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EXPOSURE METRICS FOR RF EPIDEMIOLOGY CELLULAR PHONE HANDSETS.pdf Powered By Docstoc
					Radiation Protection Dosimetry
Vol. 83, Nos 1–2, pp. 165–169 (1999)
Nuclear Technology Publishing



EXPOSURE METRICS FOR RF EPIDEMIOLOGY: CELLULAR
PHONE HANDSETS
Q. Balzano
Motorola
8000 W. Sunrise Blvd
Plantation, FL, USA

INVITED PAPER

Abstract — The parameters are described that characterise the exposure of the users of cellular phones. The parameters are
distinguished in two classes: the human and the cell phone parameters. Among the human parameters the following are discussed:
size and shape of head and neck, manner of holding the phone (left vs. right, finger tips vs. palm contact) and phone position
on the face of the user. The cell phone parameters causing the largest exposure variations are: antenna geometry (size, shape,
extended or retracted) and matching conditions; operating RF power level; proximity of tissue to RF currents on metal parts,
channel access method (analogue, pulsed, CDMA). The large variability of the RF exposure is further expanded by the variety
(ever increasing) of phone models available to users who may change service frequently or sporadically. After a brief discussion
of possible dose definitions and the uncertainty of the ‘user’ of a cell phone for a specific call, the paper analyses the critical
exposure parameters that should be investigated to characterise statistically the RF exposure of the subjects of an epidemiological
study. The improved exposure assessment of the users of cellular phones requires the cooperation of network operators and
equipment manufacturers. The statistics of the most critical parameters, those with variability greater that 10:1, can be collected
by modifying the software and hardware of the cell phone equipment. The paper suggests base station software modifications
and the introduction of cell phone ‘dosemeter’ devices that record some of the critical exposure parameters. A certain number
of these ‘dosemeters’ should be distributed among subscribers to determine the statistical variations of the RF exposure from
cell phones. The paper concludes by recommending a pilot dosimetric study independent from any epidemiological study.

INTRODUCTION                                                          On the whole, exposure assessment has been rudi-
    Epidemiological studies of users of wireless tech-             mentary and unsatisfactory. In particular, epidemiolog-
nology hold the promise to yield definitive information             ical studies of cellular telephone users have relied upon
about the long-term safety of this widespread and rela-            data on call duration and user identification from billing
tively new form of exposure to electromagnetic fields.              records (22). To date there has been no capability to
The paramount value of epidemiologic research is                   ensure that the billed customer was the person exposed
recognised by various authorities, including prac-                 during the phone call nor to measure the level of
titioners of public health (1,2), the guidelines of govern-        exposure by either direct or indirect methods. Ideally,
ment agencies (3,4), the risk assessment guidelines of the         an epidemiological study would measure dose to the
International Agency for Cancer Research (5).                      user by objective methods, but even the definition of
    Despite the importance of epidemiological data, for            dose for users of wireless devices still need to be estab-
assessment of risk for users of wireless equipment, there          lished.
is at present only one directly relevant published study.             This paper addresses many of the gaps in understand-
In that paper (6) Rothman et al. present preliminary data          ing the electromagnetic field exposure to users of cellu-
showing no effects on overall mortality.                           lar telephones. The following sections identify key fac-
    As can be seen from recent reviews (e.g. Ref. 7) most          tors that determine user exposure, place them in relative
epidemiologic research on radiofrequency electro-                  order of significance, propose a definition of dose, and
magnetic fields concerned other technical uses of micro-            recommend several technical means (constituting dose-
waves such as radar (8–10) and, to a limited degree,               meter-equipped phones and radios) to advance capabili-
exposures near broadcasting transmitters (11–13), and              ties for exposure assessment in the context of epidemio-
microwave towers (14) Exposures in these environments              logical research.
are very different from exposures in close proximity to
the RF sources of a cellular telephone. Several research           RELEVANT HUMAN AND CELL PHONE
papers have presented data on persons whose exposure               EXPOSURE PARAMETERS
was evaluated by job titles (10,15,16) and hobbies (17–19).
The results provide a mixture of possible associations               The RF exposure of cellular phone users occurs in the
with disease and evidence against such associations. In            immediate vicinity of the electromagnetic field sources,
some instances there have been efforts to estimate the             which are the currents and the charges on the metal parts
level of exposure based on measurements or estimates               of the device. In electrodynamical terms, the exposure
of radiofrequency exposure for particular jobs (8,20,21).          happens in the reactive near-field of the RF sources. In

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                                                      Q. BALZANO
this condition, there is an energy interaction between the    power levels. The output power of the phone is con-
RF sources (the phone) and the parts of the body in           trolled by the base station and most cell phone networks
contact or in close proximity to the phone. ‘Close prox-      have a 100:1 operating range for the RF power of the
imity’ means a distance no greater than /2 , or about         mobile units. Cellular networks are designed to operate
6 cm in the band 800–900 MHz. In physical terms, the          so that the voice quality of one channel (one frequency)
energy interaction, in matched conditions, minimises the      is limited by the interference of other signals using the
electromagnetic energy stored and maximises that emit-        same frequency in other parts of the cellular system. For
ted by the user’s body and cellular phone.                    this reason the network reduces the RF power of each
   The energy interaction is substantially determined by      roaming unit at the minimum level compatible with the
the geometrical shape of the phone components and the         voice quality required for a phone conversation.
proximity of body organs, as well as their dielectric and
dissipative characteristics. From these general consider-
ations it is possible to identify the relevant human para-    DEFINITION OF DOSE
meters:                                                          Since the early 1980s, the definition of specific
(a) Size and shape of the head and neck.                      absorption rate (SAR) has gained substantial support as
(b) Internal anatomy (fat, muscle, bone content) of           a valid metric of human exposure to RF electromagnetic
    head and neck.                                            energy. The SAR is defined as the ratio of the rate of
(c) Manner of holding the phone (finger tip or palm            RF energy absorption in a very small volume of tissue
    contact, left or right hand).                             and the mass of that volume. SAR is measured in watts
(d) Dielectric characteristics of human tissue.               per kilogram (W.kg−1). The absorbed energy increases
(e) Phone position on the face of the user.                   the kinetic energy of the random motion of the atoms
(f) Hand size and sweatiness.                                 and ions of the exposed biological tissue.
                                                                 With the given definition of exposure metric, it is
Other human geometric or anatomical parameters could          easy to define the dose as the sum of the products
cause minor variations in exposure, but the list above
                                                                         N
has proven theoretically and experimentally (23,24) suf-
ficient to characterise the user exposure.                       Dose =         SAR (ti)   ti
                                                                         i=1
   The list of cell phone parameters is long because of
the variety of phone shapes, sizes and antenna designs        where ti is the time interval of exposure with dose
as well as the number of RF wireless technologies avail-      rate SAR(ti).
able to the consumer:                                            Since SAR is a point and an instant function, it
                                                              remains to specify which space- and time-averaging fea-
(a) Antenna geometry (extended, retracted, size and
                                                              tures of the SAR metric must be selected for use in the
    shape).
                                                              dose formula. The time averaging should be performed
(b) Antenna isolation from phone case.
                                                              over a period of the RF signal or over a time interval
(c) Distance of metal parts (including the antenna)
                                                              during which the phone output power is constant. There
    from the body of the user.
                                                              is also the possibility of considering only the SAR
(d) Operating RF power level and frequency.
                                                              values above a threshold: a dichotomous function which
(e) Channel access technology (Frequency Domain
                                                              requires a detailed knowledge of the statistics of human
    Multiple Access, Code Domain Multiple Access,
                                                              exposure, which may not be available at the present
    Time Domain Multiple Access).
                                                              time.
   The reactive coupling between the body and the                The volumetric averaging of SAR has several poss-
phone determines the antenna impedance. The power             ible options. SAR can be averaged over a small or a
level and the radiation impedance establish the intensity     large volume of tissue. With the most refined theoretical
of the RF currents on the metal parts of the phone and        and experimental tools of RF dosimetry, it is possible
on the body of the user; these currents induced in the        to average SAR over a volume of about 0.1 cm3 or
body constitute the human exposure.                           approximately 100 mg of tissue shaped like a cube.
   Assessment of user exposure from cellular phones is        With this building block, it is possible to average SAR
difficult to model and to measure. Theoretical dosimetry       over an entire organ or major parts of an organ. The
using the most recent numerical methods requires all the      type and size of volume averaging should be selected
storage and numerical/algorithmical resources of high         after the specific pathologies of an epidemiological
power computer workstations. Experimental dosimetry           study have been identified so their aetiologies can be the
(direct measurement) requires specialised RF sensors          basis of SAR averaging criteria. This selection requires
and expensive robotic positioning systems because of          cooperation among epidemiologists, pathologists and
the critical distance factors between RF sources and          experts in RF dosimetry. In fact, the importance of the
observation points in simulated tissue.                       close cooperation between epidemiologists, pathologists
   An important dosimetric variable that also must be         and RF dosimetry experts to evaluate the feasibility of
considered is the fact that the phone operates at different   an epidemiologic study of the users of cellular phones

                                                          166
                                    EXPOSURE METRICS FOR RF EPIDEMIOLOGY
cannot be overstated. Without close consultation, the         ‘critical’ metric in the sense defined above, but can be
selection of the exposure metrics might be inappropriate      controlled as will be explained in the next section.
and defeat or dilute the purpose of a large study.               The variability of exposure with head shape, size and
                                                              tissue types can be bracketed by using numerical or
                                                              experimental dosimetry. Results to date show that the
WHO IS CALLING? WHO IS USING THE PHONE?                       maximum variation of exposure parameters from strictly
   The RF exposure of the individuals in a prospective        anatomical features is less than 2:1 (25) and thus is not a
cohort or a case-control study must be measured over a        critical parameter.
period of time, in order to evaluate the dose. Is there
available a methodology or a means to ensure that a           IMPROVED DOSIMETRIC DATA COLLECTION
specific phone is used only by one person or that a call
is performed only by one individual? At this time, the           At the present time there is readily available tech-
answer must be: No.                                           nology to enhance user device identification; there is
   People sometimes share a phone as part of an office         also the possibility of a novel ‘dosemeter phone’ that
or family plan or just during a call if the need arises.      uses state of the art instrumentation and software to
While office sharing is decreasing with the cost of the        characterise cell phone exposure.
subscriber units, family sharing is becoming more com-
mon with the penetration of the cell phones in the larg-      User and device identification
est strata of the population. For example, teenagers on
a weekend or evening outing often use a parent’s phone.          Uncertainty about user identification can be decreased
   From billing records, where only monthly air time          by use of phone ‘smart cards’, which are plastic cards
and the number of calls are collected, it is not possible     with read-only memory (ROM) electronics. When the
to reconstruct with certainty who made the phone calls.       ‘smart card’ is inserted in the cellular phone, the device
The following sections discuss the development of an          reads the identification of the card owner. This infor-
available personal identifier for call time charging pur-      mation is transmitted to the base station for billing pur-
pose, which will be helpful to reduce the uncertainties       poses and/or to decrease the prepaid value of the card
about user identity.                                          at the end of the conversation. Multiple users of a single
   In the next few years, using voice recognition algor-      phone can have individual cards.
ithms, it will be possible to disable the phone if a user        Call duration is recorded by current technology using
with a voice different from that which ‘trained’ the          beginning and ending time of the RF link during a con-
voice recogniser, tries to use the device. As cell phones     versation. This information can be correlated to phone
become the storage device of private data, a voice rec-       or ‘smart card’ number. The phone electronic serial
ogniser will be an effective way to prevent the misuse        number serves to identify the phone model. These ident-
of sensitive information stored in the phone memory.          ifying data can be recorded and made available for
The voice recognition feature also can be used to ident-      exposure assessment purposes. The various carriers
ify subscriber air time, if the change in voice character-    need to establish uniform procedures and formats for
istics and the call duration are recorded by the com-         data collection and reporting.
munication system.
                                                              A ‘dosemeter’ phone
CRITICAL EXPOSURE METRICS                                        At this time there is no reliable statistical information
   From the discussion in the previous sections it is clear   on the operational power level of phones, how people
that cellular phone user exposure involves a large num-       switch the phone from the left to the right ear and how
ber of parameters, each with its range of variability. We     they position or reposition the phone at the same side
define as ‘critical’ the parameters that change the            of the head. This information can be collected by trained
exposure of a user over a range of 10:1 or greater. With      observers or by instrumenting cellular phones to become
this criterion one can identify the following as para-        ‘dosemeters’. However, the ‘dosemeter’ phones cannot
meters to be controlled:                                      be substantially different in weight, size and shape from
                                                              real phone models.
(1)   User identification and call duration.                      In brief, these are the technical features of a poss-
(2)   RF power level from cellular phone (100:1).             ible ‘dosemeter’:
(3)   Left or right hand use (side exposure 10:1).
(4)   User positioning of phone at the face (10:1).           (1) Pressure points on the narrow sides of the phone.
(5)   Phone model (20:1).                                         These are simple elastomeric pressure switches,
                                                                  which identify the position of the thumb of the user.
  The possibility of an antenna extended or retracted             The side with the lower number of pressure points
causes a variation of about 5:1 in the SAR of certain             identifies the thumb location. One can imagine situ-
regions of the head. Therefore antenna position is not a          ations where this device is defeated, e.g. right hand

                                                          167
                                                         Q. BALZANO
      against left ear, but these should be a statistical min-       microcomputer memory and input/output ports can
      ority of the cases. If the phone is operated from a            be used to store and transmit periodically all the
      holster on the belt, all pressure points are equally           information collected by the other sensors in the
      switched on.                                                   ‘dosemeter’.
(2)   Operating power level data. The phone records the
                                                                    It is clear that the instrumentation of a number of
      power level and the duration of the RF transmission
                                                                 ‘dosemeter’ phones is a substantial task, although well
      during a conversation. The information can be
                                                                 within the present technology of cellular telephony. In
      stored and transmitted to the base station during or
                                                                 fact the prototype is currently under development for the
      at the end of a call.
                                                                 activities of IEEE Standard Coordinating Committee 34.
(3)   Antenna up or down position information. This is
      easily achieved by reading the state of a switch con-
      nected to the base of the antenna. The information         CONCLUSION
      can be transmitted at the beginning of the RF link.           This paper has outlined methodologies which, if feas-
      Changes in the configuration of the antenna during          ible,hold considerable potential to increase the statistical
      connect time will be transmitted and recorded at the       certainty of validating the RF exposure of users of cellu-
      base station.                                              lar phones. The very definition of ‘dose’ requires close
(4)   Phone position at the face of the user. This infor-        cooperation between pathologists, epidemiologists and
      mation is the most difficult to instrument for collec-      experts in RF dosimetry after the specific pathologies
      tion. It requires a set of pressure sensors around the     of an epidemiologic study have been identified. This
      ear piece of the phone and a device to measure the         point is stressed because of its fundamental importance,
      distance between the axis of the antenna and the           given the fact that there is no proven pathway to any
      head of the user. The distance is measured at the          specific pathology by RF energy at the low levels of
      base of the antenna, so the distance metering instru-      SAR typical of cellular phone exposure.
      ment is mounted on top of the phone case. There               To increase the statistical validity of the measure of
      are at least two available prototype devices that can      the RF currents in the users of cellular devices, ‘dose-
      be used for this purpose. They are currently being         meter’ phones should be developed and tested for data
      developed at the MIT Media Laboratory in Cam-              collection purposes. This entails a dosimetric pilot study
      bridge, MA, and at the Lawrence Livermore                  that should be conducted independently of any epidemi-
      National Laboratories in Livermore, CA.                    ologic study. Such a pilot study is a substantial effort
(5)   Antenna matching conditions. This information              that requires close cooperation between manufacturers
      complements the data provided by the positioning           and cell phone service providers, as well as pathologists,
      instrumentation. The antenna matching conditions           epidemiologists and experts in RF dosimetry.
      can be measured by resorting to a relatively simple           If statistically valid RF exposure data can be gener-
      RF circuit board currently being developed as part         ated by ‘dosemeter phones’, then it may be possible to
      of the activities of IEEE Standard Coordinating            design prospective epidemiologic studies to determine
      Committee 34. The RF board output DC voltages              whether cell phones cause adverse health effects in
      (giving the amplitude and phase of the antenna             humans. Without the many preparatory efforts outlined
      reflection coefficient) are read once a minute and           in this paper, epidemiologic studies of users of cellular
      stored in the memory of a microcomputer. The               phones run the risk of being incorrect or futile.

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