Section 4A: Safety Guidelines 172How is radiofrequency radiation measured?RF waves and RF fields have both electrical and magnetic components. It isoften convenient to express the strength of the RF field in terms of eachcomponent. For example, the unit “volts per meter” (V/m) is used to measurethe electric field strength, and the unit “amperes per meter” (A/m) is used toexpress the magnetic field strength. Another common way to characterize anRF field is by means of the power density. Power density is defined as powerper unit area. For example, power density can be expressed in terms ofmilliwatts (one thousandth of a watt) per square centimeter (mW/cm2) ormicrowatts (one millionth of a watt) per square centimeter (μW/cm2).The quantity used to measure how much RF energy is actually absorbed by thebody is called the Specific Absorption Rate or SAR. The SAR is a measure of therate of absorption of RF energy. It is usually expressed in units of watts perkilogram (W/kg) or milliwatts per gram (mW/g).What biological effects can be caused by RF energy?The biological effects of radiofrequency energy should not be confused withthe effects from other types of electromagnetic energy.Very high levels of electromagnetic energy, such as is found in X-rays andgamma rays can ionize biological tissues. Ionization is a process whereelectrons are stripped away from their normal locations in atoms andmolecules. It can permanently damage biological tissues including DNA, thegenetic material. Ionization only occurs with very high levels ofelectromagnetic energy such as X-rays and gamma rays. Often the termradiation is used when discussing ionizing radiation (such as that associatedwith nuclear power plants).The energy levels associated with radiofrequency energy, including both radiowaves and microwaves, are not great enough to cause the ionization of atomsand molecules. Therefore, RF energy is a type of non-ionizing radiation. Othertypes of non-ionizing radiation include visible light, infrared radiation (heat)and other forms of electromagnetic radiation with relatively low frequencies.Large amounts of RF energy can heat tissue. This can damage tissues andincrease body temperatures. Two areas of the body, the eyes and the testes, areparticularly vulnerable to RF heating because there is relatively little blood flowin them to carry away excess heat.The amount of RF radiation routinely encountered by the general public is toolow to produce significant heating or increased body temperature. Still, somepeople have questions about the possible health effects of low levels of RFenergy. It is generally agreed that further research is needed to determine whateffects actually occur and whether they are dangerous to people. In themeantime, standards-setting organizations and government agencies arecontinuing to monitor the latest scientific findings to determine whetherchanges in safety limits are needed to protect human health.FDA, EPA and other US government agencies responsible for public health andsafety have worked together and in connection with WHO to monitordevelopments and identify research needs related to RF biological effects.
