Gang Kang

Currents induced in anatomic models of the human for uniform and nonuniform power frequency magnetic fields.

We have used the quasi-static impedance method to calculate the currents induced in the nominal 2 x 2 x 3 and 6 mm resolution anatomically based models of the human body for exposure to magnetic fields at 60 Hz. Uniform magnetic fields of various orientations and magnitudes 1 or 0.417 mT suggested in the ACGIH and ICNIRP safety guidelines are used to calculate induced electric fields or current densities for the various glands and organs of the body including the pineal gland. The maximum 1 cm(2) area-averaged induced current densities for the central nervous system tissues, such as the brain and the spinal cord, were within the reference level of 10 mA/m(2) as suggested in the ICNIRP guidelines for magnetic fields (0.417 mT at 60 Hz). Tissue conductivities were found to play an important role and higher assumed tissue conductivities gave higher induced current densities. We have also determined the induced current density distributions for nonuniform magnetic fields associated with two commonly used electrical appliances, namely a hair dryer and a hair clipper. Because of considerably higher magnetic fields for the latter device, higher induced electric fields and current densities were calculated.

Calculation of induced current densities for humans by magnetic fields from electronic article surveillance devices

This paper illustrates the use of the impedance method to calculate the electric fields and current densities induced in millimetre resolution anatomic models of the human body, namely an adult and 10- and 5-year-old children, for exposure to nonuniform magnetic fields typical of two assumed but representative electronic article surveillance (EAS) devices at 1 and 30 kHz, respectively. The devices assumed for the calculations are a solenoid type magnetic deactivator used at store checkouts and a pass-by panel-type EAS system consisting of two overlapping rectangular current-carrying coils used at entry and exit from a store. The impedance method code is modified to obtain induced current densities averaged over a cross section of 1 cm2 perpendicular to the direction of induced currents. This is done to compare the peak current densities with the limits or the basic restrictions given in the ICNIRP safety guidelines. Because of the stronger magnetic fields at lower heights for both the assumed devices, the peak 1 cm2 area-averaged current densities for the CNS tissues such as the brain and the spinal cord are increasingly larger for smaller models and are the highest for the model of the 5-year-old child. For both the EAS devices, the maximum 1 cm2 area-averaged current densities for the brain of the model of the adult are lower than the ICNIRP safety guideline, but may approach or exceed the ICNIRP basic restrictions for models of 10- and 5-year-old children if sufficiently strong magnetic fields are used.

Effect of dielectric properties on the peak 1-and 10-g SAR for 802.11 a/b/g frequencies 2.45 and 5.15 to 5.85 GHz

Compliance with 1- or 10-g specific absorption rate (SAR) safety guidelines is required in various countries for all newly-introduced personal wireless devices such as Wi-Fi PCs. Even though the dielectric properties of the human tissues are known to be nonuniform and highly variable, relatively rigid adherence to prescribed dielectric properties (/spl epsiv//sub r/,/spl sigma/) is required for compliance testing of such devices. Using some typical near-field radiators, we have examined the effect of dielectric properties for SAR measurement fluids with conductivities varying by 2:1 to show that both 1- and 10-g SARs vary by less than /spl plusmn/2%-4% for the 802.11a band 5.15 to 5.825 GHz and only slightly more at the lower 802.11 b/g frequency of 2.45 GHz. This is due to higher surface SAR but shallower depth of penetration of EM fields for the higher conductivity media resulting in nearly identical SARs for cubical volumes associated with 1- or 10-g of tissue, respectively. Also studied, is the effect of lower /spl epsiv//sub r/ fluids recommended in some standards which results in slightly higher and, thus, a conservative assessment of SAR.

SARs for pocket-mounted mobile telephones at 835 and 1900 MHz

Increasingly, mobile telephones are becoming pocket-sized and are being left in the shirt pocket with a connection to the ear for hands-free operation. We have considered an anatomic model of the chest and a planar phantom recommended by US FCC to compare the peak 1 and 10 g SARs for four typical cellular telephones, two each at 835 and 1900 MHz. An agreement within +/- 10% is obtained between calculated and experimental 1 and 10 g SARs for various separations (2-8 mm) from the planar phantom used to represent different thicknesses of the clothing both for the antenna away from or turned back towards the body. Because of the closer placement of the antennas relative to the body, the peak 1 and 10 g SARs are considerably higher (by a factor of 2-7) for pocket-mounted telephones as compared to the SARs obtained using a 6 mm thick plastic ear head model--a procedure presently accepted both in the US and Europe. This implies that a telephone tested for SAR compliance against the model of the head may be severely out of compliance if it were placed in the shirt pocket.

Comparison of various safety guidelines for electronic article surveillance devices with pulsed magnetic fields

The paper uses the two methods suggested in both the ICNIRP and proposed IEEE safety guidelines for compliance testing of security systems based on time-varying magnetic fields being introduced for electronic article surveillance (EAS), radio-frequency identification, and other applications. For nonsinusoidal pulses that are often used, the two procedures are to treat the exposure as a multifrequency exposure with various frequency components or to calculate the peak induced current densities or electric fields treating the highest of the pulses of duration t/sub p/ as a single frequency, half sinusoid of the same duration and frequency 1/(2t/sub p/). Using either of the procedures, the induced current densities (J) or electric fields (E) are compared to the basic restrictions on J or E for compliance testing. Using a heterogeneous, tissue-classified anatomic model of the human body, we calculate the induced J and E for the various tissues for a realistic, EAS system for two typical nonsinusoidal pulses to show that the two methods give substantially different results. While the approximate but simpler method of treating the pulse as a half sinusoid results in peak induced J or E that may be compliant with safety guidelines, the rigorous method of treating such exposures as multifrequency exposures gives induced current densities or electric fields that may be up to twice as large, thus making such systems potentially noncompliant with the safety guidelines. Since it is more accurate, it is suggested that safety assessment based on the Fourier analysis leading to multifrequency components be used for compliance testing of such devices.

An open-ended waveguide system for SAR system validation or probe calibration for frequencies above 3 GHz

Compliance with safety guidelines prescribed in terms of maximum electromagnetic power absorption (specific absorption rate or SAR) for any 1- or 10-g of tissue is required for all newly introduced personal wireless devices such as wireless PCs. The prescribed SAR measuring system is a planar phantom with a relatively thin base of thickness 2.0 mm filled with a lossy fluid to simulate dielectric properties of the tissues. A well-characterized, broadband irradiator is required for SAR system validation or submerged E-field probe calibration for the Wi-Fi frequencies in the 5-6 GHz band. We describe an open-ended waveguide system that may be used for this purpose. Using a fourth-order polynomial least-squares fit to the experimental data gives SAR variations close to the bottom surface of the phantom that are in excellent agreement with those obtained using the finite-difference time-domain (FDTD) numerical method. The experimentally determined peak 1- and 10-g SARs are within 1 to 2% of those obtained using the FDTD both at 5.25 and 5.8 GHz.

Temperature rise for the human head for cellular telephones and for peak SARs prescribed in safety guidelines

The bioheat equation is solved for an anatomically-based model of the human head with resolution of 3/spl times/3 mm to study the thermal implications of exposure to EM fields typical of cellular telephones both at 835 and 1900 MHz. Up to 4.5/spl deg/C temperature elevation may be caused for some locations of the pinna by a cellular telephone warmed by electronic circuitry to temperatures as high as 39/spl deg/C, with temperature increases for the internal tissues such as the brain and the eye that are no more than 0.1-0.2/spl deg/C higher than the basal values. Another objective was to study the thermal implications of the SAR limits for the occupational exposures of 8 W/kg for any 1-g, or 10 W/kg for any 10-g of tissue suggested in the commonly used safety guidelines. Such SARs would lead to temperature elevations for the electromagnetically exposed parts of the brain up to 0.5/spl deg/C, with 10 W/kg for any 10-g of tissue resulting in somewhat higher temperatures for larger volumes.

Use of the impedance method for ICNIRP-related compliance testing of electronic article surveillance devices

Electronic article surveillance (EAS) systems based on the use of alternating magnetic fields at frequencies up to 10-20 MHz are being rapidly introduced into society to prevent unauthorized removal of items from stores, libraries, and hospitals. The EAS systems may take the form of one or two-sided panels of current-carrying loops or pillars at or near the exit door, and loops hidden in the ceiling and/or the mat on the floor. Another manifestation is the magnetic tag deactivation systems that are mounted as checkout counter top devices. The net result is that an individual passing through or standing close to these devices is exposed to nonuniform vector magnetic fields emanating from these EAS systems. Limits of induced current densities in the human body have been prescribed in the IEEE and ICINIRP standards that may not be exceeded for exposure of the general public or for occupational situations. Computational methods using heterogeneous anatomically-based models of the human body are acceptable to show compliance of new EAS devices. For the present paper, we have used the widely accepted 3-D impedance method to calculate the currents induced in the human body for some representative EAS devices. Since the purpose of the paper is to illustrate the approach and the kind of results that one may expect, the geometrical configurations, the ampere turns and the frequencies have been altered from those used in commercial devices.