Controlled Environment

TB 430133

higher the levels of the absorbed energy and the longer the exposure time, the more

the concern for potential tissue and/or organ damage. The scientific community has

concluded that as the SAR approaches 4.0 Watts per kilogram mass or 4.0 Watts per

2.2 pounds of tissue mass, potential health risks must be addressed, primarily because

at or about this rate there is more energy being absorbed than the body can safely re-

move through perspiration, exhalation, etc. As a result, the body's thermal regulatory

system becomes overloaded. This overabundance of heat energy may cause hyper-

thermia, and is of serious concern. This can be further aggravated by other seemingly

unrelated variables, such as the ambient temperature, the relative humidity and the at-

mospheric pressure (the elevation where the potential overexposure occurs). If the indi-

vidual has recently raised his or her own body temperature from strenuous activity, the

added stress of the impending energy will further increase one's chances for hyperther-

mia. In certain frequency ranges and/or power levels, various organs may be more sus-

ceptible to damage than the body as a whole. The human eye and the male testes are

very sensitive to heat buildup since there is not much blood flow in these particular or-

gans to carry away excess heat. An analogy to the importance of good blood flow is

that of an automobile engine. When an automobile radiator malfunctions because of a

leak or clog, the engine will eventually overheat.

The safety standard for RF radiation is expressed as the Permissible Exposure Limit

(PEL). PELs are based on a WholeBody (WB) Specific Absorption Rate (SAR) of 0.4

and 0.08 Watts per kilogram (W/kg) for controlled and uncontrolled environments, re-

spectively. This incorporates a safety factor of 10 and 50 below the threshold of 4.0 W/

kg of mass for occurrence of potential biological effects. This is the minimum rate at

which a person is overexposed to RF radiation. The amount of RF radiation absorbed

by the human body depends principally upon two factors: the intensity (i.e., power den-

sity level present) and the frequency of the radiation. The power density level is com-

monly expressed in power per unit area or milliwatts per square centimeter (mW/cm2).

(1) Controlled Environment. In a controlled environment, personnel are aware

of the potential for RF exposure as a consequence of employment. Individuals knowing-

ly enter areas where higher levels of RF energy can be reasonably anticipated. Addi-

tionally, it includes personnel who may be exposed during transient passage. For this

environment, the "limits" or permissible exposure levels (PELs) are actually based on

onetenth of the previously mentioned 4.0 Watts per kilogram level or 0.4 Watts per ki-

logram of tissue mass.

(2) Uncontrolled Environment. In an uncontrolled environment, unsuspecting

personnel or the general public could be exposed to electromagnetic radiation. For this

environment, the established PELs are based on a much tighter restriction (1/50) or

0.08 Watts per kilogram, thus ensuring a much larger margin of safety.

In DoDI 6055.1, the SAR is converted to a quantity that can be more easily measured:

power density. Power density is measured in terms of mW/cm2 or power per unit area.

Introduction-5