Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Dosimetry and Exposure This section reports on the workshop session on radiofrequency (RF) energy, dosimetry, and exposure. As discussed by Dr. van Deventer at the workshop (van Deventer 2007) there is a need to characterize exposure of juveniles, children, pregnant women, and fetuses both for personal wireless devices (e.g., cell phones, wireless personal computers [PCs]) and for RF fields from base station antennas. This characterization includes taking into account gradients and variability of exposures due to the actual use of the device, the environ- ment in which it is used, and exposures from other sources, multilateral exposures, and multiple frequencies. The data thus generated would help to define exposure ranges for various groups of exposed populations. There is a need for reliable and accurate exposure assessment for de- signs of the next generation of epidemiologic studies, such as development of an index that integrates service technology and location of use (both â RF energy includes waves with frequencies ranging from about 3000 waves per second (3Â kHz) to 300 billion waves per second (300 GHz). Microwaves are a subset of radio waves that have frequencies ranging from around 300 million waves per second (300 MHz) to 300 billion waves per second (300 GHz). â RF dosimetry is the science pertaining to coupling of RF waves, e.g., from cell phones to the human body. Because of the human anatomy, RF dosimetry must take into account the shape as well as the heterogeneity of the tissues. The unit for absorbed dose (i.e., rate of energy absorption per unit mass) is Watts/kg. â RF exposure is the quantification of the absorbed RF energy and its distribution for the various parts of the body. The absorbed energy and its distribution within the exposed body is a function of the incident electromagnetic fields described in units of Watts/meter-squared and the spatial variation of these fields. 13
14 IDENTIFICATION OF RESEARCH NEEDS geographic location and whether a phone is primarily used indoors or outdoors). Towards this end, we need tissue-characterized models of chil- dren of different ages and of pregnant women for dosimetric calculations. Specific Absorbtion Rates (SARs) for children are likely to be higher than for adults, both for cell phones and for base station exposures, due to the fact that the exposure frequency is closer to the whole-body resonance fre- quency for shorter individuals such as children (ANSI 1982; Gandhi 1979; Wang et al. 2006; Hirata et al. 2007). Better characterization of SARs for children of various age groups is, therefore, needed. Furthermore, models are not presently adequate for men and women of various heights and for children of various ages. Base stations Wireless networks are being built very rapidly, and many more base sta- tion antennas are being installed. Maintenance personnel may be exposed to fairly high electromagnetic fields emanating from base station antennas unless all of the typically four to six antennas mounted on the base station are turned off. For all of the base station antennas, the radiated power is on the order of several tens of watts, with higher powers being radiated at peak hours of the day. Though not as well characterized, particularly for multiple co-located base station antennas, the radiated RF fields for roof- tops near base stations may also be fairly high. The quantification of SAR distributions from base stations is fairly minimal and those distributions are of concern for professionals involved in maintenance of base stations, building/roof maintenance personnel, and members of the public that live in close proximity to the antennas. There are also subpopulations among the employees, which might be exposed to greater amounts of RF energy than the average population. The characterization of these subpopulations is important. Thus, the interest in base station exposures close to the antennas is driven by the potential health effects on antenna repair professionals and building/roof maintenance workers from relatively high, acute exposures, but the interest in exposures for members of the public that live in close proximity to the antennas or for the public at the ground level at larger distances is motivated by the need to address public concern about very low â Specific Absorption Rate (SAR) is a measure of the rate at which radiofrequency (RF) en- ergy is absorbed by the body when exposed to an RF electromagnetic field. The most common use is in relation to cellular telephones. â Base station antennas mounted on rooftops, on poles, or other elevated positions are the important intermediaries for cell phone communications.
DOSIMETRY AND EXPOSURE 15 level, chronic exposures that are in fact similar to those from existing TV and radio antennas albeit at different frequencies. Most of the reported studies to date have involved one base station antenna and have used mostly homogeneous models, often of simplified circular or rectangular cross sections of the exposed human. One study involving a heterogeneous, anatomically based model consisting of di- verse constituents, but still assuming a single antenna rather than typical arrangeÂments of four to six antennas, is given in Gandhi and Lam (2003). In other words, the studies to date do not pertain to the commonly used multiple-element base station radiators. Also, unlike highly localized cell phone RF energy deposition, the base station exposures involve much, if not all, of the body and would have slightly different radiator origins (for multi-Âelement base stations) and may be multi-frequency as well, par- ticularly if several different-frequency base station antennas are co-located. Furthermore, Âbecause of the whole-body resonance phenomenon, the SAR is likely to be higher for shorter individuals due to the closeness of the frequency/Âfrequencies of exposure to the whole-body resonance frequency. In addition to the rapid growth in the number of base stations since 1990, there has also been growth in other sources of RF radiation from cordless phones, wireless computer communications, and other communications systems. The last general survey of RF levels in U.S. cities was during the 1970s, and an updated survey of RF intensities would be useful background for future epidemiologic studies. There are many indoor wireless systems as well as cell phones, which are used both indoors and outdoors. Mea- surements of the differences in the exposures generated by the use of these devices in these environments will be of value in determining if there are any health effects resulting from exposures to low levels and intermittent sources of RF radiation. Mobile phones The use of evolving types of antennas for cell phones and text mes- saging devices needs to be characterized for the SARs that they deliver to different parts of the body so that this data is available for use in future epidemiologic studies. A great deal of research has been done by many laboratories worldwide to understand coupling of RF energy irradiation from cell phone antennas to the human head. For most of these studies, the â Whole-body resonance: It has been shown that each individual absorbs maximum energy from incident RF fields at frequencies that are higher for shorter individuals. Furthermore the SAR at this resonance frequency is increasingly higher for shorter individuals (Gandhi 1979). As the absorbed energy diminishes inversely with frequency in the post-resonance region, it is still quite high for the shorter individuals at base station frequencies because of the relative proximity of these frequencies to the resonance frequencies.
16 IDENTIFICATION OF RESEARCH NEEDS researchers have assumed that cell phones are held against one of the ears, and studies have used a variety of anatomically based models. Cell phones were assumed to have pull-out linear rod antennas with dimensions on the order of several centimeters. However, most of the recent telephones use built-in antennas of various shapes for which additional published informa- tion is needed. The published results on pull-out linear rod antennas are generally in agreement in that the RF energy coupled to the human head is the highest for the ear and for a limited volume (approximately 3 Ã 3 Ã 3 cm) of the brain proximal to the cell phone (IEEE 1996). As expected, the penetration of the coupled electromagnetic fields into the brain is shallow (approxi- mately 2 cm) at higher frequencies (i.e., 1800-1900 MHz). For cell phones held against the ear, the SAR drops off rapidly for the regions of the brain away from the antenna and is negligible for the rest of the human body except for the hand. Wireless technology is leading to devices such as wireless PCs, handheld devices used for video calls, and other handheld devices for text messaging. In their typical usage, the antennas are closer to the hand or other parts of the body. SAR distributions for these newer devices have been obtained using homogeneous liquid-filled flat phantom models. Though these models are reasonably accurate to get the 1 or 10 Watts/kg average SAR needed for safety compliance testing, they are incapable of providing detailed SAR distributions because of lack of detailed anatomical features, e.g., for the hand or the human lap or parts of the body close to the devices. Addition- ally, such models cannot resolve the detailed RF field distribution at the cellular and subcellular levels. Given a set of anatomical data, the RF field distributions can be modeled and estimates can be made of the effects of various wave forms and carrier frequencies. An important research gap is the lack of models of several heights for men, women, and children of vari- ous ages for use in the characterization of SAR distributions for exposures characteristic of cell phones, wireless PCs, and base stations. Presently, there is negligible or relatively little knowledge of local SAR concentration (and likely heating) in close proximity to metallic adorn- ments and implanted medical devices for the human body. Examples in- clude metal rim glasses, earrings, and various prostheses (e.g., hearing aids, cochlear implants, cardiac pacemakers). Research is therefore lacking to quantify the enhanced SARs close to metallic implants and external metal- lic adornments. â Ifeither the electric or magnetic field has a time dependence, then both fields must be con- sidered together as a coupled electromagnetic field using Maxwellâs equations.
DOSIMETRY AND EXPOSURE 17 Laboratory exposure systems There is a need for improved exposure systems for human laboratory studies. Furthermore, location-dependent field strength needs to be ac- counted for in the characterization of exposures. Most of the present-day exposure systems used in laboratory studies focus on the exposure of the head. Though exposures to the head are relevant for most cell phone ex- posures, whole-body exposures due to base stations are a research need. The laboratory exposure systems also need to include ELF and pertinent modulation protocols. There is a need for reliable and accurate exposure assessment for de- signing the next generation of epidemiologic studies, such as development of an index that integrates service technology and location of use (both geographic location and whether a phone is primarily used indoors or outdoors). For human laboratory studies there has been considerable effort to quantify the uncertainties of the different methods used in dosimetry. However, there is little information about the overall accuracy of the dosi- metric approaches with respect to reality and variability. The accuracy of dosimetric approaches is particularly important as well as the validation of results by several independent investigators to establish SAR variability. The committeeâs evaluation of presentations and discussions at the workshop has resulted in the identification of the following research needs and gaps. Research Needs 1. There is a need to characterize exposure of juveniles, children, pregnant women, and fetuses both for personal wireless devices (e.g., cell phones, wireless PCs) and for RF fields from base station antennas includ- ing gradients and variability of exposures, the environment in which devices are used, and exposures from other sources, multilateral exposures, and multiple frequencies. The data thus generated would help to define expo- sure ranges for various groups of exposed populations. 2. Wireless networks are being built very rapidly, and many more base station antennas are being installed. A crucial research need is to character- ize radiated electromagnetic fields for typical multiple-element (four to six elements) base station antennas for the highest radiated power conditions and with measurements conducted during peak hours of the day at loca- tions close to the antennas as well as at ground level. A study of the wire- â ELF: Extremely low frequency fields, such as the 50 and 60 Hz power frequency fields used in Europe and the United States, respectively. â Some commonly used modulation protocols are TDMA (time division multiple access) and CDMA (code division multiple access).
18 IDENTIFICATION OF RESEARCH NEEDS less RF fields in a properly selected sample of the population is needed to characterize and document rapidly changing exposures. 3. The use of evolving types of antennas for hand-held cell phones and text messaging devices need to be characterized for the SARs that they deliver to different parts of the body so that this data is available for use in future epidemiologic studies. 4. RF exposure of the operational personnel close to newer multi- e Â lement base station antennas is unknown and could be high. These expo- sures need to be characterized. Also needed are dosimetric absorbed power calculations using realistic anatomic models for individuals, including both men and women of different heights. Research Gaps Research Ongoing 1. Although several models are available for children and individuals of reduced stature, a research gap remains in the further development of models of several heights for men, women, and children of various ages for use in the characterization of SAR distributions for exposures characteristic of cell phones, wireless PCs, and base stations. Judged to Be of Lower Priority 2. Presently, there is negligible or relatively little knowledge of local SAR concentration (and likely heating) in close proximity to metallic adorn- ments and implanted medical devices for the human body. 3. There is a need for improved exposure systems for human labo- ratory studies including reliable and accurate exposure assessment for designs of next generation exposure systems for human laboratory studies. Furthermore, location-dependent field strength needs to be accounted for in the characterization of exposures. A very important consideration is the validation of results by several independent investigators so that reliable and accurate exposure assessments are available for both comparisons between systems and between laboratories. 4. An updated survey of the electromagnetic field strengths in the U.S. would improve our knowledge of the exposure levels for the population at large. This survey should take into account the large number of new cell phone stations, radio stations, and TV stations and a wide array of other communications devices. It would include a survey of the difference between indoor and outdoor environments.