Microwaves Exposure And Potential Health Consequences

Microwaves: Exposure and Potential Health Consequences C Marino and P Galloni, ENEA Casaccia Research Center, BIOTECMED, Rome, Italy & 2011 Elsevier B.V. All rights reserved.

Abbreviations ACTH

B(a)P BBB

CNS CRF

El-Pim1 mice EAS

EM EMFs ERK

GSM

HSP

adrenocorticotropic hormone, which stimulates the cortex of the adrenal gland and boosts the synthesis of corticosteroids, mainly glucocorticoids but also sex steroids (androgens); it is also related to the circadian rhythm in many organisms benzo(a)pyrene blood–brain barrier, which comprises the specialized system of blood vessel endothelial cells that protects the brain from harmful substances in the bloodstream central nervous system corticotropin-releasing factor, which is a polypeptide hormone and neurotransmitter involved in the stress response transgenic mice strain prone to lymphoma development electronic article surveillance, which is a technological method for preventing shoplifting from retail stores or pilferage of books from libraries; special tags are fixed to merchandise or books, which are removed or deactivated by the clerks when the item is properly bought or checked out electromagnetic electromagnetic fields extracellular signal-regulated kinase, which is a type of mitogen-activated protein kinase (MAPK); it belongs to the family of serine/threonine-specific protein kinases that respond to extracellular stimuli (mitogens) and regulate various cellular activities, such as gene expression, mitosis, differentiation, and cell survival/ apoptosis global system for mobile communications is the most popular standard for mobile phones in the world heat shock proteins; they are produced by cells under thermal (or other) stress conditions

ICNIRP

IEC IEEE MCA MN

MW ODC

RF SAR

TPA TSH

WLAN

International Commission on Non-Ionizing Radiation Protection; the scope of activities includes giving guidance for the protection of workers, members of the public, patients, and the environment; ICNIRP develops international guidelines on limits of exposure to NIR International Electrotechnical Commission Institute of Electrical and Electronics Engineers 3-methylcholanthrene micronuclei; a micronucleus test is a test used in toxicological screening for potential genotoxic agents, to quantify chromosomal damage microwave ornithine decarboxylase; enzyme involved in polyamine synthesis; its activity can be modified during alterations in cellular proliferation processes (tumor) radiofrequency(s) specific absorption rate, which is a parameter indicating the electromagnetic energy absorption in the unit of mass of the exposed biological sample, expressed in W kg1 12-O-tetradecanoylphorbol-13-acetate thyroid-stimulating hormone (also known as thyrotropin) is a peptide hormone synthesized and secreted by thyrotropic cells in the anterior pituitary gland, which regulates the endocrine function of the thyroid gland A wireless LAN is a wireless local area network, which is the linking of two or more computers or devices without using wires; WLAN uses technology based on radio waves to enable communication between devices in a limited area, which gives users the mobility to move around within a broad coverage area and still be connected to the network

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Microwaves: Exposure and Potential Health Consequences

Introduction

military and civil radar systems, all operating at different frequencies.

Basic Concepts Microwaves (MWs) are defined as EM waves (Figure 1) with wavelengths ranging from 1 mm to 1 m, or frequencies between 300 MHz and 300 GHz; often they are considered as a portion of radiofrequency (RF) fields, generally defined as covering the range of frequencies from 100 kHz to 300 GHz. Although the name may suggest a micrometer wavelength, it is better understood as indicating wavelengths very much smaller than those used in radio broadcasting. The boundaries between far-infrared light, terahertz radiation, MWs, and ultrahigh-frequency radio waves are fairly arbitrary, and are used variously between different fields of study. The term MW generally refers to ‘electromagnetic waves with frequencies between 300 MHz (3  108 Hz) and 300 GHz (3  1011 Hz).’ Both International Electrotechnical Commission (IEC) standard 60050 and Institute of Electrical and Electronics Engineers (IEEE) standard 100 define ‘microwave’ frequencies starting at 1 GHz (30 cm wavelength). Exposure sources

People may be exposed to different sources of low-level MW. The most common exposure today is related to mobile phones and wireless data transmission. Currently, this technology uses the frequencies from 450 to 2500 MHz. This is a technology in constant development, and therefore the frequency range may change in the future. The other main exposure source is occupational, for example, RF sealers, medical applications, and

Exposure Limits The thermal effects of MW fields are well-known. Such effects may be observed at frequencies over 10 MHz with high field intensities. Consistent with recommendations of national and international experts, the guidelines published by the International Commission on NonIonizing Radiation Protection (ICNIRP) in 1998 for limiting exposure to MW fields are designed to prevent whole-body or localized tissue heating. The ICNIRP guidelines for the general public were incorporated in a European Council Recommendation in 1999. The recent Directive 2004/40/EC of the European Parliament and of the Council reviews the common regulations on the health and safety requirements regarding occupational exposure to MW fields. Basic restrictions, termed ‘exposure limit values,’ are similar to those recommended in ICNIRP guidelines for the exposure of workers. Under the ICNIRP guidelines, whole-body exposures at frequencies below 10 GHz are limited to 0.4 W kg1 for workers and 0.08 W kg1 for members of the public. These restrictions are intended to prevent an adverse effect that might result from increased body or tissue temperatures. Guidelines are also provided to limit partial body exposures: for workers the maximum exposure is set at 10 W kg1 in the head and trunk and at 20 W kg1 in the limbs. The corresponding values for the general public are five times lower than these values (2 and 4 W kg1).

← Increasing frequency (ν) 1024

1022

1020

γ rays

1018

X rays

1016

1014

UV

1012

IR

1010

108

106

Microwave FM

AM

104

102

100

ν (Hz)

Long radio waves

Radio waves 10−16

10−14

10−12

10−10

10−8

10−6

10−4

10−2

100

102

104

106

Increasing wavelength (λ) → Visible spectrum

400

500

600

Increasing wavelength (λ) in nm →

Figure 1

Electromagnetic spectrum with light frequencies evidenced.

700

108

λ (m)

Microwaves: Exposure and Potential Health Consequences

Biological Effects Interaction Mechanism MW energy may change physiological function, initiate dysfunction, or cause the onset of disease in humans or animals. Mechanism must exist by which the physical forces exerted by electric and magnetic fields on charged particles alter molecules, chemical reactions, cell membranes, or biological structures. MW is a physical, not a chemical agent. The biological plausibility of initiating a process that leads to adverse health effects must be assessed with this in mind. The initial physical step in the process of altering biological function is illustrated in the following causal chain by which MW interaction effects could occur: MW ) Matter ðphysicsÞ ) Molecules ðchemistryÞ ) Organisms ðbiologyÞ ) Disease

This process is illustrated in more detail in Figure 2. Biological processes in living organisms include many interactions among electric charges (on ions, molecules, proteins, and membranes). Hence, it is clearly possible that exposure to MW might have the potential to modulate biological function. For MW to cause or exacerbate disease in humans, the electric fields would have to trigger an initial transduction step, and then begin a cascade of sequential steps that lead to a disease outcome. As Figure 2 illustrates, the causal chain begins with human exposure to MW. To complete the first step, MWs interact with biological molecules (or structures) in such a way as to alter their size, shape, charge, chemical state, or energy. In this initial ‘transduction’ step, some absorption of MW energy must occur or there would be no effect. For observable biological (and possibly health adverse) effects to follow transduction, sequential events

RF

at the molecular, cellular, and tissue levels are required. The outcome ‘Progress toward disease’ (Figure 2) is one of the many potential outcomes. This process requires specific triggering of intermediate steps. Multiple points exist in the causal chain where variations in the signal produced by a weak, preceding step might be within normal limits, and therefore, may have no functional consequences. To date, the only evident harmful effect attributable to MW is an increase in the temperature of biological materials (thermal effect). The effect of heat on the physiology and metabolism of cells is due to modification of protein structure (denaturation) and function. Heat shock can cause extensive cellular damage, particularly to membranous structures. Membrane damage may cause dispersal of the Golgi apparatus and endoplasmic reticulum, and swelling of mitochondria with concomitant loss of function. The plasma membrane is also affected by heat shock. The plasma membrane increases in fluidity, loses active transport activity, and becomes injured with lesions large enough to allow the passage of small molecules. Nonmembranous structures (chromatin, nucleoli, and actin filaments) also suffer heat-induced damage. In mammals, the ‘whole body’ response to thermal stress may be behavioral or physiological. For behavioral control, the whole organism acts consciously or subconsciously to alter its environment with regard to radiated heat energy. The physiological automatic system is largely controlled by the autonomic nervous system, requiring a sophisticated network of sensors and a variety of control mechanisms that work in concert with other autonomic functions such as metabolic demand, evaporative mechanisms, and local skin blood flow. Little is known about the initial transduction step for low (nonthermal) levels of MWs. Because the health and viability of the human body depends on the normal structure and function of large molecules (e.g., proteins,

Cell signal Transduction Interaction

RF force, induced current metric

Below noise No transduction

Modify molecules, membranes, or ion currents

Not perceptible by cells No amplification triggered

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Signal cascade or amplification

Biological response

Cell dysfunction

Changes in cell behavior Adverse effect

Signal within normal variation No functional consequences

Sensory effect Neutral effect No adverse effects

Transient, reversible, no effect

Progress toward disease Within reserve capacity, no effect

Repair, adaptation, no effect

Figure 2 The casual chain leading from an exposure to disease has multiple steps, each of which may or may not trigger the next step. For RF interactions with molecules, cell structures, or tissues, the transduction mechanism is a crucial first link in the casual chain. By definition, the electric and magnetic fields in RF waves can exert force on electrically charged particles.

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nucleic acids, carbohydrates, and lipids), potential mechanisms must predict how MWs interfere with or modify the normal synthesis, function, or degradation of these molecules. The magnitude of endogenous forces that are known to act at the cellular level to modify the structure of proteins has been measured, and can serve as a basis for comparison to forces produced by MW fields. Using the electric field strength in tissues corresponding to a specific absorption rate (SAR) of 2 W kg1, which is the maximum level allowed for cell phones (averaged over 10 g of tissue), it is estimated the associated internal electric field is B45 V m1 at 1 GHz. In evaluating possible bio-effects of this electric field, a useful unit of force is the picoN (pN). The pN is one-trillionth (1012) of a newton (N) of force. The weight of 1 cm3 of water (mass ¼ 1 g) is B0.01 N. The mass of a human cell is B109 g. Its weight is B1011 N, or 10 pN. Considering a protein molecule that has 100 unbalanced electric charges, it is calculated that the maximum force of 45 V m1 on this molecule is B0.0007 pN. This is 410 000-fold smaller than the smallest forces known to modify protein function. For example, the electric force on a 100-fold-charged protein molecule embedded in a plasma membrane is B160 pN. This is based on the normal resting cell membrane potential of B70 mV over 7 nm. That is, the typical plasma membrane voltages result in robust electric field forces on membrane proteins, which are known to mediate cell function, such as opening and closing of ion channels. Forces of 10 000 times smaller can be predicted to have no functional consequences. Plausible mechanisms are elusive for biological effects of MW below the established standard. Because of the safety factors built into guideline levels, even somewhat exceeding permissible levels of MW would yield amounts of thermal energy absorbed, which are within the adaptive capacity of the body, and would therefore be unlikely to cause dysfunction. Cellular Studies Over the past 30 years, there have been many in vitro studies on the potential cellular effects of MWs. Results so far have not provided consistent evidence for biological effects under nonthermal MW exposure conditions. There have been, for example, studies of possible genetic effects, but in most cases where positive findings were made, these effects could be attributed to a thermal insult, rather than to the MW exposure per se. The same holds true for other end points. Genotoxicity

Many studies have been devoted to MW-induced genetic effects in humans and other cell types. Most (but

obviously not all) studies have found no evidence of in vitro MW-induced genetic damage at nonthermal exposure regimes and indicate that MW has no marked synergistic or additive effect together with another environmental agent (mutagen/carcinogen). However, many studies were devoted to genetic end points that correspond to evident structural chromosome anomalies, and hence no subtle indirect effects on, for example, the replication of genes under relative restricted exposure conditions could be seen. Chromosome aberrations and micronuclei tests were adopted as classic tests. The comet assay was introduced later. However, there was no detectable effect. Signaling, gap junctions, and receptor clustering

The evidence of effects on calcium signaling from later, better conducted studies, particularly those using functionally significant measures of calcium ion concentration, does not support the concept that low-level amplitude-modulated MWs modify intracellular calcium ion metabolism. In addition, no compelling effects have been observed on calcium-dependent spike activity. Very few results are available with regard to the effects of MW exposure on nitric oxide (NO) processes, especially in the GHz range, on which to draw any conclusion. No definite conclusion can be drawn on the basis of the few data on the effects of MW exposure on gap junction formation or receptor clustering. Overall, with regard to signaling, the evidence from studies using functionally significant measures of calcium ion concentration does not support the earlier reports suggesting that low-level amplitude-modulated MW may affect calcium ion physiology. Gene and protein expression

Many studies have examined the effect of exposure to MW on stress proteins, including heat shock proteins (HSPs). HSPs appear following exposure to 42–43 1C and remain detectable for up to 72 h. HSPs impart thermotolerance to exposed cells for the same time period. Results of several experiments suggest that some of the positive effects might result from heating alone. Recent studies suggest that the MW exposure has no or very little effect on the expression of cancer-related genes (proto-oncogenes and tumor suppressor genes). Changes in cell physiology and function imply changes in gene and protein expression. An early publication on heat shock gene expression in the nematode Caenorhabditis elegans initiated further investigation of various genes known to be stress-responsive. This positive finding was later shown to have resulted from inadvertent heating, due to lack of proper dosimetry. Some studies, however, reported increased protooncogene expression in p53-deficient cells and a transient increase in Egr-1 gene expression. Although negative

Microwaves: Exposure and Potential Health Consequences

reports predominate in these gene-specific approaches in mammalian cells, positive effects cannot be ignored. Among the few other signaling pathways that have been investigated, the extracellular signal-regulated kinase (ERK) pathway was found to be altered. Again, the studies provided gave inconsistent results. High-throughput studies of gene expression in various cell types have reported a variety of results, including a lack of effect, and the up-regulation and down-regulation of various genes. However, research carried out to date is insufficient to reach valid conclusions. Several researchers have suggested that low-intensity (less than B2.0 W kg1) MW exposure, especially at the mobile phone utilization frequencies (800–2000 MHz), can change gene or protein expression in some types of cells. However, the magnitude of these changes is usually small (a few percent) and of doubtful functional significance. More recently, studies have been carried out using high-throughput screening techniques that are capable of examining changes in the expression of a very large number of genes and proteins. Preliminary results show no evidence of an MW ‘signature.’ Finally, there are no effects of MW exposure on ROS production, cell proliferation, cell cycle control, or apoptosis. Cell transformation

The neoplastic cell transformation assay is an integrative assay that is used to test carcinogenic and co-carcinogenic effects of chemical and physical agents. Its main advantage is that it reveals the carcinogenic potential of genotoxic and nongenotoxic compounds. Several authors have used models of mutant cells in co-cultures or animals to determine whether MW exposure could influence the dose-dependent promotion of chemical and physical inductors. No influence of MW exposure at any SAR level (up to 200 W kg1) was seen on 12-Otetradecanoylphorbol-13-acetate (TPA)-induced focus formation and neoplastic transformation of either physical (X-rays) or chemical [benzo(a)pyrene (B(a)P), 3methylcholanthrene (MCA)] inducers. Overall, the data consistently indicate that RF has no effect on neoplastic transformation rate at nonthermogenic levels, either alone or in combination with physical or chemical inducers. Animal Effects In vivo studies in animals have employed many end points in experiments concerning effects of MWs.

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main interest in most studies has been in the development of tumors of the central nervous system (CNS). If MW fields alone are capable of enhancing tumorigenesis in long-term animal studies, an indication of increased tumor incidence would also be expected in studies using sensitized animal models. This has not been the case, however. Most of the published animal studies suggest that exposure to low-level MW fields does not have combined effects with known genotoxic or carcinogenic agents on tumorigenesis. Sporadic studies evaluating combined effects of MW fields with several other genotoxic agents report a lack of effects of low-level fields. The finding of the first study in transgenic (tumorprone) Em-Pim1 mice, which reported over a twofold increase in lymphoma in MW field-exposed animals, was provocative. However, replication of this failed to find increased tumorigenesis. The attempted replication, however, raised a debate regarding whether some points critical to the tumor development were changed in the study design. Lack of effect of low-level fields on the final steps of carcinogenesis was also reported based on studies using transplanted/injected tumors cells. A few studies reported an increased mortality in fieldexposed animals. However, the majority of long-term studies have not found a statistically significant effect. A recent review of this subject suggested that the long-term animal studies, as a whole, appear to show a potential decrease in mortality in field-exposed animals. Since differences reported in individual studies have been small, chance may explain the conflicting results. Neither acute nor long-term field exposure was reported to induce micronuclei (MN) in vivo. New findings from two research groups suggest an increase in micronuclei frequency in bone marrow or peripheral blood of field-exposed rats. These changes were, however, transient in some cases. Although generally used as an indicator of genotoxicity, an increased MN incidence has uncertain implications for public health. Two recent studies (from the same research laboratory) report an increased ODC activity in the brain tissue of rats. Other studies using this end point have shown normal ODC activity. Increased levels of HSP in exposed transgenic worms (C. elegans) have been reported by others. The later studies were interpreted as providing support for the suggested association, although there is some indication that the reported effects might be heat-mediated. A correlation between increased HSP levels and the development of cancer has not been established. Nervous system

Cancer

The studies evaluating the carcinogenic potential of lowlevel MW fields have nearly uniformly reported a lack of effects on tumor development. Owing to the pattern of exposure associated with the use of mobile phones, the

The evidence for health effects of low-level MW exposure on the nervous system is questionable. This conclusion also was based on several studies investigating end points such as the electroencephalogram, evoked potentials, direct registration of brain cortical and

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hippocampal activity, antioxidant; antioxidant defence; and lipid and protein oxidative damage in the frontal cortex and hippocampus. Under experimental conditions, most of the positive results reported to date could be attributed to thermal effects. The diverse methods and experimental designs as well as lack of replication of many seemingly important studies prevent formation of definite conclusions concerning nervous system health effects from MW exposure. The only firm conclusion that may be drawn is the potential for hazardous consequences of high-power MW exposure. The blood–brain barrier (BBB) is a specialized area consisting of vascular endothelial cells that protect the brain from harmful substances in the circulation. The BBB permits a supply of required nutrients to the brain. The BBB is often the rate-limiting factor in determining permeation of therapeutic drugs into the brain. Additionally, its breakdown is thought to result in pathology of the CNS. Limited evidence suggests that single or short-term MW exposure at or below exposure limits increases permeability of the BBB in experimental animals. However, growing evidence suggests a lack of effect after long-term or repeated exposure.

hypothalamic-pituitary-adrenal system, the HPA axis), thyroid-stimulating hormone (TSH, from the hypothalamic-pituitary-thyroid system), melatonin (from the pineal gland), and both estrogen and progesterone (from the female reproductive system). These chemicals are released into the bloodstream and have very rapid effects on blood glucose level, metabolic rate, nervous system tone, sex, sleep-wake cycle, regulation of the immune system, and certain aspects of emotion and attention. Neurotransmitters, hormones, and cytokines mediate interactions among the nervous, endocrine, and immune system. The function of these systems shows patterns of circadian rhythmicity. The hypothalamus-pituitary axis plays a central role in neuro-immune-endocrine function, releasing hormones and neuropeptides with direct modulatory action on the immune effectors or regulating the hormonal secretion of peripheral endocrine glands. Recent investigations have found significant influences of MW on the levels of various hormones and molecules related to endocrine function. There is no consistent finding of alteration of the immune system of animals acutely exposed to MW radiation at moderate power levels (corresponding to SARs below a few W kg1).

Auditory system

Immune system

The human auditory response to MW pulses, commonly called MW hearing, is a well-established phenomenon. MW-induced sounds are similar to common sounds such as a click, buzz, hiss, knock, or chirp. Induced sounds can be characterized as low-intensity sounds because, in general, a quiet environment is required for the sound to be heard. For mobile phones, the ear is, of all anatomical structures, closest to the device. This may lead to relatively high-energy deposition in the ear, compared to other parts of the body. As a consequence of the limited evidence concerning mobile phone exposure and hearing, two projects were funded by the European Commission to investigate this end point. Results of these projects have led to the conclusion that MWs have no effects on the auditory system.

The immune system plays a relevant role in (1) protection from pathogens and (2) maintenance of tissue homeostasis. These tasks are accomplished through a complex network of cellular interactions and release of soluble factors. Toxic compounds and ionizing radiation, as well as some pathogens, perturb normal regulatory mechanisms and predispose to pathologies. Alterations in immune function may result in ineffective responses against pathogens, reduction in tumor surveillance, and development of immune-mediated diseases. These considerations have promoted several studies on the possible effects of RF/MW radiation exposure on immune system function. MWs may exert effects directly on immunocompetent cells and their cooperative regulatory mechanism, or they might influence neurohormonal regulation of the immune system. Both possible types of effect, together with activation of the stress reaction, trigger adaptation mechanisms that may or may not be beneficial for the host’s immunity. Because of adaptability and redundancy in the immune system and its regulatory mechanisms, the host can generally survive transient perturbations in single elements of immunity. Thus, subtle effects generated by MW and the concomitant stress reaction may lead to clinically undetectable immune dysfunction. Many immunological parameters (lymphocyte differentiation, antibody response, cytokine production, cell proliferation) in animals have been investigated. At best, the results are controversial. However, the role that stress may play, through thermoregulation, or confinement, or

Cardiovascular system

Growing attention has been devoted to cardiovascular effects of MW fields. However, in most cases, only small, sometimes spurious, differences between exposed and nonexposed animals were seen. Interactions with an inhibitor of nitric oxide synthase and long-term blood pressure were significantly altered by MWF. Neuroendocrine system

The neuroendocrine system consists of a number of interconnected subsystems, including the limbic system (see below), which produce a large number of hormones. Among them are corticotropin-releasing factor (CRF), adrenocorticotropic hormone (ACTH), cortisol (from the

Microwaves: Exposure and Potential Health Consequences

both, in particular on the cell-mediated immune response (delayed-type hypersensitivity), may be significant. Behavior

It has been long recognized that exposure to MW at thermal levels may affect performance of learned tasks by adult animals. Behavioral responses are altered in the apparent absence of heating. However, these observations remain controversial. Based on recent studies published by one laboratory, there is limited evidence for a detrimental effect of pulsed MW fields on spatial memory in rats. However, more recent attempts to confirm or replicate these results by independent laboratories have failed to observe field-related behavioral changes. A recent study reported an improvement in spatial memory in immature animals, but no effects on other measures of behavior were documented. Effects on Humans Potential neurological and behavioral effects

Laboratory studies with volunteers have investigated whether low-level exposure to MW fields associated with mobile phones can affect brain function and behavior. Reported reactions to assumed MW exposure include a wide variety of nonspecific symptoms. Most commonly reported symptoms are sleeplessness, fatigue, dizziness, digestive disturbances, and concentration difficulties. By and large, well-controlled, double-blind studies have shown that these clinical symptoms do not correlate with MW exposure. These symptoms may be caused by preexisting conditions, including stress resulting from worrying about perceived MW health effects, rather than the MW exposure per se. To date, only subtle and transient effects have been reported, and any implications for health remain unclear. Exposures used in these studies are similar to those of the head from mobile phone use, rather than to the much lower MW levels associated with general public exposure from base stations. An extensive systematic search identified relevant blind or double-blind provocation studies of individuals who are potentially hypersensitive to the presence of EMF. A meta-analysis found no evidence of an improved ability to detect EMF in ‘hypersensitive’ participants. Weak EMFs are unlikely to be causative factors for neurological symptoms. An investigation into possible differences in blood cells between patients reporting EMF hypersensitivity and normal patients found no differences in lymphocyte response to MW from GSM mobile telephones. Other investigators have likewise concluded that ‘based on the limited studies available, there is no valid evidence for an association between impaired well-being and exposure to mobile phone radiation.’ This statement is applicable to MW in general.

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Cardiovascular, reproductive, immune, and other systems

A majority of studies have related to exposures from cellular telephones. Although both positive and negative findings were reported in some studies, a majority of them showed no significant health effects. Most studies had methodological limitations. Although some cardiovascular effects due to MW were reported in epidemiological studies (e.g., lower 24-h heart rate, blunted circadian rhythm of heart rate), there were no major effects on a large number of cardiovascular parameters in laboratory studies of volunteers during exposure to cell phone emissions. In population-based studies of a wide range of MW frequencies, findings were equivocal for rates of birth defects and fertility, level of reproductive hormone, and generational transmission of neuroblastoma. Changes in immunoglobulin levels and in peripheral blood lymphocyte counts were reported in several studies of radar and radio/television transmission workers. Owing to variations in results and difficulties in comparing presumably exposed subjects with controls, however, it is difficult to propose a unifying hypothesis of immune system effects. Although subjective symptoms may be produced in some sensitive individuals exposed to MW, there were no clear differences in the rate of clinical symptoms between exposed and control subjects in the majority of epidemiological and laboratory studies. Consistent, strong associations were not found for MW exposure and adverse health effects. The majority of changes relating to each of the diseases or conditions were quantitatively small and insignificant.

Epidemiology General population

Epidemiological studies evaluate the effect of exposures under real circumstances in humans; they are important in the process of health risk assessment. As in other types of research, the scientific evidence must be carefully evaluated to assess the quality of all the available studies to determine whether potential sources of error might have influenced the results, such as confounding, exposure misclassification, and selection bias. The relationship between mobile phone exposure and cancer has been carefully studied in the past decade. For brain tumors other than acoustic neuroma, the current evidence does not indicate any increased risk related to phone use. However, the evidence relates only to mobile phone use over a short period of time. A conclusion about long-term exposure cannot be drawn. For acoustic neuroma, findings indicated that there might be an increased risk associated with mobile phone use over a long time period (10 years or more). Several studies are currently under way to evaluate this possibility.

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None of these epidemiological studies has provided clear evidence that MW exposure from the transmitters increases the risk of cancer or any other health effect. The reporting of cancer ‘clusters’ around RF broadcast transmitters and mobile phone base stations has heightened concern among the general public. However, given the random nature of the distribution of cancers in the population, it is not surprising that such clusters randomly appear.

Radar – In general, staff are rarely exposed to direct • emissions of radar signals from antennas. However,

•

Occupational exposure

The use of MW fields in workplaces has increased rapidly during the past decade, mainly due to the increased use of wireless communication technique, in security devices and in medical applications. However, exposure in these cases is, in general, low. These doses are in most cases consistent with recommendations of the EU directive. However, there are exceptions. In office as well as in industry and transportation environments, wireless communications are frequently used. The indoor base stations as well as different blue tooth equipment and WLAN used for man-tomachine or machine-to-machine communication have a low output power and hence the possible exposure of workers within the EU guidelines. Low exposure can also be expected when the sources are enclosed. Examples in industry include plasma metallization and polymerization, plasma deposition and etching, and MW heating (i.e., vulcanization of rubber). These processes are normally performed in closed chambers. However, leakages (especially after reconstruction) or changes in process may occur. Therefore, ongoing monitoring might be a part of the normal process. A particular device, the electronic article surveillance (EAS) system, operates in the MHz range at both continuous swept frequency and fixed pulse frequency. Normally, the staff is only passing trough and therefore exposed for a short time period. However, there might be devices situated near a permanent working place (for instance, a cashier). In that case, actions must be taken to insure that EU guidelines are followed. Particular cases

Industrial MW ovens and MW drying – Often these • ovens are closed and no access is permitted to areas where high-intensity MWs are encountered. However, leakage may occur from cabinets and connections. A regular maintenance program is recommended. MWs are also used for drying of water-damaged buildings. High-powered devices are used with an applicator that has the potential to leak. Owing to the high-intensity MW energy used, it is also possible to be exposed on the other side of a wall or a floor from where the applicator is located. Great care is required while using these devices.

•

during manufacturing, service, and repair, staff may be accidentally overexposed. Medical applications – Three primary applications in medicine are of interest: (1) physiotherapeutic use of diathermy, (2) surgical diathermy, and (3) magnetic resonance imaging (MRI). In medical clinics, RF is used in physiotherapy in shortwave or MW diathermy treatment. In these cases, the applicators are open, and possible overexposure of the staff can occur. Strict adherence to safety instructions is needed. Communications – Rooftop workers near base station antennas might be exposed to MW fields B900– 2000 MHz. Examples of high-risk workers are sheet metal workers, chimneysweepers, and painters. In these cases, the emission properties are well defined, and simple instructions are more relevant than performing measurements.

Conclusions MWs can cause biological effects when exposure is sufficiently intense. Possible injuries include cataracts, skin burns, deep burns, heat exhaustion, and heat stroke. They are primarily due to heating. There have been scattered reports of effects that do not appear to be due to temperature elevation: the ‘nonthermal’ effects. None of these effects have been independently replicated, and most have no obvious consequence for human health. Furthermore, there are no known biophysical mechanisms suggesting that such effects actually occur. Most of the research dealing with the effects of MW EMFs is still investigating nonthermal effects and thresholds for the thermal events. Several long-term studies have been performed in animals and the results may, in part, be extrapolated to humans. Following the very rapid development of mobile telephony, a major research effort was carried out worldwide (tens of millions of Euros per year). European investigators are most active (UK, Germany, Italy, France, Switzerland, and Finland, in particular). Other contributing research groups are from Japan, the USA, and Australia. Extrapolation of data obtained with the first generation of mobile telephony will be difficult if nonthermal effects are assessed. Findings to date, including results of epidemiological studies and animal studies using both short-term and lifetime exposures, have shown no evidence that exposure causes cancer or any other disease. The general weight of scientific evidence does not support an increased risk of health effects from MW technologies in normal use. Furthermore, no accepted cellular or molecular mechanisms of potential health effects have been demonstrated at these low exposure levels.

Microwaves: Exposure and Potential Health Consequences See also: Impact of Mass Casualties Resulting from Radiation Exposure on Healthcare Systems, Ionizing Radiation Exposure: Psychological and Mental Health Aspects, Magnetic Fields: Possible Environmental Health Effects, Radio Frequency Electromagnetic Fields: Health Effects, Risk of Radiation Exposure to Children and Their Mothers.

Further Reading AFSSE (2003) Report to the French Agency on Environmental Health (AFSSE) on Mobile Telephony and Health. www.afsse.fr (accessed 21 September 2009). AFSSE (2005) Report to the French Agency on Environmental Health (AFSSE) on Mobile Telephony and Health. www.afsse.fr (accessed 21 September 2009). AGNIR (2001) Possible Health Effects from Terrestrial Trunked Radio (TETRA). Report of an Independent Advisory Group on Non-Ionising Radiation. Docs NRPB, 12(2), Chilton, NRPB. AGNIR (2003) Health Effects from Radiofrequency Electromagnetic Fields. Report of an Independent Advisory Group on Non-Ionising Radiation. Docs NRPB, 14(2), Chilton, NRPB. Barnes FS and Greenebaum B (2007) Handbook of Biological Effects of Electromagnetic Fields, 3rd edn. Boca Raton, FL: CRC Press. De Pomerai D, Daniells C, David H, et al. (2000) Non-thermal heatshock response to microwaves. Nature 405: 417--418. ICNIRP, International Commission on Non-Ionizing Radiation Protection (1998) Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz). Health Physics 74: 494--522.

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Jauchem JR (2008) Effects of low-level radio-frequency (3 kHz to 300 GHz) energy on human cardiovascular, reproductive, immune, and other systems: A review of the recent literature. International Journal of Hygiene and Environmental Health 211: 1--29. Leszczynski D and Meltz ML (2006) Questions and answers concerning applicability of proteomics and transcriptomics in EMF research. Proteomics 6(17): 4674--4677. Meltz ML (2003) Radiofrequency exposure and mammalian cell toxicity, genotoxicity, and transformation. Bioelectromagnetics (Suppl 6): S196--S213. Repacholi MH (1997) Radiofrequency field exposure and cancer: What do the laboratory studies suggest? Environmental Health Perspectives 105(Suppl 6): 1565--1568. Valberg PA, van Deventer TE, and Repacholi MH (2007) Workgroup report: Base stations and wireless networksFRadiofrequency (RF) exposures and health consequences. Environmental Health Perspectives 115(3): 416--424. Vijayalaxmi OG (2004) Controversial cytogenetic observations in mammalian somatic cells exposed to radiofrequency radiation. Radiation Research 162: 481--496.

Relevant Websites http://www.elettra2000.it/download/atti_convegni/Roma2007/ Veyret.pdf Elettra 2000. http://web.jrc.ec.europa.eu/emf-net/ EMF-NET. http://www.iarc.fr/ENG/Units/INTERPHONEresultsupdate.pdf IARC, Interphone Study.