize the current state of knowledge, explain the methodologic issues

    that are involved, and aid in the planning of future studies. There have been a large number of occupational studies over several decades, particularly on cancer, cardiovascular disease, adverse reproductive outcome, and

    cataract, in relation to RF exposure. More recently, there have been studies of residential exposure, mainly from radio and television transmitters, and especially focusing on leukemia. There have also been studies of mobile telephone users, particularly on brain tumors and less often on other cancers and on symptoms. Results of these studies to date give no consistent or convincing evidence of a causal relation between RF exposure and any

adverse health effect. On the other hand, the studies have too many deficiencies to rule out an association. A key

    concern across all studies is the quality of assessment of RF exposure. Despite the ubiquity of new technologies using RFs, little is known about population exposure from RF sources and even less about the relative importance

    of different sources. Other cautions are that mobile phone studies to date have been able to address only relatively short lag periods, that almost no data are available on the consequences of childhood exposure, and that published data largely concentrate on a small number of outcomes, especially brain tumor and leukemia. Key words: electromagnetic fields, EMF, epidemiology, health effects, radiofrequency, RF. Environ Health Perspect 112:1741-1754 (2004). doi:10.1289/ehp.7306 available via http://dx.doi.org/ [Online 23 September 2004]

    The advent of mobile telephones, now used by about 1.6 billion people worldwide, has been accompanied by an upsurge in public and media concern about the possible hazards of this new technology, and

    specifically of radiofrequency field (RF) exposure. Although some epidemiologic research was conducted several decades ago on RFs in occupational settings, in general the effects of RFs in humans are an emerging area of investigation, and most studies are recent or not yet published. Furthermore, although the results of studies of mobile phone risks have received widespread public attention, their interpretation is not straightforward because of methodologic difficulties. In particular, because RFs are invisible and

    imperceptible, individuals cannot directly report on their exposure, and therefore the quality of exposure assessment needs particularly careful consideration when interpreting epidemiologic studies. In order to summarize the current state of knowledge, to explain the methodologic issues that need to be considered when assessing srudies, and to aid in planning future studies, we have undertaken a broad review of epidemiologic knowledge about the effects of RFs on human health. We have divided the literature, for this

    purpose, into studies of RF exposure from occupational sources, from transmitters, and from mobile phones.

    In this review we cover the possible effects of long-term exposure to RFs-defined as 100 kHz to 300 GHz-

    on the risk of diseases, for instance, cancer, heart disease, and adverse outcomes of pregnancy. We have not reviewed the health consequences of communications technology that are indirect or unlikely to be due to radiation. In particular, RFs can interfere with implanted medical devices, such as cardiac pacemakers,

    but the effects on health are a consequence of this interference, rather than a direct effect on the body; phone conversations by drivers of moving vehicles appear to raise the risk of motor vehicle accidents, but

    this is probably related to distraction rather than to RF exposure. Although anxieties and psychosomatic illnesses might be caused by knowledge of the presence of phones or phone masts, again, this would nor be an effect of RFs and is not discussed.

    As well as epidemiologic studies of disease causation, some studies have been published that use an epidemiologic design to investigate whether mobile phones can affect acute symptoms, such as headaches. For completeness, we have included these in this review, although such investigations are

    usually better conducted by laboratory volunteer experiments rather than by observational epidemiology, given the high degree of susceptibility to biased reporting in response to concerns.

    Because this is primarily an epidemiologic review, we have not detailed the physics and dosimetry of RFs from different sources, which are described elsewhere [Hitchcock and Patterson 1995; Independent Expert Group on Mobile Phones (IEGMP) 2000; Mantiply et al. 1997]. However, because understanding of mobile-

    phone-related epidemiology is critically dependent on understanding of mobile phone technology, we have included some information explaining this technology. We have also included, because of its importance to future research advance, some comments on the interface between physics and epidemiology, and the gaps to be bridged between these disciplines if more rigorous investigation of potential RF effects is to be achieved.

    Exposure

    Sources of Exposure

    Communications sources have increased greatly in recent years, and there is continuing change in the frequencies used and variety of applications. The first mobile phone systems were analog and used 450 and 900 MHz. Digital systems, operating at somewhat higher frequencies (1,800-1,900 MHz) and using

    different modulation techniques, became prevalent in the early 1990s. Currently, the third-generation

    systems using the Universal Mobile Telecommunication System are being introduced, which will operate in the 1,900-2,200 MHz frequency range. Occupational RF exposures occur to workers engaged in a number

    of industrial processes, particularly when using dielectric heaters for wood lamination and the sealing of plastics and industrial induction heaters. Relatively high levels of exposure to RFs can occur to workers in

    the broadcasting, transport, and communications industries and in the military, when they work in close proximity to RF transmitting antennas and radar systems. Medical exposures can come from medical diathermy equipment to treat pain and inflammation, clectrosurgical devices for cutting tissues, and diagnostic equipment such as magnetic resonance imaging.

    Distribution of Exposure in the Population

    Despite the rapid growth of new technologies using RFs, little is known about population exposure from

    these and other RF sources and even less about the relative importance of different sources. In a typical house, nonoccupational exposure could come from external sources, such as radio, television (TV), and mobile-phone base stations, as well as internal sources, such as a faulty microwave oven, in-house bases

    for cordless phones, or use of mobile phones.

    Radio and TV transmitters have a large coverage area and therefore operate at relatively high power levels up to about 1 MW (Dahme 1999). Although these transmitters could generate fairly high fields at ground level, most are not located in heavily populated areas and do not lead to high exposure of the population.

    Mobile-phone base stations are low-powered radio transmitters that communicate with users' handsets. In

    early 2000, there were about 20,000 base stations in the United Kingdom and about 82,000 in the United States. Base stations can transmit power levels of ? 100 W (Schux and Mann 2000). It is expected that the

    number of base stations will roughly double to accommodate new technology and a larger percentage of sites will have to be shared between operators, complicating exposure assessment. The power density levels inside a building can be from 1 to 100 times lower than outside, depending on the type of building

    construction (Schüz and Mann 2000). In addition, exposure can vary substantially within the building. For example, exposure was found to be about twice as high (and more variable) in the upper compared with the lower floors of a building (Anglesio et al. 2001). Driven by a typical pattern of use, the exposure from base stations shows a distinct diurnal pattern, characterized by lowest values during the night and by two maxima during the day, the first from 1000 hr to 1300 hr and the second from 1800 hr to 2200 hr (Silvi et al. 2001).

    There have been few and limited efforts to characterize population exposures; all of them have been small (usually areas around 10-20 base stations) (Anglesio et al. 2001; COST281 2001; Schüz and Mann 2000).

    The total power density from the base stations was slightly higher than, but comparable with, the background power density from all other RF sources combined.

    Mobile phones operate at a typical power of 0.25 W. Analog systems operated at higher power levels than

    the newer digital systems. Similarly, older cordless phones operated to the analog standard, whereas modern ones operate to the digital with a transmitted power of a base around 0.09 W in a home but higher in a business setting. The actual exposure of the user depends on a number of factors such as

    characteristics of the phone, particularly the type and location of the antenna; the way the phone is handled; and most important, the adaptive power control, which may reduce the emitted power by orders of

    magnitude (up to a (actor of 1,000). Factors that influence adaptive power control include distance from the base station, the frequency of handovers, and RF traffic conditions. Thus, the emitted power is higher in rural than in urban areas and when the user is moving (e.g., in a car). In areas where there is a great deal of phone use, phones may operate more than half of the time at the highest power levels. To compensate for the shielding effect of materials, power levels of phones are, on average, higher when a phone is used

    indoors than outdtxirs. RF absorption is maximal on the side of the head to which the phone is held, greatest close to the antenna, and decreases to less man one-tenth on the opposite side of the head

    (Dimbylow and Mann 1999).

    In an occupational setting, higher exposures occur, albeit infrequently; for example, radar exposed workers in the U.S. Navy had potential for exposures > 100 mW/cm^sup 2^ (Groves et al. 2002).

    Epidemiologic Considerations in Exposure Assessment

    General. In the absence of information on what biologic mechanism is relevant, it is unclear what aspect of exposure needs to be captured in epidemiologic studies. Because halting is the only known effect of RFs, most research has assumed that the metric of choice must be a function of the specific absorption rate

    (SAR). Metrics used in epidemiologic studies of other agents, such as cumulative exposure, average exposure over specific time intervals, and peak exposure, need to be considered. Given the uncertainty

    about the relevant interaction mechanism, the dose needs to be assessed not just as external field intensity but also as SAR for specific anatomical sites. Integrating exposure over time is further complicated by the fact that sources vary markedly over very brief time periods relative to the time periods of interest.

    Epidemiologie studies thus far have relied on rather crude proxies for exposure, such as job title, proximity to a base station, or use of a mobile phone. Refinement of exposure assessment is critical to improved

    epidemiology. This requires a bridge between the rather disparate worlds of epidemiology and physics. Although it is of interest to know about sources of variation or uncertainty in general, the critical need in epidemiologic studies is to identify those variables that are most important in determining exposure levels and most amenable to capture within populations.

    A key element in linking the complexity of the exposure sources and patterns with the needs of epidemiology is a meter that is capable of monitoring individual exposure. Such meters have now been developed [National Radiation Protection Board (NRPB) 2003].

    Ideally, the dose, time pattern, and frequencies (wavelengths) of exposure from all key sources should be estimated for each individual in the study. Dose- and durationresponse analyses are important to

    assessment of etiology but have often been absent in the existing literature (Swerdlow 1999). In addition, the possible lag period between exposure and disease manifestation needs to be considered. Handheld

    mobile phones were not used regularly until the 1990s. Thus, studies published to date have had little power to detect possible effects involving long induction periods or effects from long-term heavy exposure

    to mobile phones or base stations.

    Methodologically, it would be desirable to conduct studies to clarify the relative contributions of different spheres of life. Such knowledge would allow epidemiologists to design studies that incorporate all important sources of RF exposure, or at least determine how much it matters that the occupational studies to date have taken no account of residential or mobile phone exposures and vice versa.

    Occupational exposures. Most occupational epidemiologic studies have based their exposure assessments

    simply on job titles and have included no measurements (Tables 1-4). It is possible that some jobs (e.g.,

    radar operator) are adequate indicators of RP exposure. However, many job titles that have been previously considered to indicate exposure may provide a poor proxy for RF exposure.

    In addition to improving exposure assessment in individual studies, there is the potential to develop job-

    exposure matrices, with the rows corresponding to relatively homogeneous groups with respect to RP

    exposure, defined by job title, perhaps specific work location, calendar time, and other recordable work history, and the columns corresponding to RP exposure metrics.

    Transmitter exposures. All published epidemiologic studies of transmitter exposures have based exposure

    assessment on distance from the transmitter. The relation between exposure and distance from the antenna is usually very complex, especially in urban areas. Close to the antenna, the field is very low because of the directional antenna characteristics. As one moves away, the Geld pattern can be

    complicated, with peaks and valleys in field intensity with increasing distance from the antenna.

    Estimation of community exposure to RFs from transmitters may, however, be amenable to refinement. Geographic information systems allow for precise assignment of residence, topography, and some other likely determinants of exposure. Historical information on power output from the transmitters may well be available. This information combined with personal measurements may provide refined measures of

    exposure that can be applied retrospectively, with empirical validation.

    Mobile phones exposures. Studies on mobile phones have used the simple dichotomy of user versus nonuser, with some incorporating information on years of use, number of phone calls per day, and duration

    of calls. Some studies have separated analog and digital phone use. Few have included use of cordless phones, which also generate RFs but from which exposure pattern is different.

    Ongoing studies are attempting to incorporate information on intensity of use, place of use, position of the telephone, type of telephone, and calendar period of use. Each of these extensions need to be evaluated, however, to determine a) whether they are truly an important determinant of exposure and b) whether they

    are amenable to accurate historical reconstruction through recall or some type of written record. There is little benefit in knowing that the intensity of exposure varies by a parameter that cannot be captured, or

    gathering relatively precise information about, say, model of mobile phone, if no useful exposure variable can be derived from it.

    Mechanisms

    Heating of cells and tissues from RF exposure might have benign or adverse biologic effects. These effects,

    which reflect an imbalance in the amount of heat built up in the body and the effectiveness of mechanisms to remove it, can be due to either elevated temperatures o'r increased physiologic strain from attempts to remove the heat. Of particular concern for whole-body heating are effects in the elderly, people taking

    certain kinds of drugs, and the fetus and infant. Cardiovascular mortality, birth defects, and impaired ability to perform complex tasks are among the outcomes that have been associated with whole-body heating.

    The sensitivity of different tissues and cells to thermal damage from both localized and whole-body heating

    varies. The central nervous system, testis, and lens of the eye seem to be particularly sensitive, the last due to a limited capacity to dissipate heat rather than a greater sensitivity of its cells to heat-induced damage.

    Laboratory studies suggest that adverse biologic effects can be caused by temperature rises in tissue that exceed 1?C above their normal temperatures (Goldstein et al. 2003). In addition to the absolute increase in

    temperature, duration of heating and thermoregulatory capacity of the body are important determinants of the harmful levels of tissue heating. High rates of physical activity and warm and humid environments will

    reduce tolerance to the additional heat loads.

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     Table 1. Cohort studies of risk of cancer in relation to occupational or hobby RF exposure: description of studies.

    There has been concern about possible carcinogenic effects of RFs below levels that cause detectably harmful heating. RFs are not sufficiently energetic to destabilize electron configurations within DNA molecules. Thus, there is no direct link between RF exposure and genotoxic effects such as DNA mutations, DNA strand breaks, or other genetic lesions. Experimental evidence from animal and laboratory

    studies at the cellular level confirms the lack of genotoxic effect of RFs (Krewski et al. 2001; Moulder et al. 1999). Similarly, an investigation in rodents did not find support for the suggestion that growth of tumors

    induced by other agents may be promoted by RPs from mobile phone signals (Imaida et al. 2001; Mason et al. 2001).

    Repacholi et al.. (1997) evaluated the effects of RFs on tumorigenesis in a moderately lymphoma-prone Eµ-

    Pim1 oncogenetransgenic mouse line. Exposure was associated with a statistically significant 2.4-fold

    increase in the risk of developing lymphoma. Utteridge et al. (2002) recently repeated this study with a larger number of mice and with several refinements in the experimental design and did not demonstrate any

    difference in the incidence or type of lymphomas that developed between control and treated groups. Questions have been raised about the conduct and reporting of both studies and the inconsistency has not been resolved (Goldstein et al. 2003). Additionally, extrapolating the transgenic model to humans remains controversial.

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     Table 2. Cohort studies of risk of cancer in relation to occupational RF exposure: results for brain tumor and leukemia.

Outcomes

    A particular public concern appears to be that the use of handheld mobile phones may be linked to the

    occurrence of malignant disease, especially brain cancer and, to a lesser extent, leukemia. Other tumors such as acoustic neuroma that occur in the head and neck region have also been investigated. Each of these conditions is rare. The incidence of malignant tumors of the brain in the general population is around 10-1 5 per 100,000 each year (Benin et al. 2003); the annual incidence of benign extracerebral tumors such as meningiomas is about 3 per 100,000, and benign tumors of the cranial nerves, such as acoustic

    neuromas, are rarer still. Because tumor incidence is so low, investigators have so far relied on case-control

    studies or, in a few instances, retrospective cohort studies. In addition, different tumor subtypes are likely to

    have different causes, as evidenced among brain tumors by the different molecular pathways leading to malignant astrocytomas on the one hand and benign meningiomas and acoustic neuromas on the other (Inskip et al. 1995). Similarly, there are a variety of types of leukemia, each probably with differences in causation, making it even more difficult to ascertain sufficient numbers of homogeneous tumors for study. Epidemiologie assessments have been further complicated because the environmental risk factors for

    malignant and benign brain tumors (Inskip et al. 1995), and hence potential confounders, are largely unknown beyond high-dose ionizing radiation. For leukemia (Petridou and Trichopoulos 2002), knowledge of potential confounders is greater but still limited. Other risk factors, besides ionizing radiation, include exposure to chemotherapy, cigarette smoking, and benzene, as well as constitutional chromosomal abnormalities among children in particular.

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     Table 3. Case-control studies of risk of brain tumor and leukemia in relation to occupational RF exposure.

    Table 4. Analyses of routinely collected data on brain tumor and leukemia risk in relation to occupational RF exposure.

Available evidence suggests that induction of brain tumors occurs over decades after tumorigenic

    exposures early in life. Latency of tumors varies from months to years depending on how aggressive tumor growth is and the location of the tumor. Epidemiologic studies should therefore in principle allow for a lead time between potentially causal exposure and disease, although in the absence of biologic or epidemiologic evidence it is unclear what length this should be for potential RF effects.

    Other chronic diseases such as cardiovascular disease, as well as symptoms, both acute and chronic, have been studied in relation to RF exposure. Headaches and other cranial discomforts including sensations of local warmth or heating, dizziness, visual disturbances, fatigue, and sleeplessness are the main symptoms reported by users of mobile phones. All of these are common symptoms in humans.

    Review of Studies on Occupational Exposure

    Cancer

    Information on cancer risks in relation to occupational RF exposure comes largely from three types of epidemiologic study: cohort studies, investigating a wide range of cancer (and noncancer) outcomes in

    groups with potential RF exposure (Tables 1 and 2); case-control studies of specific cancer sites,

    investigating occupational RFs as well as other exposures (Table 3); and analyses of routinely collected data sets on cancer incidence or mortality, in which risks of cancer have been assessed in relation to job title (Table 4). The most extensive literature addresses brain tumors and leukemia.

    Considering study size, design, and likely quality of RF assessment, the most informative studies (Groves et

    al. 2002; Milham 1988; Morgan et al. 2000) provide little evidence of an association with either brain tumors or leukemia. The one possible exception was an increased risk of nonlymphocytic leukemia in radar-

    exposed navy veterans (Groves et al. 2002) restricted to only one of three highly exposed occupations (aviation electronics technicians), but this finding was divergent from that of an earlier study of U.S. naval personnel (Garland et al. 1990). Two U.S. case-control studies of brain tumor etiology have shown elevated

    odds ratios (ORs) of around 1.5 in relation to jobs believed to have RF exposure. However, the study by Thomas et al. (1987) was based on interviews with relatives of dead cases and hence was unable to identify exposure with much certainty. The other study (Grayson 1996) assessed exposures by a job-

    exposure matrix based on historical reports of incidents of exposure above permissible limits (10 mW/cm^sup 2^). No clear or consistent trend was found in risk of brain tumor in relation to exposure score.

    A widely cited study of U.S. embassy staff in Moscow and their dependents with possible RF exposure was only published as a précis by a third party (Goldsmith 1995); this leaves the study methods unclear, but few brain tumors or leukemia occurred, and half were in dependents who lived outside the embassy.

    A key concern across all these studies is the quality of assessment of RF exposure, including the question of whether it was truly present at all and, if so, for what proportion of the cohort. Although the published

    studies do not give consistent evidence for an increased leukemia or brain cancer risk, they cannot be counted as substantial evidence against a possible association. Most of the studies suffer from severe imprecision, with the cancers of greatest interest rarely found in cohort studies of modest size and the exposure of interest rarely found in geographically based case-control studies. The cohort studies generally

    lack data on other relevant exposures, including non-radio frequencies of radiation, as well as on RF

    exposures outside the workplace (e.g., mobile phones). The studies based on routine data are vulnerable to publication bias given the many data sets worldwide that could be used to address this issue. Several of

    these studies did not follow workers after they left the job of interest (Garland et al. 1990; Grayson 1996; Szmigielski 1996), with the potential for bias if individuals left employment because of health problems that later turned out to be due to cancer; this might especially be a problem for some types of brain tumor, which can be present for long periods before diagnosis. In addition, several studies have had substantial methodologic inadequacies-for instance, one study that found apparently increased risks for many different

    cancers used more sources of exposure information for cancer cases than for noncancer subjects and was analyzed improperly (Szmigielski et al. 2001).

    Breast cancer. Several studies have investigated the risk of breast cancer in relation to RF exposure. A

    cohort study of radio and telegraph operators in Norwegian merchant ships by Tynes et al. (1996) found a relative risk (RR) of breast cancer of 1.5 [95% confidence interval (CI), 1.1-2.0), based on 50 cases in

    women working in this occupation, and stronger for women ? 50 years of age [2.6 (95% CI, 1.3-5.5)]. An

    elevated RR found also for endometrial cancer suggests that reproductive and hormonal factors (for which full adjustment could not be made), not RFs, may have been responsible for the increased breast cancel-

    risk. A large case-control study based on job titles from death certificates in the United States found no trend in risk of breast cancer in relation to probability or to level of occupational RF exposure (Cantor et al.

    1995). A case-control study in the United States of men with breast cancer found an OR of 2.9 (95% CI, 0.8-10) in radio and communication workers (Demers et al. 1991), based on seven cases in exposed men, and with a low response rate in controls. A study of U.S. embassy personnel with potential RF exposure

    found two breast cancers, with 0.5 expected (Goldsmith 1995). Other studies of male (Groves et al. 2002) and female (Lagorio et al. 1997; Morgan et al. 2000) breast cancers, with few cases, did not report

    increased risks. The available data are insufficient to reach any conclusion on whether RF exposure is related to breast cancer risk, but the results of Tynes et al. (1996) do support continued evaluation of the possibility.

    Testicular cancer. Testicular cancer was considered in a U.S. case-control study (Hayes et al. 1990). A

    significantly increased risk was found for self-reported occupational exposure to microwave and other radio

    waves (OR = 3.1) but not for self-reported radar exposure or for radar or other microwave exposure

    assessed by an occupational hygienist based on job history. A cluster of testicular cancer (observed/expected ratio = 6.9) was reported m six police officers m Washington State (USA), who routinely used handheld traffic radar guns (Davis and Mostofi 1993). In a large U.S. Navy cohort with radar exposure, testicular cancer mortality was lower than expected [standardized mortality ratio (SMR) = 0.6 (95% CI, 0.2-

    1.4) in the group with potential for high exposure (Groves et al. 2002).

    Ocular melanoma. Ocular melanoma was associated with self-reported exposure to microwaves (excluding

    domestic microwave ovens) or radar [OR = 2.1 (95% CI, 1.1-4.0)] in a case-control study (Holly et al. 1996).

    Stang et al. (2001) found an increased risk of ocular melanoma in subjects with self-reported occupational

    exposure for at least 6 months and several hours per day to RFs (14% of cases, 10% of controls) and for occupational exposure several hours per day to radio sets [OR = 3.3 (95% CI, 1.2-9.2)]. There was no

    relation of risk to duration of this exposure, however, and risk was not increased for radar exposure [OR = 0.4 (95% CI, 0.0-2.6)]. The study was small and combined subjects from two different study designs.

    Lung cancer. A nested case-control study of electrical utility workers in Quebec (Canada) and France

    thought to be exposed to pulsed electromagnetic fields found a significant excess of lung cancer (Armstrong et al. 1994) and a dose-response gradient with increasing cumulative exposure. Adjustment for crude

    indicators of smoking and other factors left the results little changed. In an attempt to addtess a similar exposure in a cohort of U.S. electric utility workers, limited because of the ill-defined agent addressed in the

    original study, no increased risk of lung cancer was found (Savitz et al. 1997). No other studies of RFs have reported associations with lung cancer (Groves et al. 2002; Lagorio et al. 1997; Milham 1985, 1988; Morgan et al. 2000; Muhm 1992; Szmigielski 1996; Szmigielski et al. 2001 ; Tynes et al. 1996).

    In conclusion, there is no cancer site for which there is consistent evidence, or even an individual study providing strong evidence, that occupational exposure to RFs affects risk. The quality of information on exposure has generally been poor, however, and it is not clear that the heterogeneous exposures studied should be combined in etiologic studies. This, combined with imprecision and methodologic limitations, leave unresolved the possibility of an association between occupational RFs and cancer.

    Other Outcomes

    Adverse reproductive outcomes. A wide range of potential reproductive consequences of RF exposure have been investigated (Table 5), with a focus on exposures of physiotherapists to therapeutic short wave diathermy (typically 27.12 MHz). Depending on the type of equipment used and the location of the operator in relation to the equipment, substantial peak exposures can occur (Larsen and Skotte 1991). Many of the studies analyzed levels of exposure, on the basis of duration of work and type of equipment used

    (shortwaves or microwaves).

    There are isolated suggestions of an association between RF exposure and delayed conception (Larsen et al. 1991), spontaneous abortion (Ouellet-Hellstrom and Stewart 1993; Taskinen et al. 1990), stillbirth

    (Larsen et al. 1991), preterrh birth after exposure of fathers (Larsen et al. 1991), birth defects in aggregate (Larsen 1991), and increased male-to-female sex ratio (Larsen et al. 1991). Almost always, however, either the finding was not corroborated in other studies of comparable quality, or there are no other studies available. The evidence is strongest for spontaneous abortion (based on two independent studies with some support). Potential confounding by other aspects of work activity (e.g., physical exertion) needs to be

    considered, however.

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     Table 5. Summary of literature on RF exposure and reproductive health outcomes.

    Semen parameters have been examined among men with varying forms of military exposure to microwaves and radar (Table 5). Three of these studies found reductions in sperm density (Hjollund et al. 1997;

    Lancranjan et al. 1975; Weyandt et al. 1996), with variable results for other semen parameters. Several of these reports were based purely on volunteers, with no attempt to sample from a defined population

    (Lancranjan et al. 1975; Schrader et al. 1998; Weyandt et al. 1996), and those that did provide information about response proportions (Grajewski et al. 2000; Hjollund et al. 1997) had substantial nonresponse. However, given the well-known susceptibility of spermatogenesis to even subtle heating, the possibility of reduced fertility in exposed men is reasonable to evaluate.

    Overall, problems of exposure assessment temper any conclusions regarding reproductive outcomes, and no adverse effects of RFs have been substantiated.

    Cardiovascular disease. Several methodologically weak studies from the Soviet Union addressed

    microwave exposure and acute effects on cardiovascular physiology (e.g., hypotension, bradycardia, tachycardia) as part of a set of illdefined conditions (Jauchem 1997). Additional studies of considered symptoms among a range of potentially exposed groups including radar workers, pilots, radio broadcasting workers, and electronics industry workers. The variability in research methods, exposure characteristics,

    and outcome measures makes it difficult to draw conclusions: there are sporadic reports of symptoms among some groups of workers, but no obvious pattern is present.

    Major clinical outcomes have been examined less frequently. In a mail survey of U.S. physical therapists

    (Hamburger et al. 1983) men more highly exposed to microwave and shortwave radiation, based on indices including length of employment and frequency of treatments, tended to report a significantly greater prevalence of heart disease, with ORs of 2-3. Selective response to this survey must be considered among

    possible explanations for the associations that were observed. In U.S. Navy veterans potentially exposed to radar (Groves et al. 2002) and in a cohort of nearly 200,000 Motorola workers (Morgan et al. 2000), heart

    disease SMRs were well below 1.0, and analyses of mortality (Groves et al. 2002), hospital admissions, and disability compensation (Robinette et al. 1980) did not support greater risk with greater potential exposure.

    Other cohort studies reporting cardiovascular mortality have had small numbers (Lagorio et al. 1997; Muhm 1992).

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     Table 6. Summary of studies on transmitters and cancer.

Overall, the literature on RFs and cardiovascular symptoms and disease provides little suggestion of an

    association but is at too rudimentary a level to draw firm conclusions.

    Cataracts. Laboratory research indicates that the lens of the eye is highly sensitive to heat, and damage can occur from even a single acute exposure. Hence, there is a potential mechanism for RFs to lead to

    increased cataract incidence. Epidemiologic research has been limited, however, especially with regard to exposure assessment.

    Based on hospital records of U.S. military veterans (deary et al. 1965), men with cataracts were no more

    likely than men with other medical conditions to have been radar workers (OR = 0.67, p > 0.10). Age was adjusted using broad groupings, with little change to the result.

    In two studies in the U.S. military, ocular examinations were conducted on microwave-exposed and

    unexposed workers, without knowledge of exposure status by the examiner. In one (Cleary and Pasternack 1966) a tendency toward increased minor lens changes was found among exposed workers, characterized as the equivalent of 5 years of advanced aging in the exposed compared with unexposed workers around

    60 years of age. In the other (Shacklett et al. 1975), prevalence of lens opacities was similar in exposed and unexposed individuals matched on age.

    In an Australian study of workers who built and maintained radio and TV transmitters, compared with

    unexposed workers from the same geographic regions (Hollows and Douglas 1984), posterior subcapsular opacities were in excess in exposed workers (borderline significant), but nuclear sclerosis prevalence was

    similar in exposed and unexposed workers. It was not specified whether evaluators were aware of exposure history. Exposures were estimated to be from 0.08 to 3,956 mW/cm^sup 2^, with brief, intense exposures thought to be quite common.

    The study designs above are limited with respect to exposure assessment and selection of unexposed workers. Solar radiation exposure, a known risk factor for cataracts, was not considered and could have differed between RF-exposed and unexposed workers. Not all of the opacities were of direct clinical

    importance, but they would be pertinent to a pathway that could lead to cataract later in life. The plausibility of a causal relation supports more extensive investigation.

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