超GHz遠方界ばく露の偏波を考慮した乳幼児モデルに対する全身平均SARの計算

摘要

With much concern about health effects of children for radio-wave exposure, analytical studies on whole-body averaged specific absorption rates (WBA-SARs) in anatomical based child models have actively been conducted. Based on an anatomically-based Japanese model of three-year-old child, we developed linearly reduced models corresponding to nine-month Japanese 9-month and 0-month infants, and showed the frequency characteristics of 0.3-6GHz WBA-SARs for far-field exposure. In this study, in view of electromagnetic exposure polarization we calculated WBA-SARs up to 15 GHz in Japanese 3years, 9-month and 0-month infant models with a resolution of 1 mm/0.25 mm, and compared them with those estimated from our previously proposed formula for WBA-SARs. As a result, we found that horizontal exposure polarization provides larger WBA-SARs in infant models than vertical one does, and the WBA-SARs have flat characteristics in the frequency range from 1 GHz to 3 GHz, which become almost kept constant at over 10 GHz after decreasing with frequencies. It was also found that the estimated WBA-SARs from the formula are conservative despite exposure polarization. The finding obtained for WBA-SARs in newborn models suggests that the reference level at 0.8 GHz in general public exposure should be employed also for the levels at over GHz bands.%電波の子供に対する影響の関心から，数値モデルに対するSAR（Specific Absorption Rate）計算が盛んにおこなわれている．筆者らは先に，情報通信研究機偶の開発になる3歳児相当の解剖学的数値モデルを等比率で縮減構築した9ケ月幼児および0ケ月新生児モデルを用いて，FDTD法により遠方界ばく露における乳幼児の全身平均SARを300MHz-6GHzの周波数範囲に亘って明らかにした．本稿では，照射電波の偏波を考慮した乳幼児モデルに対して全身平均SARを15GHzまでの周波数範囲に亘って計算し，筆者らの提案した全身平均SAR予測式による超GHzでの推定結果との比較から，乳幼児の全身平均SARは垂直偏波に対して水平偏波を浴びた場合は高くなること，全身平均SARは1GHzから3GHzまではほぼフラットで周波数の上昇と共に一旦低下し上昇する傾向にあるが，10GHz以上ではほぼ一定となること，予測式の結果は偏波に拘わらずconservativeであったこと，などがわかった．新生児に対する全身平均SARの計算と予測式によるconservative推定は，0.8GHzでの現行ICNIRP参考レベルをそのまま超GHz帯において適用すべきことを示唆する．