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阅读短文： 技术是一把双刃剑，电磁波的应用也是这样．它在使人类的生活发生日新月异变化的同时也存在副作用--电磁污染．例如，频率超过0.1MHz的电磁波的强度足够大对人体构成威胁．按照有关规定，人体所受到的电磁辐射强度（即单位时间内垂直通过单位面积的电磁波的能量）不得超过某一规定值I e ． 已知某无线通信设备发射电磁波的功率为P，设该通信设备向四面八方均匀地发射电磁波，且电磁波在传播过程中无能量损失．由数学知识可知，球面的面积S=4πγ 2 （式中r为球半径），球半径与球面总是垂直的． 根据上述资料，可估算出人体到该通信设备发射电磁波处的安全距离至少 ______，这时人体所受到的电磁辐射强度才不会超过规定值．
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