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Impacts of RF radiation on the human body in a passive RFID environment

Conference Paper

Impacts of RF radiation on the human body in a passive RFID environment

Abstract and Figures

In this paper, we present an experimental study on the SAR of the human head in an environment with a typical RFID reader system. The results predict that 2 RFID reader antennas at distances of 10 cm from the human head operating additively, will contribute to a SAR in the human head of up to a maximum of 2.02763 W/kg, which is above the limit for safe exposure of RF radiation as allowed by FCC is the US (1.6 W/kg).
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Impacts of RF Radiation on the Human Body in a Passive RFID Environment
Darmindra D. Arumugam*
(1)
, and Daniel W. Engels
(1)
(1) Department of Electrical Engineering, University of Texas at Arlington, Texas, USA
E-mail: darumugam@uta.edu
Introduction
The characterization of RF propagation is a theoretical method for evaluating the
workings of electromagnetic radiation in a given environment. Recently, there have been
numerous explorations of the fundamental characterizations of passive radio frequency
identification (RFID) systems in electromagnetic structures [1-4]. These studies have
concentrated on various topics not limited to the holy grails of passive RFID systems, i.e.
metallic and moist environments. Because of the nature of passive systems, attenuation in
these environments is relatively high and often causes an RFID tag to hover towards the
no-read zones. This is typically propelled by the lack of power being harvested by the tag
or attenuation of the tag reply signals. An area of research that has garnered interest is the
use of passive RFID systems in and around human tissues, but this interest has not
previously resulted in significant research.
Human tissues are composed primarily of
molecules, which tend to absorb RF
energy. The rate of absorption is a dosimetric measure known as the specific absorption
rate (SAR). The purpose of this paper is to study and quantify the SAR of the human
body, more specifically the human head and shoulders for the passive UHF RFID reader
environment. UHF RFID reader systems operating under FCC regulations may create RF
energy absorption that could represent a significant impact to the human body, when
present in high densities and/or close proximities.
In this paper, we present the theory and analytical evaluations to study the SAR in the
human head at 1W, 10W, and 100W power output of a 7.4dB gain reader antenna. These
evaluations are presented for distances of 10cm, and 100cm from the human head. The
visualizations presented for the SAR are quantized meaningfully to depict the impacts of
a passive RFID environment on the human body, more specifically the human head. We
also show that an additive absorption environment with just 2 RFID readers at a
proximity of approximately 10cm from the human head presents a SAR above ,
the maximum value allowed by FCC in the US.
Specific Absorption Rates
A living organism subjected to a static RF field or to a non-radiating near field will
typically extract energy from the radiating source. However, the quantitative descriptions
of these mechanisms by which this extraction takes place, is very different at higher
frequencies [5]. At these higher frequencies, both the electrical and magnetic fields of the
incoming electromagnetic waves, after reflection at the boundary conditions, are further
decreased due to energy dissipation [5]. Both and fields decrease exponentially in
nature with distance from the boundaries as depicted in Equation 1, where the skin depth
() is defined as the distance at which the field decreases to  
  of its value just
inside the boundaries [5], where is the ratio of conduction current to displacement
current in the given media.
 
and  




 
(1)
It is widely known that the skin effect becomes much more apparent and significant for
humans and larger vertebrates at higher frequencies [5, 6]. Specific absorption rates, or
SAR, is the rate at which the body or tissues absorbs RF energy when exposed to an
electromagnetic field. A dosimetric measure that has been widely adopted is the time
derivative of the incremental energy () absorbed by, or dissipated in an incremental
mass () contained in a volume element () of a given density () [5], which is
expanded in Equation 2.
 



 






(2)
Experimental Test Setup
The setup used to evaluate the SAR in the human head is presented in Figure 1, where a
rectangular patch antenna with a gain of 7.4dB is positioned directly in front of the
human face, and the SAR is evaluated for antenna distances of 10cm and a 100cm from
the human face. At each instance, results are observed for 1W, 10W, and 100W of
radiated power. The simulation environment used here is Feko 5.3, which is a software
utilizing advanced computational electromagnetic (CEM) methods.
Figure 1: Experimental Test Setup for the Human Body
SAR in the Human Head
Figure 2 presents the SAR in the human head cross-section of the eyes for the radiated
source power of 1W, 10W and 100W at a distance of 100cm. Here we notice that the
maximum absorption is below the accepted limits as required by FCC in the US. Further
evaluations for radiated power at 100 W at a distance of 100cm also reveal limits below
the FCC regulations.
Figure 2: SAR for the human head at 100cm distance of 1W, 10W, and 100W radiated power
To study the impact of an environment with a high density of RFID readers or readers at
close proximity, we study the direct effects at a distance of 10cm instead. These results
are depicted in Figure 3 for the case of 1W, 10W and 100W.
Figure 3: SAR for the human head at 10cm distance of 1W, 10W, and 100W radiated power
From Figure 3 we notice that even at distances of 10cm from the human head, the
induced SAR is below FCC regulated limit of exposure for the human body. However,
we also realize that this limit is breached somewhere within the boundaries of 1W and
10W at this distance from the human face. To study in more detail the level at which this
breach occurs, and to better quantify this problem for an environment pervaded with
RFID systems, we examine the SAR details for all 10cm and 100cm scenarios.
Figure 4: SAR (W/kg) versus radiated power (W) at distances of 10cm and 100cm
Figure 4 is a plot of the of the SAR results from the experiments at 10cm and 100cm for
all radiated power levels. It can be easily conceived from this figure that the FCC limit is
breached between the 1W and 10W radiated power levels at reader antenna to human
face distances of 10cm. Using the equation for the trend-line as depicted in Figure 4, we
can solve for  to arrive at . Since, the FCC limit for RFID reader
systems are at 1W of radiated power, it is clear that 2 reader antennas placed 10cm away
from the human face would provide much higher SAR (Equation 3) than that allowed by
FCC in the US.
  

  (3)
Conclusion
In this paper, we present an experimental study on the SAR of the human head in an
environment with a typical RFID reader system. The results predict that 2 RFID reader
antennas at distances of 10cm from the human head operating additively, will contribute
to a SAR in the human head of up to a maximum of , which is above the
limit for safe exposure of RF radiation as allowed by FCC is the US ().
References
[1] Arumugam, D.D. (2007) ‘Characterization of RF Propagation in Metal Pipes for
Passive RFID Systems’, Master Thesis, University of Texas at Arlington.
[2] Arumugam, D.D. and Engels, D.W. (2007) ‘Characterisation of RF Propagation in
Metal Pipes for Passive RFID Systems’, Int. J. Radio Frequency Identification
Technology and Applications, Vol. 1, No. 3, pp. 303-343.
[3] Arumugam, D.D. and Engels, D.W. (Accepted 2007) ‘Characterisation of RF
Propagation in Rectangular Metal Pipes for Passive RFID Systems’, Int. J. Radio
Frequency Identification Technology and Applications.
[4] Arumugam, D.D., Gautham, A., Narayanaswamy, G. and Engels, D.W. (Accepted
2007) ‘Impacts of RF Radiation on the Human Body in a Passive Wireless
Healthcare Environment’, IEEE APiPH Conference 2008.
[5] Polk, C.; Postow, E., Handbook of Biological Effects of Electromagnetic Fields,
CRC Press, LLC, ISBN 0-8493-0641-8, 1996.
[6] Poljak, D., Human Exposure to Electromagnetic Fields, WIT Press, LLC, ISBN 1-
85312-997-6, 2004.
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