Encyclopedia of Environmental Science and Engineering, Volume I and II

786 NON-IONIZING RADIATIONS

In addition, power and frequency “windows” have been

reported, that is enhanced biological responses have been

elicited within narrow bands of incident power and radiation

frequency.

What is often overlooked in any description of the bio-

logical effects of microwave radiation is that such radiations

have produced benefi cial effects. Controlled or judicious

exposure of humans to diathermy or microthermy is widely

practiced. The localized exposure level in diathermy may be

as high as 100 mW/cm^2.

Exposure Criteria

Schwan in 1953 examined the threshold for thermal damage

to tissue, notably cataractogenesis. The power density nec-

essary for producing such changes was approximately 100

mW/cm^2 to which he applied a safety factor of 10 to obtain

a maximum permissible exposure level of 10 mW/ cm^2.

This number has been subsequently incorporated into

many offi cial standards. The current American National

Standards Institute C95 standard requires a limiting power

density of 10 mW/cm^2 for exposure periods of 0.1 hr

or more; also an energy density of 1 milliwatt-hour per

square centimeter (1 mWh/cm^2 ) during any 0.1 hr period

is permitted. The latter criterion allows for intermittency

of exposure at levels above 10 mW/cm^2 , on the basis that

such intermittency does not produce a temperature rise in

human tissue greater than 1C. More recently, Schwan has

suggested that the permissible exposure levels be expressed

in terms of current density, especially when dealing with

measurements in the near or reactive fi eld where the con-

cept of power density loses its meaning. He suggests that

a permissible current density of approximately 3 mA/cm^2

be accepted since this value is comparable to a far fi eld

value of 10 mW/cm^2. At frequencies below 10 100 KHz

this value should be somewhat lower and for frequencies

above 1 GHz it can be somewhat higher.

The most recent proposal of the American National

Standards Institute (ANSI) specifi es a frequency dependent

criterion, with a minimal elvel of 1 mW/cm^2 in the so-called

resonant frequency range of the human body (approximately

tens of MHz to several hundred MHz) and higher permis-

sible levels at lower and higher frequencies.

The performance standard for microwave oven specifi es

a level of 1 mW/cm^2 at any point 5 cm or more from the

external oven surfaces at the time the oven is fabricated by

manufacturer. 5 mW is permitted throughout the useful life

of the oven.

Because Soviet investigators believe that effects on the

central nervous system are more appropriate measure of

the possibly detrimental effects of microwave radiation

than are thermally induced responses, their studies have

reported “thresholds” which are lower than those reported

in Western countries. Soviet permissible exposure levels

are several orders of magnitude below those in Western

countries.

The Soviet Standards for whole body radiation are as fol-

lows: 0.1 mW/cm^2 for 2 hr exposure per day and 1 mW/cm^2 for

a 15–20 min exposure provided protective goggles are used.

These standards apply to frequencies above 300 MHz. Recent

reports indicate that the Soviet Union has raised the above

mentioned value of 0.01 mW/cm^2 to 0.025 mW/cm^2 ; also, the

Soviet value of 0.001 mW/cm^2 for continuous exposure of the

general population has been raised to 0.005 mW/cm^2.

There appears to be no serious controversy about the

power density levels necessary to produce thermal effects

in biological tissue. The nonthermal CNS effects reported

by the Soviets are not so much controversial as they are a

refl ection of the fact that Western investigators have not

used the conditioned refl ex as an end point in their inves-

tigations.

Measurement of Microwave Radiation

Perhaps the most important factor underlying some of the

controversy over biological effects is the lack of standard-

ization of the measurement techniques used to quantify

results. To date, unfortunately, there seems to be little

promise that such standardization will be realized in the

near future.

The basic vector components in any electromagnetic

wave are the electric fi eld ( E ) and the magnetic fi eld ( H ).

The simplest type of microwave propagation consists of

a plane wave moving in an unbounded isotropic medium,

where the electric and magnetic fi eld vectors are mutu-

ally perpendicular to each other and both are perpendicu-

lar to the direction of wave propagation. Unfortunately the

simple proportionality between the E and H fi elds is valid

only in free space, or in the so-called “far fi eld” of the

radiating device. The far fi eld is the region which is suf-

fi ciently removed from the source to eliminate any inter-

action between the propagated wave and the source. The

energy or power density in the far fi eld is inversely pro-

portional to the square of the distance from the source and

in this particular case the measurement of either E of H

suffi ces for their determination.

Plane-wave detection in the far fi eld is well understood

and easily obtained with equipment which has been cali-

brated for use in the frequency range of interest. Most hazard

survey instruments have been calibrated in the far fi eld to

read in power density (mW/cm^2 ) units. The simplest type of

device uses a horn antenna of appropriate size coupled to a

power meter.

To estimate the power density levels in the near fi eld of

large aperture circular antennas one can use the following

simplifi ed relationship

W

A



16 4

2

P

D

P

p

(near field),

where P is the average power output, D is the diameter of

the antenna, A is the effective area of the antenna and W is

power density. If this computation reveals a power density

which is less than a specifi ed limit, e.g. 10 mW/cm^2 , then no

further calculation is necessary because the equation give the

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