1994 JAN-JUN-HIGH SCHOOL BUILDING COMMITTEE

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�I a s Power-density calculations, depending i lothe frequency and other variables. H - 106 : ANSI exposure limits for uncontrolled 102 - CONTROLLED ENVIRONMENT vironments are lower than those for c —UNCOWROLLEDEN"RONMENT 5 trolled environments, but to compens + 10 i for that the guideline allows exposure let I. to 1O+ muchinthoslonger�me periods nments to be averaged o _ P (generally to -- _ _- "u � minutes). This long averaging time ntc. •i E — SDs u that an intermittently operating RFsou J E (such as an Amateur Radio transmitter)t )' 10 -- E =.== -- - :_ -_= -- a K sow a much lower power density than w AE tD conlimuous-duty station for ngivenl ,_� — -- - lev� and antenna con fi urat ion. ioime averaging isbase)ou [he coucc --- ytoeE EH > that the human bod can withstand greater rate of body heating (and thus,HE•H ? higher level of RF energy)for a short tinthan for a longer period. However, tin-- — -- - --- ,o-' averaging may not be appropriate in coi to' siderations of nonthermal effects of R 10-3 10-2 10-1 100 101 tot 103 10 105 tab energy. FREQtH NCr.MH: The ANSI guideline excludes any Iran mitter with an output below 7 watts becaus such low-power transmitters would not b Fig.4—Proposed 1991 ANSI RF protection guidelines for body exposure of humans. able to produce significant whole-bod z heating.(However,recent studies show lha handheld transceivers often produce power densities in excess of the ANSI standard presence of a steady RF carrier more read- maximum permissible exposure levels. 100-300 MHz.The ANSI standard sets the' within the head). a ily than to an intermittent, keyed or modu- Unlike earlier versions of the standard,themaximum E-field limits between 30 and There is disagreement within the scien- lated energy source.There is some evidence 1991 draft recommends different RF ex- 300 MHz at a power density of I mW/cra Me community about these RF exposure that while EMR may not directly cause posure limits in controlled environments (61.4 volts per meter) in controlled? Guidelines. The ANSI guide" still cancer, it may sometimes combine with (that is, where energy levels can be ac- environments—but at one-fifth that levd Mended primarily to deal with thermal chemical agents to promote its growth or curately determined and everyone on the (0.2 mW/cm2 or 27.5 volts per meter)in effects, not exposure to energy at tower inhibit the work of the body's immune premises is aware of the presence of EM uncontrolled environments. The H-field levels. A growing number of researchers system. fields) and in uncontrolled environments limit drops to 1 mW/cm2 (0.163 ampere' now believe athermal effects should also be None of the research to date conclusive) where energy levels are not known taken into consideration.�1 Y ( gY o or per meter a[ 10 - eratio i p ) 0 300 MHz in controlled: n.Several European proves that low-level EMR causes adverse where some persons present may not be environments and 0.2 mW/cm2 (0.0128` wunlries and localities in the United Stales health effects. Although there has been aware of the EM fields). ampere per meter)in uncontrolled environ•`t have adopted stricter standards than the much debate about the meaning and sig- The graph in Fig.4 depicts the new ANSI mems. Higher power densities are permit.• Proposed ANSI guideline. ).. nificance of this research, many medical standard. It is necessarily a complex graph ted at frequencies below 30 MHz(below:- Another national body in the United authorities now urge"prudent avoidance" because the standards differ not only for 100 MHz for H fields)and above 300 MHz,; Slates, the National Council for Radiation of unnecessary exposure to moderate or controlled and uncontrolled environments based on the concept that the body will not Protection and Measurement(NCRP),has high-level electromagnetic energy until but also for electric fields (E fields) and be resonant at those frequencies and wi8; dso adopted recommended exposure guide- more is known about this subject. magnetic fields (H fields). Basically, the therefore absorb less energy. ?. hacs• NCRP urges a limit of 0.2 mW/cm2 lowest E-field exposure limits occur at fre- In general,the proposed ANSI guide Goe," for nonoccupational exposure in the Safe Exposure Levels quencies between 30 and 300 MHz. The requires averaging the power level over ill= 300 MHz range. The NCRP guideline How much EM energy is safe?Scientists lowest H-field exposure levels occur at periods ranging from 6 to 30 minutes(pG Piers from ANSI in two notable ways:It have devoted a great deal of effort to decid- es into account the effects of module- ing upon safe RF-exposure limits. This is n on an RF carrier, and it does not avery complex problem,involving difficult empt transmitters with outputs below public health and economic considerations. watts. The recommended safe levels have been re- Table 3 vised downward several tiwo in ...cut